Does the prop behave differently with altitude?

[carusoam]

Thoughts on tip speed....

The link that Ross supplied regarding propellor selection gives tip speed as a two part calculation:

Rotational velocity (prop spinning) + Linear velocity (speed of airplane) = total velocity

It involves a little vector math, but explains why our tip velocities are actually a bit higher than just the rotational velocity.

They give an example of a glassair with an 84" prop rotating at 2700 rpm and exceeding the limits of efficiency.  The solution was to dial back the rpm to increase airspeed....

Best regards,

-a-

[201er]

So how do the turbo guys fly their machines in the flight levels? Is it a different prop or same one? Since the engine is getting complete aspiration, can you fly the same RPM settings as a non turbo at low altitudes? Or do you need to increase RPM at high altitude to compensate for thinning air?

[carusoam]

201er,

According to the link supplied by Ross...

Max Power (MP) + Max RPM (limited by "total" tip speed) = Max airspeed. 

It is also a more expensive way to fly.  This is more about all out speed then it is about efficiency.  Thank Al Mooney for giving us this choice...

The EPI guys indicate that prop efficiency losses are overcome by AOA adjustments (within reason).  The real efficiency loss is by total tip speed approaching and exceeding the speed of sound.  Slowing the prop down is the way to logically stay outside of this efficiency red box....

For us N/A guys....We are power limited by altitude, our best speeds occur well below 10,000'.  Our best efficiencies are still way up high...

Best regards,

-a-

[jetdriven]

You can turbocharge the engine but you can't turbocharge the wing or prop. 

Your engine makes more horsepower with more RPM, given the same manifold pressure.  Turbo guys can dial the RPM back and run more manifold pressure, and gain some of the frictional losses back while maintaing the same crankshaft horsepowerr.   NA guys can only run a best power mixture, WOT, and use RPM to increase power.

Quote: 201er

So how do the turbo guys fly their machines in the flight levels? Is it a different prop or same one? Since the engine is getting complete aspiration, can you fly the same RPM settings as a non turbo at low altitudes? Or do you need to increase RPM at high altitude to compensate for thinning air?

[ChrisH]

My limited (and perhaps wrong) understanding  is that by looking at some simplified power/thrust computations that account for RPM, 

Simplified equations look like:

Power required to turn the prop = D⁴ P N³ ρ 

Thrust gained from prop = D⁴ P^2/3 N² ρ

D - Prop diameter

P -Prop pitch

N - RPM

ρ - density

So, for turbo guys, at altitude the Pitch is changing to compensate for lower air density to consume the same power, but you only get part of that HP back in thrust (pitch change raised to the 2/3). 

Based on my experience, at least in my 231, prop and engine settings are more about keeping the turbo/cylinders cool than it is about prop efficiency.

[sleepingsquirrel]

So the turbo guys do exactly the same  as me, except they fiddle with the knobs until the cylinders are cool enough.

[orangemtl]

Quote: ChrisH

My limited (and perhaps wrong) understanding  is that by looking at some simplified power/thrust computations that account for RPM, 

Simplified equations look like:

Power required to turn the prop = D⁴ P N³ ρ 

Thrust gained from prop = D⁴ P^2/3 N² ρ

D - Prop diameter

P -Prop pitch

N - RPM

ρ - density

So, for turbo guys, at altitude the Pitch is changing to compensate for lower air density to consume the same power, but you only get part of that HP back in thrust (pitch change raised to the 2/3). 

Based on my experience, at least in my 231, prop and engine settings are more about keeping the turbo/cylinders cool than it is about prop efficiency.

[Hank]

Quote: sleepingsquirrel

This is what it looks like when I fly! Kind of a mystery as to how it's done!

[thinwing]

I think that Squirrel is a genius and should be president!!!..Fiddle with the knobs!!All that headachey aero physics and calculus that orange shot up...forgetit!!!..fiddling with knobs too difficult???..make it even simpler...fly a magic carpet..no knobs at all!!!I personally am gonna forget even the flyingcarpet and have an "out of body experience"tonight after too much mental stimulation!!!kp couch

[jlunseth]

The power tables in turbo POH's generally phase out lower RPM settings as the aircraft approaches its service altitude.  For example, at lower altitudes you might find cruise settings for 2200 or 2300 RPM's (where max speed is, say, 2700 RPM's) that produce 75% HP, but at higher altitudes the power tables only show the higher settings (2600 and 2700).  This is partly because even turbocharged engines begin to run out of ability to make 100%HP as they approach critical altitude.  So if the service ceiling is 25,000 and the critical altitude (point at which the engine no longer makes 100%HP) is 15,000, from 15 to 25 the engine is operating more like an NA engine - it needs to be WOT and full speed to produce as much power as it possibly can.  So one reason for the higher RPM settings at higher altitudes in a turbo is to allow the engine to produce power.

The other piece is that the prop becomes less efficient at high altitudes because of the lack of air density.  Therefore, it is important to allow the prop (an airfoil) to travel as fast as the engine will allow, in order to increase the speed of the air passing over the prop, in just the same way that increasing the speed of the aircraft increases lift from the wing (an airfoil) by increasing the velocity of the air over the wing. 

However, I have played around with this a little up in the flight levels with my aircraft.  I have tried cruising as low as 2400, and as high as 2700, and I see no change in cruise speed as long as HP remains constant.  I have an EDM930 that readouts out %HP by algorithm so am able to keep HP at close to the same level.

So I know what the theory is, but it does not appear to make a significant difference in reality, in my aircraft.