Does the prop behave differently with altitude?

[201er]

Since the air is thinner at higher altitude, does prop efficiency and performance change? For exmaple I've always been taught to avoid cruise at highest RPM (above 2500). But at higher altitude you may have to choose between low power or highest RPM. Is it "less bad" to fly 2600/2700RPM at 8000+ feet than it is down at 2000' cause the air is thinner? Or is it just that you don't have much choice?

[philipneeper]

how do i subscribe to these post?

 

[Hank]

Philip--

Log in. Look at the top of the page, just below the buttons for Home, Forums, etc. Click on 'Subscriptions.' I don't subscribe, but that's how it works. You won't see View New Posts, Subscriptions, members, # New PM(s), Search on the Home page, but you will on this page after you log in. It's even written in blue, with little micro-icons for each selection.

[Skywarrior]

Quote: Hank

Philip--

Log in. Look at the top of the page, just below the buttons for Home, Forums, etc. Click on 'Subscriptions.' I don't subscribe, but that's how it works. You won't see View New Posts, Subscriptions, members, # New PM(s), Search on the Home page, but you will on this page after you log in. It's even written in blue, with little micro-icons for each selection.

[201er]

Hmm... anyone wanna tell me about props? lol

[sleepingsquirrel]

Not really, it still spins round and round and beats the air into submission then convinces the airplane to move forward just like at sea level. One of the laws of motion causes a force to be applied forward called Thrust which is equal and opposite the force applied to the air in a  backwards direction. Because the air is thinner at altitude a fixed pitch prop would become less effective at moving the same mass of air  to provide the same thrust at the same RPM than it would at sea level.  A variable pitch prop can be adjusted at altitude to take bigger bites and if the RPM is the same move equivalent masses of air providing thrust and performance like at sea level.

 Because the airplane is free to move in air the propeller is actually screwing its way through the air,  the higher the pitch the more distance it moves through the air for each revolution.

This is the best I've got for starters.

 

[philipneeper]

Quote: sleepingsquirrel

Not really, it still spins round and round and beats the air into submission then convinces the airplane to move forward just like at sea level. One of the laws of motion causes a force to be applied forward called Thrust which is equal and opposite the force applied to the air in a  backwards direction. Because the air is thinner at altitude a fixed pitch prop would become less effective at moving the same mass of air  to provide the same thrust at the same RPM than it would at sea level.  A variable pitch prop can be adjusted at altitude to take bigger bites and if the RPM is the same move equivalent masses of air providing thrust and performance like at sea level.

 Because the airplane is free to move in air the propeller is actually screwing its way through the air,  the higher the pitch the more distance it moves through the air for each revolution.

This is the best I've got for starters.

 

[201er]

Well since the "bite" is different relative to air density and thus altitude, can it be said that this has some kind of effect on the L/D of the prop? Does this put it into a more or less efficient operation than comparable RPM at sea level? Does this affect more than a few knots or purely theoretical?

[sleepingsquirrel]

Well, if the airplane is tied down , it's a big fan. If the airplane is free to move it's a big screw.

[John Pleisse]

A propellor.......... is a wing.

[sleepingsquirrel]

Quote: 201er

Well since the "bite" is different relative to air density and thus altitude, can it be said that this has some kind of effect on the L/D of the prop? Does this put it into a more or less efficient operation than comparable RPM at sea level? Does this affect more than a few knots or purely theoretical?

I'm pretty sure that the AOA has to be increased to get the same lift (thrust) at the same speed through the air  (RPM) as the air gets thinner. The L/D is a function of prop design. I'm sure we change the pitch as we go higher and faster to get the AOA of the propeller at its highest  LIFT(thrust) and least drag. The curve is just like L/D for any airfoil section. You could increase the pitch beyond a point and operate the propeller inefficiently,  airspeed drops, burn more fuel and just really make a lot of noise. Like when checking the propeller on the ground. Stick you hand out the window I'll bet you can feel the difference! Watch your airspeed indicator

 

[sleepingsquirrel]

 

A propeller is a machine that moves you forward through a fluid (a liquid or gas) when you turn it. Though it works the same way as a screw, it looks a bit different: usually it has two, three, or four twisted blades (sometimes more) poking out at angles from a central hub spun around by an engine or motor. The twists and the angles are really important.

Photo: A propeller is like a cut-off screw and works much the same way.

[sleepingsquirrel]

http://www.mh-aerotools.de/airfoils/propuls4.htm

 

This link has a formula where one can see a direct link between Thrust and density of the medium. Denser fluid more thrust ,less dense fluid less thrust.

[carusoam]

Air is a compressible fluid....it is far from following the ideal gas law.

[sleepingsquirrel]

Yep, milllions,(maybe billions)  of big ole bouncy balloons being pulled through the prop (static), or pulling ourselves through millions (maybe billions) of big ole bouncy balloons ( dynamic) . Some of the energy imparted to the air is in compression. Eventually we reach a steady state condition where we can begin to plot things like thrust at different altitudes.

One could imagine the propeller blade pulling balloons through the prop disc one at a time slow motion. If the balloons were really tight they would not deform as much as if the balloons were flacid. (low altitude,high pressure vs high altitude,low pressure)  to demonstrate compressibility?

Eventually we will be able to count the number of balloons for a given RPM and propeller blade AOA.

[Shadrach]

Quote: 201er

Since the air is thinner at higher altitude, does prop efficiency and performance change? For exmaple I've always been taught to avoid cruise at highest RPM (above 2500). But at higher altitude you may have to choose between low power or highest RPM. Is it "less bad" to fly 2600/2700RPM at 8000+ feet than it is down at 2000' cause the air is thinner? Or is it just that you don't have much choice?

[Skywarrior]

Quote: sleepingsquirrel

[sleepingsquirrel]

Quote: Skywarrior

[Jeff_S]

Having just been to Kill Devil Hills and getting a good reminder of everything the Wright Brothers went through to make that first flight, it was clearly pointed out there that the concept of an "air screw" as ported over from the maritime experience clearly would NOT work for an airplane propellor, and it had to be considered much more of a wing.

And can I say how cool it was to fly into First Flight Airport and walk those 40 paces that represented the first flight? 120 feet doesn't sound like much, and let me tell you, it's not. But cool to stand in that exact same spot (more or less) and imagine what it would have been like.

[orangemtl]

Is it not the case that some of the decreasing efficiency of the prop with altitude is offset by decreasing resistance to forward motion with higher, less dense air? Granted, it is also more difficult to produce lift---else, Mooneys could go up to 70,000 feet!

It's been a few decades since Physics I and Physics II, but:  I think there's a bit of an offset here, that flattens the decreasing net efficiency curve. Someone educate/correct me on this one, please.

[Hank]

When I was at Kitty Hawk in July, the experience was anything but "cool"! It's a great place to visit, though, especially if you like walking. Bring your lunch, town is even further than the airport, and in a different direction. I can only wonder why the Park Service does not have a food vendor.

Propellor efficiency falls off with air density; as you climb, density drops and takes prop efficiency with it. We compensate some for that by holding constant RPM and allowing the prop's AOA to change. One thing many people ignore is how fast the very tip of the propellor is moving. Angular velocity is a simple concept, but as the tip of the prop approaches the speed of sound, prop efficiency experiences a sudden, large decrease; fortunately the designers have done their job and our governors are not supposed to allow the propellor tips to go trans-sonic.

Circumference of a circle = 2*pi*R, or pi*Diameter. Multiply 74" prop diameter * 3.1416 * RPM to get in/min speed of prop tip.

Speed of sound at sea level is approx. 760 mph; 760 mile/hr * 5280 ft/mile * 12 in/ft ¸ 60 min/hr to get everything in in/min.

Try to stay below ~85% of this figure for decent prop efficiency, or around 85% for max speed. Of course, higher RPM will require higher fuel flow [MP] regardless of mixture regime. Speed is never free, we even pay [in climb fuel burn] for our descent speed.

[sleepingsquirrel]

Quote: orangemtl

It's been a few decades since Physics I and Physics II, but:  I think there's a bit of an offset here, that flattens the decreasing net efficiency curve. Someone educate/correct me on this one, please.

[Hank]

Quote: orangemtl

Is it not the case that some of the decreasing efficiency of the prop with altitude is offset by decreasing resistance to forward motion with higher, less dense air? Granted, it is also more difficult to produce lift---else, Mooneys could go up to 70,000 feet!

It's been a few decades since Physics I and Physics II, but:  I think there's a bit of an offset here, that flattens the decreasing net efficiency curve. Someone educate/correct me on this one, please.

[Hank]

Quote: sleepingsquirrel

[sleepingsquirrel]

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