Flaps on takeoff
Flaps on takeoff
<HTML>I Read a question recently posted that asked about flaps on takeoff.
I have seen their use - 10 degrees of flaps - recomended only occasionally in the flight manuals of several cessnas - some of mine recomended their use and some did not.
The only apparent recomended use is 10 degrees for "soft field" only - not necessarily "short field" - It seems that the use of flaps help get you out of the mud or tall grass-soft field but after that - as soon as you are airborne that you give up too much forward movement for the extra lift you have induced. - That is the flaps (10 degrees) gets immediate lift to get the weight off the landing gear - wheels, but with too great of a drag penalty. I have seen it used to get out of the mud then retracted, very carefully and slowly.
I would be open to any further insite into the use of flaps on takeoff, mud or not.
Thanks in advance
Re: Flaps on takeoff
<HTML>Generally speaking most single engine Cessna aircraft can use 10 degrees of flap for take off but it is real value is for ground run reduction, as the lift advantage is usually overcomed by the drag of the flaps before you clear the FAA's mystical 50' tree. Cessna does not recommend retracting the flaps before clearing the tree because the climb tend to stagnate for a few seconds as the pilot readjust the attitude for the no flap condition and it is a bit of a zoo especially for low time pilots. If you do a quick drag verses lift analysis you would find that the Vx (best angle of climb speed) with the flaps is quite low and would leave you in very nose high attitude and big trouble if the fan quit. To compensate for that potential problem Cessna tends to recommend the use of a speed substantially higher than the "real Vx with flaps" which basically negates the value of using flaps for anything but a soft field (minimum ground run) takeoff. Remember, these are generalizations and I'm sure that for certain aircraft, under certain conditions these generalizations will not be true, but I think you will find these thoughts to be true for your basic 150/152/172/182 birds. Now if you really want to start a deep "discussion" get the Piper drivers to discuss the "popping the flaps" theory for short field take offs. If you don't know about this one it's probably best to let a sleeping dog sleep, if you know about it then it is almost for sure to make for a good hangar discussion.</HTML>
Re: Flaps on takeoff
<HTML>The early Cessna 170B's had a 20-degree takeoff flap setting recommended by Cessna. Later B-models had a 10 degree setting, but performance data was not produced for that setting. Instead pilots were instructed to use the 20-degree data. :} "Popping" the flaps is not a Piper exclusive. Any manually operated flaps can be used that way.
While flap use will shorten ground run, it will lengthen climb distance requirements to achieve altitude. So the choice to make is whether ground run available is more critical than total distance to the obstacle. If the idea is simply to get up out of tall grass (so you can accelerate in ground effect) or out of mud or a rough strip so as to avoid pounding your airplane to death,....then certainly a takeoff with flaps can be a valuable technique. Just be aware that the aircraft will not match a flaps-up climb rate. (At least it won't until you reach the recommended flaps-up climb speed,...after which you may retract flaps and increase pitch simultaneously without further penalty.)</HTML>
Re: Flaps on takeoff
<HTML> Extending 10 or 20 flaps makes the wing a different shape. Approximately 80 percent or more of the stall speed reduction of the white arc comes from 10 flaps (meaning if Vs = 40 and Vso = 35, 10 of flaps gives a new stall speed of about 36).
We're NOT talking about split flaps (like the 310) which only cause drag, we are talking aboutt those magnificent fowler flaps found on many Cessnas.
Flaps somewhere around 10 to 20 make a lot of sense if:
it's not gusty
you're at low density altitude
you're at low weight
you have some headwind
the runway's short
In gusts and heavy crosswinds, flaps make the plane weathervane and feel turbulence more, and holding Vx is maybe 10 knots closer to stall speed than Vy (even given stall speed with flaps down
At high(est) density altitude, anything except Vy will cause a descent. So hot tahoe days with lots of weight is a bad time to use any flaps.
At low weights, flaps create more lift than drag to a point.
Some 172 manuals say not to use any flaps for short T/O because the flap advantage is lost on climb over a 50 foot obstacle. This is true at max gross and with no headwind, but otherwise may be false. At lighter weights and with headwind, we wish we had the curvy, slow flying wings of older, slow taildraggers. To get this, drop a little flap.
Lastly, if the runway's short, you'll want to use flaps a) to
get the wheels off quick and b) to reduce the stall speed if you decide to abort or you crash off the end
The effect of flaps in ground effect is even more pronounced.
The downside is that above Vx, the flaps start creating more drag than they're worth in some cases. So when you go to Vy, make sure you pull them up. Another downside is you don't want them in gusts or heavy crosswinds (weathervaning problem and
if you're at Vx you're close to stall speed). And yet anoher downside is a pitch up they provide (esp 182) and how that radically goes away and simultaneously the stall speed decreases if you pull them out too quickly.
I've flown my 172 with less than half fuel and just me and the lowest power setting was 1700 rpm and about 12 of flaps at 70 mph. If I sped up or slowed down or added flaps or took any out, the plane would sink (even though I would never move the throttle).
Go up and try it! This is the max endurance configuration at that weight. It's useless, unless you just wanna hang out...:-P
So I use 10 flaps and Vx T/O a lot to help noise abatement, fly a tight pattern, avoid seagulls at the dump at the end of the runway, and I even avoided a incursing airplane on my runway at MRY one time that way.
There's a good book "Aerodynamics for Naval Aviators" that talks about the different kinds of flaps, the drag vs. lift provided, induced drag, equations for calculating Vs and Va and Vx and Vy at different weights, etc. Maybe check that out? I think everything I mentioned here is in that book.
By the way, you'll notice the 182 says 20 flaps for short T/O. With all that extra power, the 20 flaps realy launches the plane straigh up! Clears obstacles like a charm! I really like the 182 for this... On the other hand using any flaps in a 150 at max gross is not in the manual for T/O and seems to be a joke (the thing hasn't enough power to climb reasonably at anything less than Vy and flaps up).
I hope that helps
Re: Flaps on takeoff
That's a lot of anecdotal information. Much of it incorrect. Example:"On the other hand using any flaps in a 150 at max gross is not in the manual for T/O ..."
Reality: Some C150 manuals actually recommend 10-degrees of flaps for soft/short field takeoffs ...and despite your comment, ...the ONLY performance weight calculated for that situation IS "gross" weight.
Use of flaps do not affect the relationship between Vx and stall speed. Even if they did, it would be inconsequential to the use of them for takeoff.
Use of flaps does not increase "weathervaning". Slower airspeeds (which flaps may allow) can reduce rudder effectiveness and require greater rudder input,...which might be the reason you THOUGHT they do.
"If I sped up or slowed down or added flaps or took any out, the plane would sink (even though I would never move the throttle..." Mark if you don't move the throttle, ...yet "sped" up,...the only way to do that without reducing drag is to reduce your pitch (lower your nose). Of COURSE that would result in a loss of altitude. NOTHING to do with flaps.
What information source do you have to support your statement "Approximately 80 percent or more of the stall speed reduction of the white arc comes from 10 flaps..."? By definition, the lower end of the white arc defines the CAS at which the aircraft will stall with FULL flaps (not 10-Degrees), and that calulation was also done at Full Gross Weight.
There are other errors in your message that cause me to believe you are expressing personal feelings/opinion rather than any educated aerodynamic statement of facts that your message implies.
I'm not trying to provoke any argument, ...I'm just trying to encourage accuracy, if were going to make such statements.</HTML>
Re: Flaps on takeoff
<HTML>I don't believe that lowering pitch necessarily results in a loss of altitude. To explain this, I'll cite an example of the extreme case of L/D:
Lowering pitch when your airspeed is less than best climb may result in a descent turning into a climb with no movement of the throttle. This is called a "stall recovery." Try going up in a plane with full throttle, and pull back on the yoke until you hear a horn and feel a buffet. Then hold it at that pitch and
look at the VSI. Just keep holding that pitch as long as you want, and watch the VSI show a descent. You can use rudders to
maintain straight ahead and not fall off a wing. I call this the "falling leaf". Then try lowering the nose to a slightly lower pitch. The VSI will show less descent. If you lower the nose to near Vy on the ASI, you will probably get a climb. If you lower it past that, you may be in a dive, and now your descent rate increases, the previous induced drag vector becomes a forward vector, and airspeed builds rapidly.
I know this from doing it every single day. I know the L/D configuration from doing it myself in an airplane. I, Mark Boyd, do solemnly swear that I flew over 6 hours in my 172 adding a little bit of flaps as the weight went down and I burned fuel.
I was trying to figure out the "linger configuration." I don't recommend this, since it's totally useless, but I didn't just do it for 5 minutes. I never do this now because the green arc is at 2100, and I'm sure there's a reason the manual doesn't have any info below 2100 RPM. (George can probably tell me about the effects of extended low RPM, I would be grateful to know).
I also know flaps cause a plane to weathervane more because I have sat on the ground with full flaps and the plane has tried to weathervane (while stopped) and hasn't done so as much when the flaps were up. I also have experienced something called "adverse yaw" which is when a plane seems to yaw to the right or left when a aileron is dropped. Sit on the ground on a windy day and position a plane 30 degrees to a headwind and drop the flaps to full. If it's windy enough (and not too windy) the plane will weathervane only with flaps down. I KNOW this because I have DONE this.
I have a 172, the nose wheel was on a oil pan, and I unfettered it. As I was preflighting, I dropped the flaps to full, and when I finished my walkaround, I noticed the nose had moved. I moved it back to the way it was, and it moved again. I pulled the flaps up, moved the nose back, and it didn't move any more! See, I could write for "stupid pet tricks."
I concluded that flaps make the plane weathervane more, even when stopped.
All of these experiences were nice. They helped me form my personal feelings and opinion.
Plus they matched what I had read in some books about flying. One book, the Pilot's Handbook of Aeronautical Knowledge, has a nice chart, figure 1-15, that talks about best lift over drag. It then talks about Angle-Of-Attack and Lift. It seems to directly contradict the idea that lowering the nose and speeding up would result in a loss of altitude.
There is another section that talks about "Effect of Airfoil Shape on Lift and Drag" on page 1-7. It talks about how lowering flaps approximates an airfoil with more lift and more curvature. It says that "more lift is produced (up to a point)." In my circumstance, that point seemed to be 12 or so of flaps at that weight and thrust.
Page 2-2 of the same Handbook talks about the magnificent fowler flaps. The "Aerodynamics for Naval Aviators" book has actual charts which show these flaps and compares them and their settings and shows, among other things, that split flaps seem to mostly add induced drag and not so much lift, and that slight flaps (like 10 degrees) on fowler, give a great stall reduction (approximately 80 percent, but hey maybe +/- whatever you think approximately is, say +/- 80). This makes sense because as the PHOAK also notes on pg. 2-2, AOA also increases as flaps are added, so as flaps are increased past 20 or so, the AOA is increasing but the vector of lift is more rearward (induced drag) and less upward (good lift). Unfortunately the ANA is such a useful book that someone "procured" my copy so I can't give you the exact page number like I did for the PHOAK. Too bad, it explains the relationship between weight and stall speeds and Va and Vy as well, which is really useful.
By the way, the other things George says our absolutely true as far as I know. The bottom of the white arc is full flaps and max gross (also furthest forward C.G.? DON'T quote me on that ). But, some manuals have the stall speeds for other flaps, gear down, bank angles, fwd vs. rearward C.G., and different weights.
The 1965 Mooney M20C mark 21 owner's manual gives stall speeds for 0, 15 and 33 flaps (67/64/57, I think they're plain flaps, not Fowler). In fact, with some higher math, the airline guys and the jet engineers think they can calculate every single one of these speeds accurately for all the configurations (even with forward slats and droops and such)! I'm not at work so I can't get a jetliner manual but for example the sample 737 speed card on page 238 of "The 737 Super Guppy 300-400-500 Simulator Checkride survival manual" by Mike Ray, available at UAL HQ in CO (if you want to fly out for one) mentions VREF of:
for different flap settings. The diff between 0 and 15 is 35 knots, and the diff between 15 and 40 is 13 knots. From this I'm guessing the first 15 of flaps lowers Vref by a lot more than the next 25 degrees.
But we don't have a jet, so we can go up in our little Cessna and try it. It's easiest at most aft legal C.G. in a spinable plane to demonstrate this (otherwise you can't get the yoke far enough back to stall). Try a power off stall with 10 flaps and with 40 flaps. Check the ASI, refer to the CAS table in the manual, and see what results you get. I got about 80% when I did it. Just my personal feelings and opinion, which I was pleased to find matched everything I've read.
"Some C150 manuals actually recommend 10-degrees of flaps for soft/short field takeoffs ...and despite your comment, ...the ONLY performance weight calculated for that situation IS
Absolutely true. However, I was talking about climb over 50 foot obstacle. For short and soft otherwise, the 10 flaps may help get weight off wheels in ground effect. For short with 50 foot obstacle, however, I don't think 10 flaps is in the 150 book. But don't believe me, get a 150 manual and a 182 manual. I may be wrong and the 150 manual may recommend 10 flaps for climb over 50 foot obstacle, but I don't think so.
The point I was trying to make (without maligning 150s, great airplane) is that flap settings for climb over 50 foot obstacle might correlate to excess power. With lots of excess power and climb rate, more flap settings may give better climb over 50 foot obstacle (or maybe not). If this is true, maybe if you have more excess climb for other reasons (less weight, headwind, lower than sea level density altitude like alaska) you can use more flaps.
On the other hand, maybe if your old, runout engine is trying to
get off a hot runway carrying a dead Caribou, flaps aren't gonna help you as much as Vy. The clearest example is one where you are at a weight/altitude such that Vy gives marginal climb while out of ground effect (we practiced this two days ago a few times in a twin commander with one engine sim out), but Vx gives only a sink. OK, so takeoff at Vx is a no-go. So do you use 10 flaps? Is there a new best L/D airspeed with 10 flaps that's lower than Vy? Is this new airspeed Vx? I'd be very careful here because Vx is a power on speed, not a power off speed, so a lack of excess power for whatever reason seems to invalidate this number. Vy tends to be much closer to best glide (in most airplanes I've seen) so excess power would seem to be less of a factor (the speed that gives best climb also gives about the best glide, so how can you go wrong flying that, regardless of excess power?)
So the last post brings up many good points:
10 flaps seems to be good for getting weight off the wheels,
Some manuals ONLY calculate V speeds for max gross
slower airspeeds at touchdown require more rudder for X-wind
bottom of the white arc is Max Gross stall w/ full flaps
And one point I don't understand:
"Use of flaps do not affect the relationship between Vx and stall speed" (hmmm, I dunno what that means. Vx will remain the same multiple of stall speed regardless of flaps? Vx and stall speed remain the same all the time with the same flaps regardless of weight? Vx and stall speed remain the same regardless of weight? Short field Flap setting should remain the same regardless of headwind, density altitude, or weight? OK, I dunno)
And one thing which I disagree with wholeheartedly:
reduce your pitch (lower your nose). Of COURSE that would result in a loss of altitude
One thing I think we can all agree on is that this is a fine newsgroup and we appreciate the discussions (sometimes heated) which reflect the passion with which we fly and love our hobby and profession. It is very, very fun to have such educated and experienced people to discuss and sometimes argue the finer points with.
Another thing we can agree on is that Cessna's rock. I flew a speed canard (Long-EZ type) today for four hours and I tell you, it was sketchy. Roll and pitch rate of a 152, and there the similarities end. Only the engine keeps it from sinking, lands at 80 kts, no flaps, little rubber shock thing in the front, tiny tires so when they touch the nose pitches down hard and bounces, and that was the "experienced pilot" landing it! I didn't land once! My god this is a certified airplane? Don't they label any of the switches? How about a god-blessed weight and balance chart? The pilot in the front seat has to put in a ten pound weight? You can't see the engine when you start it, oil sprays back, not on the windshield, power gives no airflow over the elevator (front canard), you can't keep the nose off on landing.
"I've never done stall practice with two people, and I don't know about spins in this plane, so maybe we won't do that ok?"
And a freakin' hammer for beating your way out of the cockpit? Crazy! Give me two huge doors any day!
I'll tell you, I don't think the Van's RV-9A flies any slower, it has more room, and every other performance factor says it's a great plane (although I've never flown one). I wish they made those into certified trainers...
So I LOVE the docile little lovebird Cessna's. And they'll never pry a Cessna out of my cold, dead fingers...
Re: Flaps on takeoff
<HTML>So the AFH (Airplane Flying Handbook) says on page 7-12 that
"the degree to which flaps should be extended will vary with the airplane's handling characteristics, as well as the wind velocity"
"will vary"... not very prescriptive
But page 7-13 says "To maintain good control, the approach in turbulent air with gusty crosswind may require the use of partial wing flaps." But doesn't say why...again not very prescriptive...
Re: Flaps on takeoff
<HTML>OK, last two thoughts for now
Maybe Old wives tale - the max aileron deflection is the same as the best L/D flap setting
#2 - flaps are behind the main wheels so they create a weathervane even while stopped, like a parachute
No evidence, just fuel for the fire :-)
I guess the FAA had a big flap over this subject a decade ago, recommending full flaps for landing and resulting in many heated discussions...
Re: Flaps on takeoff
<HTML>Thanks for all the input -
I have a better understanding of at least some of the variables in this area, and I'll share many thoughts with others, including instructors and students, because it has been the topic of several sincere questions.
I also can see a few areas where I and Others that I have spoken with may have some misunderstanding - because when we made Individual statements - We have made them with certain assumptions, or mental notes or prequalifications, and I'm not sure the assumptions were conveyed clearly or at all - thus taking Our valid statements " out of context " and that many times seems to make things even more unclear.
Thanks again and Thanks in advance for any additional info
Re: Flaps on takeoff
<HTML>Happy to share,
but I must say no hangar talk or books replace the actual experience of flight. I think the best things to do are to figure out ways to experiment safely in a plane. For example, I once took off and landed using only throttle and rudders. I had tried it before but cheated (used flaps and trim also). But this time I used just rudders and throttle for a landing, then rudders and throttle to taxi back, then took off with throttles and rudder, flew a wide pattern, and landed (still only throttles and rudder).
I had tried this several times, with various safety pilots/instructors. It probably took ten different times to figure out how to control phugoid oscillations with power, how flaps affect pitch, shallowing the final approach, trim, etc. I finally settled on very low weight, forward CG, and a long runway. This worked fine, but took a LOT of finesse for the final result (without flaps, trim, yoke, or moving weight in the cockpit).
In the end, it took a calm day so the stall horn didn't come on for takeoff and the landings were very flat (shallow glide and pretty much 3-point touchdown).
So I would tell you to go out and experiment. In the multi training, we do a drag demo and Vmc demo, but most people have never done the approximate equivalent in a single. So go up there and get to Vy and the lowest power for level flight. Then fool with the flaps and gear combinations and record what the VSI says. If you have a coupla flights, try it at different weights.
Try it with different C.G.'s (you can move weights around in flight). Look back at the trim tab at different C.G.s. You'll get to see really subtle differences that aren't in the manual.
Try a climb test. Take your plane as high as you can go, calculate the density altitude there, and record it. You'll get to see exactly how much rudder helps climb or hurts it if off center, you'll see how much minor pitch oscillations make a difference for that last 50 fpm climb, and you'll have fun.
You'll also get an idea about the excess power of that engine compared to a new one. If it's a great engine, you might need oxygen (hehe).
Go to altitude, get into a full forward slip (full rudder), and while maintaining heading and altitude, gradually pitch up until you can't hold heading. This is faux-Vmca for a single-engine airplane. That's what a serious crosswind landing looks like, and why you can't land slower than that airspeed crossed up like that if you expect to land straight down the runway.
If you have a spinnable plane, try spins at most forward and most rearward legal C.G. Secure all the loose objects, and make sure you are in the C.G. range for utility and spins, as recommended, and take a CFI and/or parachutes if you like. It's fun, and you'll find that at more forward C.G., the elevator doesn't have enough authority to hold a spin without some power. And spins left are easier to induce, and if you do it right, you can do an aileron spin. Oh, and you'll notice your gyros are all screwed up, so if you thought you were gonna use them at night or IFR after the spin recovery, think again.
Take a CFI up to altitude and cut the mixture, stall, and stop the prop. You might notice it glides better. Then try an in-air restart. If it doesn't work, you'll get to practice dead stick landing (hehe). You were over an airport with a big runway, right? (Castle works great for this).
Try accelerated stalls, secondary stalls, trim stalls, etc. Try the "falling leaf" with forward and aft C.G.
You'd get all of these things in a good CFI course (I did), but depending on your comfort level you may be able to do some/most of this solo.
So there is no replacement for experience. And it is great to "know thy airplane".
Re: Flaps on takeoff
<HTML>Mark, I still feel you are having some misconceptions about flaps.
You said, "Some 172 manuals say not to use any flaps for short T/O because the flap advantage is lost on climb over a 50 foot obstacle. This is true at max gross and with no headwind, but otherwise may be false."
I don't know which manuals you are quoting, or if you are quoting them correctly. I have three single engine Cessna manuals here on my desk, the '53 C-170, the '64 C-182, and the '64 C-206. They ALL recommend 20-degrees of flaps for obstacle clearance takeoffs. Although flaps decrease rate-of-climb performance (due to drag) they shorten the distance necessary to clear an obstacle.
You said, " I don't believe that lowering pitch necessarily results in a loss of altitude." You went on to switch from talking about apples to talking about oranges. You stated, "Lowering pitch when your airspeed is less than best climb may result in a descent turning into a climb with no movement of the throttle. This is called a "stall recovery." ........You then go on to describe what actually is a recovery from the back side of the power curve....but you erroneously call it a stall recovery. Not the topic at all. Trust me. If you reduce your pitch by lowering your aircraft's nose the rest of the airplane will follow.
Your other illustration using an experience you had with a twin commander is an example of your misunderstanding of the concept. Multi-engine airplanes have performance charts predicated upon the loss of a powerplant. Those charts either prohibit or do not approve the use of flaps for takeoff above critical weights at high density altitudes. This is because the drag produced by the use of flaps,...even small amounts of flaps....will hurt single-engine climb performance to the degree that flight cannot be maintained with any flaps extended and that the reduction of those flaps (such as for the purpose of increasing speed for climb) will only result in a descent and possible contact with the ground. Your statement that while practicing with an engine simulated out proved that the aircraft would not climb at Vx only confirms the fact that multi-engined airplanes cannot climb at speeds less than Vxse (which is a higher speed than Vx). Vx is lower than Vmc in most twins and THAT is why takeoff at Vx is not a good idea. In fact Vx is rarely even published for multi-engined airplanes. (Turbine equipment charts however, sometimes equate V2 to Vxse.) Your example is not applicable to operation of single engined airplanes and the incorrect use of it only muddies the water. It's part of what I meant when I said "anecdote".
Your quotes referencing Aerodynamics for Naval Aviators and Pilot's Handbook of Aeronautical Knowledge were clearly intended to bolster your argument, but you misunderstand those references as well. Your perceptions following your ad hoc flight tests led you to incorrect conclusions. Think about this. Every device which creates lift also creates drag. At any given speed, the more lift produced,...the more induced drag is also produced. Certainly you and I can agree that initial applications of flaps will produce additional lift resulting in very little additional drag, but that greater applications of flaps will result in greater lift as well as greater drag. But in any situation where we have an equilibrium of thrust, drag, weight, and lift,....i.e., steady state flight,....and we then change the direction of that thrust by lowering our nose,...then there will be a loss of altitude. The fact that the aircraft was trimmed for a set speed which subsequently increased by virtue of our having lowered our nose, and subject to the previous trim position, influenced the nose to return to a higher pitch which in turn resulted in a gain of altitude,...is no cause to claim that lowering your nose did not result in a loss of altitude. It did. Your instrumentation may not have allowed you to observe it, but it happened.
Nor does a descent which is caused by being on the back side of the power curve become arrested and turned into a climb by lowering your nose. It does so only after further loss of altitude is traded for additional speed which brings you to the forward side of the L/D curve, and THAT allows you to trade the additional speed for altitude.
Your example about flaps causing weathervaning is also faulty. Scientific method would require that the wind which you claim acted upon the flaps to cause the nosewheel to slide on the oil pan be measured and controlled. There were no contols in your experiment to guarantee that the exact same wind components were applied to the airplane with flaps up. How do you know, for example, that it wasn't the wind acting upon the vertical stabilizer that caused your airplane to weathervane when the flaps were down? Wind which diminished or changed direction slightly when the flaps were up. Imagine this: Your airplane is sitting with a direct crosswind. The total effect of wind upon your flaps is negligible because the wind is acting upon the length of your flaps,...not upon the width or chord. Even if the flaps were full down only the outboard edge or end-rib of the windward flap would feel any wind. The opposite flap would feel nothing at all. The fuselage would blank out the wind. Are you trying to convince us that the 8 or so inches of that end-rib located about 2 feet aft of the main gear is more influenced by the wind and exerts greater force upon the airplane than the 10 or 12 square feet of vertical stabilizer and the 20-25 square feet of fuselage and tail-cone located much further aft and therefore with much greater leverage? Phooey.
And further, while in flight (and after all, it IS in flight that all this really matters, not on the ground with the nosewheel in an oil pan) the relative wind is the factor with which to contend. Relative wind is directly opposite direction of flight. Any effect of that wind upon flaps aft of the center of gravity will in fact tend to keep the airplane stabilized and facing forward, not weathervaning to the side as you suggest. Any weathervaning perception while in flight will be the result of drifting downwind (due to crosswind) across the runway, while the pilot steers upwind (into the wind) to stop that drift. THAT is what will be perceived as weathervaning (even tho' it's not). Quoting from the Cessna 182 and 206 manuals regarding use of flaps in a strong crosswind, use "minimum flap setting necessary for field length, to minimize the drift angle immediately after takeoff." They are referring to the drift caused by the crosswind upon the fuselage and tail surfaces, not the flaps. The reason for minimum flap useage is to prevent the increased lift from those flaps from reducing tire contact traction with the runway which may allow drifting (not weathervaning). They recommend accelerating to a speed slightly higher than normal (another reason not to use flaps because in addition to drifting their aft center of lift will also encourage "wheelbarrowing"),... and then abruptly pulling the aircraft off the ground.
Actual weathervaning doesn't occur until touchdown and during rollout and even then is usually only a factor on a wet or icy runway. At that point, once again, we're back to that 8 square inches versus those 25 square feet. (In fact, even if you were correct, the opposite, downwind flap (not seeing a crosswind because of the fuselage) would offset the upwind flap thereby straightening the plane out rather than causing weathervaning.) The reason to retract flaps quickly in such a situation is to place as much weight as possible upon the wheels/tires for increased traction to counter the effect of the crosswind upon that huge tail, and to improve braking effectiveness. (In some jets, especially rear-fuselage mounted engine models such as Hawkers, DC-9's, etc. there is actually a negative weathervane effect due to the large amount of fuselage forward of the gear. Flaps are left down in those during rollout because spoilers are deployed to dump the lift and get weight upon the wheels. The flaps provide additional aerodynamic drag for stopping. Drag, by the way, which tends to keep the airplane pointed straight down the runway,...not weathervane.)
You said you didn't understand my comment "Use of flaps do not affect the relationship between Vx and stall speed." (This was in response to your claim that using Vx is "maybe 10 knots closer to stall" "even with flaps down.") It's simple. FAR 1.2 defines Vx. It says it means the speed for best angle of climb. It does not mention any relationship to stall speed. Use of Vx does not change the stall speed of the aircraft whether flaps are used or not. (The use of flaps will change stall speed, but Vx won't have any effect whatever.)
In discussing climbs to clear obstacles you stated, "The downside is that above Vx, the flaps start creating more drag than they're worth ... ...and if you're at Vx you're close to stall speed)." Well, I suggest that 1) if you're attempting to clear obstacles that you shouldn't BE above Vx ...and 2) you are NOT close to stall speed...especially if you're doing this with recommended flaps because the use of those flaps actually reduced the aircraft's stalling speed. (Perhaps you are confusing the published power-off stall speed, with the stall speed when flying Vx with full power. They are quite different. In the 182 they are separated by more than 17 mph.)
I admire your spirit of exploration Mark. I agree that it's fun to go play and get to know your airplane intimately. But it's important to remember that we don't have the instrumentation and control over our experiments necessary to derive new operational data. Perceived performance differences are not valid reasons to toss the proven and approved data and techniques recommended in the approved aircraft flight manuals. And keep in mind, that many who read your suggestions may take our comments and believe it's actually OK to go out there and spin airplanes at both ends of the CG envelope not realizing that errors in log bookkeeping or out of date Wt and Bal reports, and an occasional arithmetic boo-boo can put even an experienced aerobatic pilot in a flat spin from which there may be no recovery. I'd suggest that if anyone is going to go spin airplanes, they keep them away from the extreme edges of the CG envelope. Especially stay away from the aft end. (And don't under any circumstances spin a Traumahawk. But that shouldn't be a problem. We're all Cessna folks.) .02c</HTML>
Re: Flaps on takeoff
<HTML>Hmm...I'll have to find my 150 and 172 manuals. Interesting that the 170 says 20 of flaps. It says use 20 flaps for short field T/O with 50 foot obstacle, or for soft field? Please let me know. That surprises me, since the 170 doesn't seem like it would have that much excess climb. Is the 170 max gross lower than the 172 (2100 or 2200)? I'm hypothesizing that the amount of flaps recommended for short T/O over an obstacle depends on excess lift available.
Apples, oranges, ok. I think we've figured out that there are times (stalls and any time behind the power curve) when lowering the pitch improves climb performance, at least in the long run (due to increased airspeed). In violent agreement here... That horse is ready for the glue factory.
Do flaps weathervane or is it really just extra lift which is lifting weight off the wheels and then letting the tail weathervane? Sounds like chicken/egg. Again time for the glue factory, this horse is DEAD.
"if you're attempting to clear obstacles...you shouldn't BE above Vx .." True. But you might need to be above published Vx. Vx and Vy converge at high density altitude (airplane absolute ceiling). Most manuals don't show this, but just publish one Vx. Be aware, at very high density altitudes using the short field procedure may yeild less or no climb over an obstacle compared to some higher speed. I'm not suggesting you "toss the proven and approved data and techniques recommended in the approved aircraft flight manuals," I'm pointing out that without the mystical 50 foot trees, calm winds, max gross weights, and standard days, the data and techniques may not work. If you understand the relationship between weight, wind, density altitude, climb, and obstacles you'll feel safer (and make better decisions). I've taken off and landed my 172 at 9000 ft density altitudes (Mammoth, Tahoe, etc.). Dangerous? At 600 pounds under max gross, it was fine. Full tanks and three more people, and it'd be stupid. Sadly my flight manual doesn't have performance for different weights, but I KNOW lighter is better.
"(Vx is) NOT close to stall speed" Depends on how you define "close". Vy of 82, Vx of 68, clean stall 48, dirty stall 40. Even if we assumed 10 flaps gave us all that stall reduction, 82-48 = 34, 68-40= 28. At a Vx short field 10 flap climb in gusts and shear, I have less of a margin. I get an occasional horn sometimes when I take off in gusty winds and fly Vx 10 flaps. Does it bother me? Not so much, but I'm more careful not to be too slow...ok maybe this is a very minor point...
"may take our comments and believe it's actually OK to go out there and spin airplanes at both ends of the CG envelope"
I totally agree that you have to BE within the CG range. THINKING you are within the CG range is not enough. Sort of like the difference between having enough gas and THINKING you have enough gas Be careful out there!
Got a glider lesson from the women's altitude glider holder in Hawaii. She said she did a W&B, put weights in tthe front, and went up for spin training in a glider. Got into a many turn spin and finally got out of it at an altitude she was not willing to share. Turns out the W&B was very old and several inches off.
A friend of mine has a Piper Cub. Says he flew to a shop to get a new crank for a friends engine, then he and his friend flew home, and on the way tried some spins. He said the plane spun like a top! Hard to recover too. OOOps, that crank in the back really did it, huh?
But whatever you do, enjoy going out and DOING it! I'm only writing any of this to get you excited about reading the AFH, PHOAK, and AFNA. Great stuff! Totally gives you more capabilities and understanding. But then go out and DO it. Then come back and tell us how it went ;-)
Re: Flaps on takeoff
<HTML>Mark: "Interesting that the 170 says 20 of flaps. It says use 20 flaps for short field T/O with 50 foot obstacle, or for soft field? Please let me know. That surprises me, since the 170 doesn't seem like it would have that much excess climb. Is the 170 max gross lower than the 172 (2100 or 2200)? I'm hypothesizing that the amount of flaps recommended for short T/O over an obstacle depends on excess lift available. "
Ans: The 170 has a gross wt of 2200 lbs. The manual says use 20 flaps for obstacle clearance. I don't think there's any such defined animal as "excess lift", "extra lift", or "excess climb". Climb rates/angles ARE dependent upon excess horsepower available, however. The more horsepower you have over and above that necessary to maintain level flight in ANY configuration may be used toward climb performance. You are correct if you mean that stock 170's do not have an abundance of excess horsepower available. (Although they have an engine that claims to put out 145 hp, they will only do so at 2700 rpm. The props certificated on the airplane limit takoff rpm to less than 2330, which equates to about 120 hp.) Like most airplanes, they will get into places you'll have to dis-assemble and truck them back out of, especially the "B" model with it's slotted, semi-Fowler flaps. (Not true Fowlers.) The performance figures for the 170 B indicate that it requires 20% more distance for any given takeoff than it requires under the same circumstances for landing. I believe the author of that document to be an optimist.
Mark: "Vx and Vy converge at high density altitude (airplane absolute ceiling). Most manuals don't show this, but just publish one Vx. Be aware, at very high density altitudes using the short field procedure may yield less or no climb over an obstacle compared to some higher speed."
Reply: If you are at absolute ceiling and are looking at a 50 foot obstacle in the windshield you'd best have your affairs in order. If you are at absolute ceiling and actually considering a takeoff, wondering whether or not to use flaps for takeoff in order to clear a 50 foot obstacle, you'd better check your oxygen mask for oxygen flow and turn the valve on, or ask a bystander to help you find your way back to the asylum. (And yes, most lightplane manuals only publish V-speed data for gross wt., and many only publish Vx for flaps up, even though they recommend flaps for obstacle clearance. Go figure.)
Mark: "I've taken off and landed my 172 at 9000 ft density altitudes (Mammoth, Tahoe, ..."
Ans: Yeay! Two of my favorite places! (And for a real thrill, don't miss Telluride! Just do your homework.)
Mark:"Depends on how you define "close". Vy of 82, Vx of 68, clean stall 48, dirty stall 40. Even if we assumed 10 flaps gave us all that stall reduction, 82-48 = 34, 68-40= 28."
Opinion: The Private Pilot Flight Test Guide requires the applicant to demonstrate airspeed control MUCH better than + or - 28 kts. If one can't maintain airspeed within + or - 5 kts, one shouldn't be flying airplanes alone. ;Þ
I've spent 35+ years in this business as a professional pilot, CFI, factory demo and production test pilot. I've never failed to learn from these airplanes.
As always, Enjoyed the discussion, Mark.
Re: Flaps on takeoff
Well, you asked the question about flaps, and Mark and George have taught us all more about the subject than we could ever have imagined. I admire both gentlemen's ability to debate the technical aspects of the subject with such vigor.
Now, I started flying in the early 60's (telling my age) with my uncle who was a fish spotter pilot on the Chesapeake Bay in Virginia in a 140. We took off and landed in a 1000' pasture. I thank God now, after hearing Mark and George that I didn't have all this knowledge, because we used 20+ (manual) flaps on takeoff. In fact, I have thought for years and still do, that is the only way to depart a turf runway. It sure didn't seem to hurt the plane, because we did it for years. My Uncle's instructions were simple, get the plane off the ground, its suppose to be in the air. Buy the way, I still land my 172M in that same pasture and take off with 20 flaps.
Oh! Well! Enough about flaps all ready.Probably too many conditions in the POH because of Lawyers anyway.
Re: Flaps on takeoff
<HTML>Boys,all that intercourse on flaps was very interesting. but lets not forget about common sense.Every pilot flying a plane with flaps should have been checked on the use of them for takeoff/landing,and, crosswind takeoff,and,landing.I'm not going to go into my flying background,it would take too long,suffice to say i've been flying since 1946,and still at it in my Cessna 182, flying skydivers out of a grass strip 2000'long.Flaps are needed for short field takeoffs(20)all up wheight,landing full flaps,and depending on wind velocity using only half the runway.If you know how your plane handles under different conditions,and different flap settings, use your common sense,or get checked out on how to use your plane. I find when reading some of these articles, that ther are a lot of people out there that don't really know what they are doing in a plane.True! No one was born with a plane straped on his butt,but,there is no substitute for practice,and,expeirience.Now i'll shut up blue skys. Capn.willy</HTML>
Re: Flaps on takeoff
I appreciate all the comments-
I was looking for backround and " recomendations " - as I have heard the flap question alot over time and sometime from students and new private pilots - sometime some sincere open discussion from instructors.
sincere questions -
and I agree that I and maybe others could spend more time getting checked out or get additional instruction along these lines, but to a fairly new or low time aviator its not clear where to start and it is confusing why there seem to be conflicting recomendations - with some additional insight ( this time I think I spelled it right ) - It makes instruction and practice all the more meaningfull.
And I ask alot even if I thought that I knew enough - Its easier to ask before the flight, and I have even in ultralights which "seemed relatively docile" - and some of the suggestions have enlightened or surprised me ( and others ) .
Anyway - I'll enjoy the practice, although I'm somewhat conservative, and I do appreciate the input.
Re: Flaps on takeoff
<HTML>... when all else fails... - seems a little late to wait untill all else has failed ?
...read the manual...
1969 Model 172 and Skyhawk Owners Manual says - " normal and obstacle clearance take-offs are performed with wing flaps up " - sounds simple enough.
then goes on to say "...10 (degree) flaps will shorten the ground run approximately 10%, but this advantage is lost in the climb to a 50-foot obstacle. There-fore, the use of 10 (degree) flaps is reserved for minimum groung runs or from take-off from soft or rough fieldswith no obstacles ahead." - still sounds simple enough.
then goes on to say "...If 10 (degree) flaps are used for a minimum ground run (with no obstacles), the flaps may be retracted as the airplane accelorates...
...when an obstacle is envolved, use the best angle-of-climb speed of approximately 65 MPH with 10(degree) of flaps." ???
Didn't that say with 10 degree flaps with obstacles ahead ? - I thought the manual just said - 10 degree to be used with no obstacle ?
HMMMMM maybe just a 49 foot obstacle ?
Anyway - thats what I read in the manual and what seems like conflicting info in other manuals also, So I thought it a good Idea to ask and expand on the subject - because I have also heard other pilots ( not limited to new pilots or students ) perplexed by the same question, and not have as clear of an understanding about the subject - I also thought that the subject merited the input of other experienced pilots. But that's just My opinion.
Enjoy flying ( flaps or not )
Thanks again all - for sharing info and your experiences
Re: Flaps on takeoff
<HTML>Well, I guess we've come full circle, huh?
Perhaps it's been stirred until it's muddy again, but Here's what I thought I was expressing more clearly:
The FAA-approved flight manual for the airplane is the final word. Not an "Owner's Manual" which is produced by a mfr's marketing department. The FAA-approved manual will give the official performance figures for the aircraft and is the manual that must ultimately be the one relied upon as to operating the aircraft. (And I didn't mean to wait until the obstacle is looming in the windshield to read it. I meant when all other advice fails, to rely upon the FAA approved docoument.)
The comments you found in your "owners manual" Ken, are confirming what has been discussed here. Namely, 1) Flaps will shorten a ground roll, 2) Flaps will be a detriment in a climb to an obstacle, 3) Flaps may be raised whenever one is above the flaps up stall speed.
What they may not state specifically (and what may be the source of confusion) is that when you raise the flaps, the loss of lift (momentary sink) and the Vx airspeed relationship caused by that act of raising those flaps (more on this later) further injure your ability to clear an obstacle. So most recommended procedures instruct you to leave any flaps used for a short takeoff roll, in that position until the obstacle is cleared.
On light planes, this can be difficult to measure (and also to operate to), but IAS is affected by flap position. Most large planes have airspeed correction tables relative to flap position. They may have different Vx (Vx-flaps up) than they have at Vxfto (Vx-flaps @ takeoff position). Raising flaps from takeoff will present the pilot with the challenge of re-acquiring a new Vx appropriate to the zero-flap configuration. This will inevitably cost in terms of climb performance. So pilots are advised to leave the flaps in the takeoff position until the obstacle is cleared. (And those distances required for that are carefully plotted so the pilot may make the decision prior to pulling out onto the runway, ...as they are also calculated in our little Cessna FAA approved data as well.) Larger airplanes may have a choice of whether or not to use flaps for takeoff, based upon runway length VS density altitude. In fact, at higher density altitudes anything other than a zero flap takeoff might actually be prohibited because the use of those flaps will prevent attaining required climb performance, especially with the loss of an engine. In such cases the pilot must determine whether the available runway is sufficient for takeoff, or whether he must reduce his takoff weight, or wait until temperatures fall.
Smaller planes such as single-Cessna's simply don't have the instrumentation and performance documentation to list all the factors involved. (Their airspeed indication systems are also not up to the task to differentiate between flaps at takeoff or flaps up.) But the principle is the same. Any flaps used for reducing takeoff roll will be a liability in a climb to an obstacle. But reducing them prior to reaching any obstacle is throwing fuel on the fire.
So the choices available to us are: Use the FAA-approved aircraft data to 1) Determine that the runway length is sufficient for takoff, 2) Determine the distance to any obstacle is sufficient according to that data, 3) if the first is not true, then determine if the use of flaps will allow the aircraft to get off the runway surface in the distance available, and 4) will the distance to the obstacle be sufficient with the use of flaps according to the FAA approved data. If not, then we need to wait for better atmospheric conditions (density altitude) or we need to pick a landing spot with longer available distances next time. ;Þ</HTML>
Re: Flaps on takeoff
<HTML>OK George - advise taken - and I have misunderstood at least one or two things along the way -
Speaking of a misunderstanding, I need to ask the obvious - Owners Manual - I guess that I have taken the owners manual as "the" flight manual.
? What is an FAA approved flight manual and/or where and how can most people (and myself) acquire one.
I am asking specifically for a 1969 C172K, but where is a Private pilot and a Student to find this info - any place I have taken lessons and recurrent training (Cessnas, Pipers, and then some) during the last few decades had only the (or a copy of ) owners manual available.
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