I need performance help

I need performance help

<HTML>I have a 1965 172.  The top speed it achieved was 110mph at 2400 rpm.  The engine was due for its first overhaul at 2000 hrs (200 hrs overdue), so I had it overhauled with new crank, cam and Superior cylinders and have 80hrs on engine.  Now it achieves 115mph at 2550 rpm (pushed to the firewall.)  I can ususally be found around 5,500 msl (give or take 1000 ft) and the spec shows at 2550 rpm I should see 127mph needless to say I can't hit higher rpm to hit the 2700 rpm and 136 mph spec.  A new tach was put in at overhaul because the old one was way off during the cold winter.  I burn about 8-9 gph and 1 qt every 14hrs.  Is it my engine or airframe or are these specs in the POH inflated?  Any advice or experiences are welcomed.

James</HTML>

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Re: I need performance help

<HTML>Your POH specs are for a new plane. Compared to old 172s I'v been in, yours sounds pretty good. What is the condition of your prop? What are the breakin directives for your new cylinders?</HTML>

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Re: I need performance help

<HTML>Hello James

I also have a 1965  172  had it for 2 yrs now.That's about what I was getting before I decided to have my engine overhauled.I was getting 120 mph @ 2450rpm on a non turbulent day. By the way who did your overhaul for your engine.I 'm going to go with  Western Skyways In Colorado.And do you know what your VSI is on take off what's your static RPM.Do you have a standard prop on it or do you have a climb prop or even a cruise prop it all makes a differents.Can you tell me what you paid for your overhaul.Western Skyways is 17000 w/ millinium cylinders.
Let me know I would appreciate any ovehaul experience.

Thank You</HTML>

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Re: I need performance help

<HTML>Thanks for the input guys.  I will try to answer all your questions.
PAUL-The prop was checked as well at overhaul.  We sent it off to be balanced and painted.  We took our plane back from overhaul with 5 hrs of break-in already performed.  Our A&P insist that he performs the first few hours after overhaul.

VAUGHN-Our work was done at a small shop in northern Alabama.  The shop has been doing the overhauls and 100hr checks on all the rental aircraft at our airport and the owner of the aircraft recommended him to us.  We paid $15,500, but we had to have a new cam and crank.  VSI with just me (220 lbs) on a cool fall day and 600 ft msl is about 600-700 ft.  Add that other person and I am stuck at 500 ft.  My static rpm is 2200 and I do have a standard prop.</HTML>

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Re: I need performance help

<HTML>Hello, James!
  What you are witnessing is an engine that is "propped" for cruise flight.  Your C145/O300 engine only achieves 145 horsepower at 2700 rpm, but your prop has enough pitch in it to prevent that rpm from being developed in level flight.  This is customary and preferred.  You don't want to run around at 100% rpm all the time do you?  That'd be pretty hard on your engine.
  Most people assume that the full 145 horsepower occurs on takeoff, but that's not the case.  If you look at your tach while holding the brakes with full throttle,...what you are observing is called "static" rpm.  In your airplane that's probably around 2250-2350 rpm.  It is specified in your aircraft Type Certificate Data Sheet (TCDS or TCS) that the prop should be a certain model, a certain length and at static power to be within a certain minimum/maximum rpm.  Typically your airplane is only developing around 120 horsepower at takeoff.  To increase that horsepower, you'd have to install a prop with a flatter pitch which would let that engine run up to 2700 rpm.  Unfortunately, that would be contrary to the aircraft TCS, which is a ruling document.  It would also cause your engine to overspeed in cruise flight without drastically reducing throttle (rpm), which in turn would cause a loss of cruise airspeed.
  When your airplane was produced, the happiest combination of engine/prop/airframe was carefully proportioned.  The only simple adjustments available to the average owner is to install either a climb/standard/or cruise prop, which will allow slightly better performance in that particular prop's regime, but at a loss of performance in the other parameters.  Aerodynamics always ends up in a compromise.  For most average uses, a standard pitched prop is the best selection.
  FYI, your aircraft's allowable prop combinations were a McCauley Met-L-Prop with a diameter of 76 inches, and pitch of: Climb=52", Standard=53", and Cruise=55".  For an example, an EM7653 stamped on your prop's hub would indicate a hub/blade shape/profile/model (EM), a diameter (tip to tip) of 76", and a pitch of 53" (what distance the prop would travel forward in one complete rotation.)</HTML>

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Re: I need performance help

<HTML>Thanks for the input George. 
I do have a standard prop, and my static is about 2200 rpm.  Are you telling me that the numbers that I am looking at in my POH are not going to be numbers that I will be able to achieve from a speed standpoint.  I realize that if I don't get the rpm I won't get the speed, but a person told me today that if the plane can't hit achieve the speed (dirty I guess) I won't hit the rpm.  Am I just dreaming thinking I should be able to hit 2700 rpm (if I wanted to) in cruise and the rated speed?
If I should be able to get closer to the stated #'s where should I start?</HTML>

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Re: I need performance help

<HTML>The aircraft was never designed to achieve 2700 rpm in LEVEL flight.  The only way you're likely to see that rpm is in a descent with a wide open throttle.  (Hint: Where do you see the word "level" in the performance tables?)
A fixed pitch prop is a compromise.  If you want to achieve max engine rated rpm in level flight, you're going to have to have a very fine-pitched prop (i.e. a custom prop and therefore requiring an STC to be legal on your aircraft) or a constant-speed prop.  (Which in your case will require lots of other mods and certification issues.)  The bottom line is:  Your airplane is a compromise.  With a fixed pitch prop you can have either a short-takeoff climber,  a mediocre performer that will cruise economically, or a long-range cruiser with poor takeoff/climb performance.  You can't have it all in the same airplane without going to a constant speed or a variable pitch prop.
  What is confusing to most people is that they have a concept that a 145 hp engine should be able to put out that hp with whichever prop is normally installed.  But that's simply not the case.  Real life is: An airframe mfr (Cessna) chose to install an engine mfr's (TCM) engine which COULD produce 145 hp under certain rpm conditions.  But Cessna realizes that to run an engine at full rpm constantly would not only violate TCM's continuous power rating on that engine, but that the fule burn and maintenance costs would quickly escalate if the engine spent it's (short) life at full power.  So Cessna chose a prop that would allow max open throttle at typical altitudes, yet would result in an rpm that would place the engine in it's 65%-75% max continuous cruise power settings (about 2450 in this case).  Meanwhile, Cessna realizes that some people will want to know what speeds to expect in descents at higher rpms, so they publish it in a performance table.  (You are simply making the mistake of thinking that such rpms/speeds are attainable in continuously level flight.)</HTML>

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Re: I need performance help

<HTML>THANKS GEORGE.  What you were saying in the first response just sunk in with your last response. Thanks for the input.  I will be happy that my engine purrrrrsss and my airframe is solid.  You have saved me alot of  $$$$ chasing numbers.

James</HTML>

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Re: I need performance help

<HTML>Try installing a EGT/CHT.  I have a 172G and I've had amazing
results

900 ft/min climb at 1750# at sea level WVI

110 knots at 9500 feet, 1750#

The plane is a rocket, with 200 hrs on the engine and
just cleaned plugs and perfect mag timing.

Even at 9500, full throttle and 50 deg below EGT gives
me 2600 rpm and 7.2 GPH.

Clean plugs, timed mags (30 rpm drop), finessed mixture,
power descents at 200 ft/min, and flying a little higher
on longer trips should help.  I get 4.5 hours and still
land with 7 gallons in the tanks (3 unusable).

Pulling weight out helps too.  I have one radio/VOR and
fly with half tanks for day VFR 2 hours or less.  Then
I load any bags all the way in the back baggage compartment
for better speed (aft CG).  It does wonders.

Oh, but leave the wheel pants on...</HTML>

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Re: I need performance help

<HTML>
  James,
I do have the same performance problems as you have
We do operate a Cessna 172M model with a Lyc O320 but now took delivery of a 172D with a Cont.O300 and find the performance quite disappointing and are in the process of installing a variable pitch propeller
This at least would enable us to get max. RPM on take-off, at the moment we are
  getting 2300rpm 25" mp and at a density altitude of 3000ft this translates in 108 HP on take-off start Prior to lift-off the rpm comes up to 2380 but this is still only 112HP and I am surprised to get airborne after all
Could you advice me what density altitude you are operating on take-off
Regards Bram Water</HTML>

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