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Question About Climbing Performance

Thu Jun 20, 2002 4:20 pm

Hi all

I'm current doing a Physics project on "Why climb performance decreases with increasing altitude for light aircraft". Can somebody explain to me why, as you get higher you cannot climb as fast? I know that the air gets less dense as you ascend how does this affect engine performance and the wings performance?


RE: Question About Climbing Performance

Mon Jun 24, 2002 6:07 am

Climb performance decreases with altitude because of reduction of the engine power available with altitude... obviously, reciprocating engine aircraft cannot achieve very high altitudes and can only be performing better with the use of such things as "turbochargers" etc... Turbopropeller aircraft achieve higher altitude capability, but eventually run into propeller efficiency problem in the decreased air densities... Turbojets achieve higher altitudes than the others because of their engine power... but then they run into problems with the aerodynamic design and lift properties of their wing...
So with an airplane, you can be either "power limited" or "wing limited" as with regards to the altitude you could achieve...
I remember some 20 years ago, flying DC8-60 and 70 series, the DC8-61 had the same wing as the DC8-71, the DC8-63 had the same wing as the DC8-73, despite the tremendous difference in thrust of the CFM-56 as compared to the JT3D original engines, at equal weight, the maximum flight level remained the same regardless of the type of engine... here is some explanation...
At higher levels, the indicated airspeed decreases, even though you have a given constant Mach number, say .82 as it was for the DC8 in cruise... the wing creates lift proportional to the indicated airspeed etc... at one point, the wing cannot produce sufficient lift to carry the aircraft any higher unless a higher angle of attack is achieved, detrimental to drag... Then the wing runs into low speed at high speed buffet limits, which are limiting on both ends of the margins, barber pole (Mmo/Vmo) on one side, stall on the other side...
If you want to expand and research the subject I recommend the manual called "Aerodynamics for Naval Aviators" at the U.S. Govt. Printing Office, it is an excellent publication, and rather inexpensive to acquire...
All the best to you...
(s) Skipper
Posts: 173
Joined: Thu Jun 21, 2001 11:07 am

Another Question

Mon Jun 24, 2002 7:00 am

I've been looking at this question for some time. and waiting for someone with more experience than I to answer it. The answer I was looking for was how does altitude affect the wing's performance. Question is, if you have a wing that stalls at 100 IAS at sea level, would it be safe to assume that the same wing would have the same stall speed of 100 IAS at say 30,000ft?

I am thinking that you would have the same wing performance at sea level as at 30,000ft as long as you had the power to maintain sufficient IAS.

Ziggy  Confused

RE: Question About Climbing Performance

Mon Jun 24, 2002 10:50 am

Skipper hit the nail right on the head. To answer Ziggy's questions, yes the "indicated" stall speed would be identical, but the true airspeed at which the wing stalled would be higher. (The rule of thumb is approximately 2% per 1000 feet above SL.) This is the primary reason why for any given weight the landing rollout distances increase with altitude. As the airport elevation increases, the aircraft's "true groundspeed" at touchdown is greater and the greater the speed, the longer it takes to stop. As an example, take a SL airport on a standard day. If you touchdown at 60 KIAS, your GS will also be 60 KTS. Now, take that same 60 KIAS touchdown at Denver and your KTGS will be about 10% greater or roughly 66 knots - hence the longer landing roll out. If you had the power, the wing would still stall at 100 KIAS as FL300, but the true airspeed at that altitude would be around 160 knots!

As far as climb performance degradation goes, loss of power is the primary culprit as far as the aircraft we fly. Normally aspirated aircraft lose power with altitude. An aircraft's climb ability is directly proportional to the amount of "excess" power that it has available vs. what is needed to maintain level flight. For example, if a 200 HP normally aspirated airplane requires 100 HP to maintain level flight it would (at SL, ISA day) has 100 "excess" HP to use for climb. At 10,000' MSL, the engine might only be able to produce 130 HP, leaving it with a 30 HP surplus. This is the reason why turbocharged aircraft perform so well - you would be able to maintain SL power up until you reached the "critical altitude" for your particular installation. In some cases this is as high as 18,000' MSL.

This is also why light twins typically perform so poorly on one engine. Take, as an example, a Twin Comanche with two 160 HP engines. If that airplane required, say, 150 HP to maintain level flight it would have 170 "excess" HP to climb with. If it lost an engine, it would have lost 50% of its available power, but with just 10 "excess" HP, it may have lost 95% of its ability to climb. Light twins have two engines because they need two engines!

As far as turbine powered aircraft go, the same rules apply. Turbine-powered aircraft are, in essence, normally aspirated, in that they lose power with altitude also. Granted, aircraft designers can play a few games, such as "flat-rating" powerplants, but regardless, they still behave as if they were normally aspirated. Additionally, with turbine-powered aircraft you have to consider the effects of high-speed aerodynamics. Skipper recommended a great book - I highly recommend that you get it for your aviation library.


RE: Question About Climbing Performance

Mon Jun 24, 2002 1:32 pm

Dear Ziggy -
Actually, because of compressibility factors etc... at high altitude, the stall speed would actually be a little bit higher... A rule of thumb for an airplane, if at sea level at a given weight the plane stalls at 100 knots IAS, it is likely to stall 35 knots higher at... FL 350 (35,000 feet)... add 1 knot per 1,000 feet is a rule of thumb that will never get you in troubles...
I know what is coming - next question - well that is only 100 + 35 = 135 KIAS at FL 350... the problem is that in turbulence (or in turns) your airplane starts to be "heavier" because of "G" forces. If a plane banks 45 degrees the wings are carrying the weight of the aircraft x 1.5 times "G" factor... So now if our plane is 600,000 lbs, the wings need to create lift for... 900,000 lbs, the stall speed would be the one for... a 900,000 lbs piece of lead... dont tell me that it is because the flight attendants are overweight...
The result is that oftentimes, the low speed "margin" (stall buffet) is likely to be something like... say 250 KIAS, yet your indicated airspeed is 288 KIAS, and your Mach number is .85, giving you about 490 KTAS... (true airspeed)...
I have learned something to protect me from high speed / low speed margins, with all planes I flew - when in turbulence - is,
(1) keep wings level if you can -
(2) place a little platic bug on your "low buffet speed" -
- bearing in mind that your VMO-MMO "barber pole" is the high speed buffet -
(3) keep you KIAS speed exactly middle of barber pole and that little bug...
- this is an easy and realistic way to figure a "real" turbulence speed... call it an "home made Vra/Mra" turbulence speed...
- and tell the darned flight attendant not to fill your cup to the rim...
All the best, Ziggy...
(s) Skipper


RE: Question About Climbing Performance

Mon Jun 24, 2002 3:26 pm

I just want to clarify a point here. What Skipper says about compressibility and high speed aerodynamics is absolutely correct. However, the effects of compressibility can be pretty much ignored at speeds less than about .3 Mach, so you guys don't have to run around adding 1% per 1000' when you're out flying your Turbo-Tripacer. As far as the 1% rule of thumb goes - that too pretty much depends of the particular aircraft. Pilot's use the term "Coffin Corner" to refer to the situation in which an airplane has climbed to such a high altitude that the difference between the low speed stall buffet and the high speed Mach buffet is only a few knots. Aircraft with the newer technology wings tend to have more margin between the High Speed Mach Buffet and Low Speed Stall Buffet. In our case, our low speed buffet at FL390 at normal cruise weights would be down in the neighborhood of .55 Mach, the high speed buffet is at the barber pole or .875 Mach. If I run it up to FL450, the low speed buffet would increase to .56 Mach, there is no change to the high speed. I've seen aircraft where this wasn't the case, for example the DC-9. In my post, I ignored the effects of compressibility in my post to keep things simple.
Posts: 173
Joined: Thu Jun 21, 2001 11:07 am


Tue Jun 25, 2002 3:03 am

Thanks for the replies, although it does open up a whole new can of worms. I will certainly add "Aerodynamics for Naval Aviators" to my collection and also hope it will contain the information I'm looking for.

Ziggy  Big thumbs up

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