My plane got slower as it got higher??

My plane got slower as it got higher??

I am a firm beleiver that there is no such thing as a stupid question.  The ony way we learn things is to ask when we don't know.  With that being said, here is my stupid question smile

On December 17, 2003 I took a flight in my plane to celebrate the Wright Brothers first flight.  I climbed my lowly Skyhawk up to 10,500 feet for no other reason that to say I was that high in my plane.  I tooled around at the altitude for a short time and noticed something.  My airspeed was only indicating about 95mph instead of the 115mph I see at, lets say, 5000.  I didn't think much about it at the time.

Last night I took a 100 mile cross country to attend an AOPA safety seminar.  I flew out at 5500' and cruised at 115mph.  On the way home I flew at 8500' and only indicated 95 mph.

I watched the GPS and took into account the near zero wind at that altitude and have concluded that thats all the faster I was going.  I surmised that as I got higher, with the change in airpressure, that maybe the airspeed indicator would not give me an accurate reading.  But as I compared that to the GPS I don't know that I beleieve that now.

So the stupid part of the question is this, does a plane such as mine (non-turbo charged) slow down like that with altitude due to the airpressure being less and the engine therefor not producing as much power?

Below 5000 I stay in the green arc of the tach at 2450 RPM.  Above 5000 I run it at 2600 rpm which is in the yellow arc.

I would love to hear you take on this....

    Dave

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Re: My plane got slower as it got higher??

Dave, you stated  "I surmised that as I got higher, with the change in airpressure, that maybe the airspeed indicator would not give me an accurate reading."  Very good deduction--that is exactly what is happening--hard to believe you didn't learn this in pilot training--no offense meant.  As you gain altitude, indicated airspeed decreases but true airspeed increases (with the same power setting).  Sounds like you may have also let the power setting slip a little if there was a 20 mph difference between IAS at 5000 vs. 8500 msl.  Does your airspeed indicator have an outer ring that slides--called a TAS computer?  At any rate, please read up on the difference between True airspeed and Indicated airspeed---it will make you a better and safer pilot.
Bob

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Re: My plane got slower as it got higher??

Dave,
One other thought that may help; as you climb, the engine produces less power due to the thinner air, so in order to produce the same HP output at higher alts. you must increase the throttle setting as you climb.  In fact, you will have to increase the RPM's also to maintain a certain % of power (i.e. 75% power).  At some point, the throttle will hit the firewall and then the rpm's are gonna start decreasing as you climb any further past that point.  That alt. is probably your best cruising alt., but you should consult the POH for specifics.
Bob

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Re: My plane got slower as it got higher??

Thanks Bob.  No offense taken.  I learned to fly 20 years ago.  Not that that should be an excuse but up until recently most of my flights were confined to the local area and before 3000'. 

   Dave

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Re: My plane got slower as it got higher??

I have another part of this question now.  With this being said, what do you normally cruise at on a cross country flight?  Let's assume a normally aspirated engine, something like my Skyhawk or a Cherokee.

   Dave

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Re: My plane got slower as it got higher??

Dave,
Have you ever gone for a walk or hike at 6,000' or higher?
Assuming you're a "low lander" like me, you'll find that you're out of breath in pretty short order.  You may not be walking/running as fast as you otherwise would.
Here on the west coast with the Sierras (Lake Tahoe) only 4 hrs (driving) away.  Many people here are familiar with the sensation...I've found that to be a pretty good analogy to explain why the normally asperated planes have a service ceiling.
Bigger, more efficient lungs do better at higher altitudes, up to a point.  Mt Everest type climbers have to take oxygen (as do pilots at those altitudes).  The engines need turbo chargers to exceed their normaly asperated service ceilings.  Turbo chargers were origonally invented for airplanes.
When you were on your celebratory flight...If you kept climbing, or trying to climb...you would have eventually slowed to your stall (fall) speed.  The previous owner of my Skylane relayed a story to me of when he got cleared from 19.5K to 21.5K.  The controller told him, "If you can get there, you're cleared".  As he was just reaching 21K', he suddenly found himself recovering from a stall at 17K'.  I guess it was pretty exciting.

On my x/c flights...as a general rule, the longer the flight the higher the cruise alt.  If I'm going to exceed a couple of hours, I'll usually go to 8.5K'+
Your POH should have some tables in it telling you what your fuel usage whould be at given altitudes at different power settings. 
Personally...I like it up there  :-)
Michael

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Re: My plane got slower as it got higher??

I have a Turbo  210 and typically cruise at between 12,000 and 15,000 feet - as indicated above, the advantages of a turbo are apparent at these altitudes.  One advantage of cruising at this altitude is that you're generally above much of the GA traffic which is often well below this.  A rule of thumb that I have used in the past is that no more than 10% of your anticipated trip length should be in the climb mode. A two hour trip combined with a 500 ft/min climb would suggest 6000 feet.

Regards,

Tom.

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Re: My plane got slower as it got higher??

Dave

I normally calc my cruise at 110 kn, usually I can maintain 115 kn or slightly more but the 110 is a little conservative for planning purposes.

Several years ago I only flew locally, but now I frequently fly in the 150 to 250 nm range. After I got used to the cross country they make my 172 more fun and usefull. On that note I determine my approx cruise alt by the winds aloft almost always - only on very short flights do the winds aloft receive a low priority.

even on a 140 nm trip I routinely make, the winds aloft make a  big difference. I recently departed to 3000 ft then after crossing some pressure lines I spent the time to climb to 6500, and then the trip home was at about 2500. On very windy days I've had ground speeds only around 80 kn - but then again the trip home my ground speed was over 150 kn. 

An interesting thing though when the headwinds have been quite strong they always seem to die down ( at least a little ) before I head home - on the other hand It seems like if I had a tail wind going there it has turned into a more severe headwind  for the trip home. I know you can do the math, and I keep in touch with the weather svc, and when I travel at certain times of the day thats the what I could expect, but it's still interesting

The only other thing that receives a higher priority - when family or passengers are on board - is at what alt the " smooth air " is at.

Ken Wanagas

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Re: My plane got slower as it got higher??

Your whiz  wheel (E6B) flight computer can give you TAS at any altitude.  Just need to know IAS (indicated air speed), Altitude, and temperature to do the caculations.
Never hurts to pull out the old E6B once in a while to keep in practice.

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Re: My plane got slower as it got higher??

Thanks everyone for the advice.

Ken;
I really enjoyed flying the other night at 8500', but acording to my POH at that altitude, at 2600' RPM (70% power) I should see a cruise of 124mph.  HA HA, no way, my plane was bearly doing 95mph..  It's probably time to have the engine overhauled, it's been 30 years...

Clay;
Whats an E6B?  Kidding.  I haven't used mine in almost 15 years smile  My GPS has an E6B screen on it, thats what I use smile  I know, I know, I should practice once in a while with the real thing smile

       Dave

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Re: My plane got slower as it got higher??

David,
Was that 124 MPH in the POH in Indicated Airspeed (IAS) or True Airspeed (TAS)?  If it was listed as TAS, then you were going a lot closer to it than you think, if the 95MPH you mentioned was IAS.  IAS decreases as you climb, even as TAS increases.  Read up on them.  Also, how do you know your tach is accurate? OR the airspeed indicator itself?

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Re: My plane got slower as it got higher??

After 30 Years I've slowed down a little too.

Ken

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Re: My plane got slower as it got higher??

Bob; the tach and cable were replaced last year.  They are new.  I routinely match the IAS with my GPS, in both no wind and wind situations where I know what the wind is.  I the airspeed indicator beleive it to be with a few MPH of the GPS.

The 124 MH is IAS accoriding to the manual.  I beleive part of the problem has to do with the engine being so old and it's probably not producing as much power as it once did smile

   Dave

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Re: My plane got slower as it got higher??

Dave; I think the message is getting a little mixed here. Lets consider an aircraft with a normally aspirated engine and a fixed pitch propeller at full throttle climbing from sea level to altitude at a constant IAS. As we increase in altitude the rate of climb will decrease - this is due to the reduction in air density decreasing the effective power available. Simplistically - less air - less mass flow/charge weight - less power. Rate of climb is dependant upon the difference between power available and power required. Power required is a function of TAS and increases slightly compared to the more significant loss in power available. Eventually we will have reached a point where we are no longer able to climb. If our climb speed at this point was Vy this is our absolute ceiling.

This change in density also effects or air/fuel ratio. The engine is set up for sea level operation when we climb - less air / same fuel - engine runs rich usually necessitating the requirement to lean as we climb. Most manufactures recommend leaning in the climb above 5000ft AMSL.

In regard to stalling at 21k the IAS for the stall is essentially constant below the tropopause. If it stalls at 50knots indicated at sea level it will stall at 50knots indicated at 21k. Please note that the speed for maximum excess power (Vy) will usually always be greater than your stall speed.

As for the cruise. If you look at your POH you will see that to maintain a constant horsepower cruise, say 75%, you will need to increase RPM as you increase your cruising altitude. This will give you a relatively constant TAS cruise which will imply a decrease in indicated airspeed. At typical 172 speeds you will lose 1.5 to 2 knots indicated per 1000ft if you maintain a constant horsepower (not throttle position) cruise.

A rough rule of thumb for calculating TAS is: CAS(use IAS) + CAS (use IAS) in nautical miles per minute(divide by 60) multiplied by altitude in 1000s of feet ie. 90KIAS at 10,000ft is approximately 90 + (90/60x10) = 105KTAS - rough but close enough.

Your POH will probably show that above 7000 or 8000ft you will be unable to achieve a 75% cruise ie. you will be at full throttle. If you climb above this level both TAS and IAS will now decrease. If the winds aloft are favourable (tailwinds) it still may be more advantageous to climb and accept the lower TAS but higher groundspeed due increased tailwind. If your engine is a bit tired you may find you are not able to achieve the POH figures and you will reach full throttle at a lower altitude for a given hp cruise.

I hope this is not too long and complicated. As with most things in aviation there are always exception and variations, the above is a simplification for what is hopefully a practical explanation.

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Re: My plane got slower as it got higher??

Thanks to all who have answered.  I've re-learned a few things that I've long ago forgotten smile

    Dave

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Re: My plane got slower as it got higher??

Got a friend that gets pretty slow the higher he gets.

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Re: My plane got slower as it got higher??

Dave, most Owner's Manuals etc. give cruising performance not in IAS, but in True IAS  (same as TAS.)

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Re: My plane got slower as it got higher??

Yes, your engine develops less power at that altitude.


If you were at 10,500 ft and it was 5 degrees C there.  and it was 29.92 baro that day.  Your density alt was 11,730 and pressure alt was 10,500.


If that is about true your indicated airspeed would be 95 Knots and your TRUE AIRSPEED would be 113.2 knots.


As for winds, the scientific wild guessers are usually not right.  So you have to cruise in three or four directions to get an idea of the winds with your GPS.  But if you are heading home and climbing who has time for that stuff.

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Re: My plane got slower as it got higher??

Alex,

The only winds you are usually concerned about are the Head/Tail wind component on your current route. Your GPS will effectively give you an immediate indication of your groundspeed. If you know your TAS then you also immediately know your Head/Tail wind component. There is no requirement to cruise in three or four directions. ie. if you were climbing at 90KIAS passing 5000ft and you groundspeed was 110Kts off the GPS. Your TAS by the rule of thumb mentioned above would be 98kts therefore your tail wind is approximately 12kts (rough figures).

This is handy because if you continue to climb to 9000ft indicating 90KIAS and your groundspeed was still 110kts you have picked up approximately 6kts in TAS but nothing in groundspeed so the tailwind component must have decreased. Whether you stay there or not depends on many factors, turbulence, terrain, ability to hold the same cruise TAS at both levels, etc........

(for 70KIAS climb use ~1.2kts per 1000ft ie. 70/60=1.16nm per minute)

If you were that way inclined you could compare your track made good to your heading and your Head/Tail wind component and deduce an actual wind at that point in time by mental dead reckoning. Most modern GPS have a manual TAS input to enable wind to be displayed.

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