I grabbed these numbers from the 182T-G1000 POH:
SL 71%(163hp) 130KTAS 130KCAS 134KIAS
10k 71%(163hp) 141KTAS 121KCAS 125KIAS
As you can see, true airspeed goes up at altitude while indicated airspeed goes down for the same horsepower output.
Horsepower required for same airspeed at higher altitude
- Thread starter birdus
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SL 71%(163hp) 130KTAS 130KCAS 134KIAS
10k 71%(163hp) 141KTAS 121KCAS 125KIAS
As you can see, true airspeed goes up at altitude while indicated airspeed goes down for the same horsepower output.
EdFred
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Yes, because my max allowed RPM is 2575.
sarangan
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Assuming you are able to maintain 75% power at 10,000 ft your IAS will be roughly 90% of the sea level value. If your cruise speed at sea level is 150 KTAS, your speeds at 10,000 ft will be 165 KTAS and 135 KCAS.
This follows from the drag and power equations (with many assumptions), which I can share if anyone is interested.
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Ari
I'd be interested. Including in whether a fixed-pitch prop changes things much. My limited understanding is that a constant-speed prop has more consistent efficiency of turning horsepower into thrust at different airspeeds and, maybe, altitudes.
EdFred
Taxi to Parking
That's not what I get. It's about 20% difference, not 10%.
https://www.dauntless-soft.com/PRODUCTS/Freebies/TrueAirspeedCalculator/
0, 29.92, 15, 150: TAS = 150
10000, 29.92, -5, 150: TAS = 174
10000, 29.95, -5, 129: TAS = 150
Also follows the rule of thumb of 2%/1000'
David Megginson
Pattern Altitude
The same percentage power results in (more or less) the same calibrated airspeed and fuel burn for all the altitudes where normally-aspirated piston-engine planes fly. That means that if your ASI reads 112 KIAS when you're flying at 60% power near sea level, it should read about 112 KIAS when you're flying at 60% power at 10,000 ft (your true airspeed will be much faster at 10,000 ft, of course, and with a fixed-pitch prop, your RPM will be much higher as well).
sarangan
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Your 20% difference between IAS and TAS is correct. But that's not what I was talking about. The 10% is the change in indicated airspeed compared to sea level for the same power setting. In other words, if you get 150 KIAS at sea level, you will get 135 KIAS at 10,000 ft assuming the same power.
EdFred
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sarangan
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Propulsion power = crankshaft power * propeller efficiency = drag * TAS
Drag = Cd * A * 0.5 * rho * TAS^2, where Cd is drag coefficient, A is area, and rho is density.
Therefore, propulsion power = Cd * A * 0.5 * rho * TAS^3 (the cubic power of speed is what makes it very very difficult to fly fast)
At 10,000 ft, density is 75% of sea level. If we maintain the same propulsion power, TAS will increase by (1/0.75)^(1/3) which is 1.1, or 10% increase.
At 20,000 ft, density is 50% of sea level. If we maintain the same propulsion power, TAS will increase by (1/0.5)^(1/3) which is 1.25, or 25% increase.
Pitot (dynamic) pressure has the same drag equation. In other words, the pitot tube is like a miniature airplane "flying" through the same air.
Dynamic Pressure = Cd * 0.5 * rho * TAS^2. The drag coefficient is different for the pitot tube, but that does not change the results.
At 10,000 ft, the 75% density and 1.1x TAS gives a dynamic pressure of 0.9, or 10% decrease. So your IAS will read 10% lower compared to sea level.
At 20,000 ft, the 50% density and 1.25x TAS gives a dynamic pressure of 0.78, or 22% decrease. So your IAS will read 22% lower compared to sea level.
All of this assumes that the drag coefficient does not change, which is not exactly true.
Considering a Cessna Skylane, at 2000 ft the published cruise performance at 70% power is 146 KTAS. At 10,000 ft, at 70% power, the cruise speed is 155 KTAS. This is a 7% increase in speed. Considering that we are comparing it to 2000 ft (instead of sea level), this is consistent with the calculated 10%. If you can somehow maintain the same 70% power up to 20,000 ft, the airspeed will be close to 180 KTAS.
The indicator will show the following: At 2000 ft, it will be 146 KIAS. At 10,000 ft, it will be 130 KIAS. At 20,000 ft, it will be 115 KIAS.
Edit:
I realized a small error in the above calculation. KIAS is not just dynamic pressure, but dynamic pressure minus static pressure. Density and pressure both decline with altitude by roughly the same amount. So, I don't believe it changes the conclusions that much.
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birdus
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I was assuming (in my head) a constant speed prop, although I can see how you could accomplish the same thing either way: higher RPM with a fixed-pitch prop or coarser pitch with an adjustable-pitch prop. I've gotten a couple great answers, but this one is probably the most concise. I'm definitely learning some interesting things in this thread. Thanks!
NordicDave
En-Route
yup. My turbo-normalized 182P makes ~165 TAS up high.
Prior to Covid while flying over the Sierra Nevada’s at 16,500MSL, applied nearly full sea level power and climbed 750ft/min to 17,500MSL.
This “draggy boat” of an air frame does pretty well up high with sea level cruise power settings.
Important to watch TAS carefully with Turbo aircraft, so not exceed v-speeds. Keeping it simple, I map TAS as an overlay to Airspeed to watch the upper V-speeds.
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birdus
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MauleSkinner
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You would get the same IAS if the thrust was the same, but the thrust <> horsepower.
AlphaMike
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I've always understood that V speeds are given as IAS and not TAS.. This is because the aircraft behaves similarly at the same IAS no matter what the TAS is. Is this not true?
David Megginson
Pattern Altitude
So basically, you're describing the difference between CAS and EAS (?)
MauleSkinner
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Up at higher altitudes (note the reference for “turbo” aircraft), TAS can become the limiting factor.
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Checkout_my_Six
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Depends…..for many turbo’d engines it’s cht’s temps and the lack of an inner cooler that limits altitude and HP.
MauleSkinner
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“Can”.
sarangan
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That is still true to a large extent. But there are other limitations such as flutter which are related to TAS.
FastEddieB
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Here that is graphically:
The TAS axis will vary plane-to-plane, but the shape of the graph should be the same for any NA plane. The lesson is, for any given desired power output, the higher the better, until you get to an altitude where that power can no longer be maintained.
MauleSkinner
Touchdown! Greaser!
Of course, part of the lesson is that you have to take off at that altitude, as climb and descent are not part of the lesson.Here that is graphically:
The TAS axis will vary plane-to-plane, but the shape of the graph should be the same for any NA plane. The lesson is, for any given desired power output, the higher the better, until you get to an altitude where that power can no longer be maintained.
David Megginson
Pattern Altitude
Here's a table of corrections from CAS to EAS. As you'll see, the difference at 10,000 ft and 150 kt is neglible (less than a knot), but it starts to matter a lot at higher altitudes or faster airspeeds.
birdus
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I'll post my math again so you can point out my error:
200 0
194 1000
188.18 2000
182.5346 3000
177.0586 4000
171.7468 5000
166.5944 6000
161.5966 7000
156.7487 8000
152.0462 9000
147.4848 10000
Based on your math, at just over 33,000 feet, engines begin producing negative horsepower. How does that work?
Arnold
Cleared for Takeoff
The beauty of the Ovation.
David Megginson
Pattern Altitude
A bit above 65% at 10,000 ft DA for my O320-D3G, according to my Piper PA-28-161 POH.
David Megginson
Pattern Altitude
It's not quite linear, and varies a bit by aircraft. Best look at your POH or at your Engine Operator's Manual. But generally speaking, 8,000 ft DA is a reasonable ceiling for 75% power with a normally aspirated engine.
David Megginson
Pattern Altitude
Power = Thrust * Velocity. At higher density altitudes and thinner air, you're producing less thrust but moving at a higher velocity (TAS), so the net power and fuel consumption stay the same (up to your engine's ceiling for any given power setting).
David Megginson
Pattern Altitude
Up to about 10,000 ft DA and 200 KCAS, you can act as if the IAS doesn't change for any practical purpose. At 75% power, mid-weight, with no vertical air motion, my Piper PA-28-161 indicates about 110 KIAS at both sea level and 8,000 ft. The fixed-pitch propeller RPM is much higher for 75% power at 8,000 ft of course, but the fuel consumption is the same.
Above 10,000 ft DA and/or 200 knots, you have to start considering compressibility errors to get an equivalent airspeed (EAS) from CAS. I shared the chart for that earlier in this thread.
dtuuri
Final Approach
Ok, I'm not sure what you're arguing. Did I (and Wolfgang Langewiesche) say something untrue? Or are you saying YOU can act as if IAS doesn't change for any practical purpose? The OP wants to know if the same power at 10K' will give the same IAS (AoA) as at SL and I cited Stick and Rudder for the answer "NO".
You are using a compounding formula. General consensus is that's a simple percentage.
The engine has to run. At some point, it can't.
Looking at the graph excerpt from the POH in post #61, 75% power at 7500DALT gives 131MTAS, which translates to 117MCAS. 75% power at 0DALT gives 121MTAS which translates to 121 MCAS, which is not the same as 117MCAS.
David Megginson
Pattern Altitude
I think you just misread Langwiesche. I assume you're referring to this passage in Chapter 20 ("Thin Air"):
Langwiesche isn't talking about the same power setting; he's talking about the same throttle position. In 1944, most (all?) light pistons didn't come with power-setting tables, so there was no use talking about % power for his audience. But there are some clues:
- He says the RPM will go down at 10,000 ft for a fixed-pitch prop, but at a constant power setting, it would go up.
- In the next paragraph, he writes that true airspeed has dropped from 100 mph to 93 mph, but at a constant power setting, it would have gone up.
- In the next paragraph, he writes that fuel consumption has dropped from 5 gph to 3 gph, but at a constant power setting, it would stay the same.
birdus
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Great post!
David Megginson
Pattern Altitude
Yes, it's fair to say that individual aircraft might have their quirks. For my 1979 Piper PA-28-161, the POH says that 75% power with a 2,325 lb load will give me 112.5 KTAS at 0 ft DA (== 112.5 KCAS) and 127 KTAS at 8,000 ft DA (== 113 KCAS). Not exactly the same, but too close to matter, especially considering that the position of my CG or the presence of a rising or falling airmass can have a much bigger effect on airspeed. For the past 19 years, I've consistently seen roughly the same KIAS in my Piper from sea level to 10,000 ft at any given power setting/gross weight (except in updrafts and downdrafts), and obsessive monitoring of my fuel consumption (and later, a portable optical tach) have confirmed my power settings.
Also note that in real life, I see a few knots slower than the POH promises at both altitudes. Probably time for some new paint.
dtuuri
Final Approach
Nope. Keep on reading, lots of more good gems. Like this for example:
"Next in the logics of high altitude, there is an engineering fact that is sometimes overlooked when pilots discuss an airplane's altitude behavior: At higher altitude, the airplane needs more power. In any given flight condition— such as flat cruising flight or slightly mushing flight or nearly stalled flight—it will fly faster at the higher altitude; it will fly faster without additional drag. But it nevertheless will need more power."
birdus
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If I wanted to do the computations in real-time while I was in my airplane, climbing, at each thousand-foot increment, how would I do it?