Van's Air Force
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Prop Efficiency
- Thread starter TXFlyGuy
- Start date
you are trying to over-simplify things. RPM is not a very important driver to prop efficiency (note I am not saying thrust) provided your tip speeds are subsonic. The prop has to be matched for the engine and the airframe at a given airspeed and rpm. You can't look at prop efficiency by itself. This is a very complicated problem and your question can't really be answered without a lot more information.
Carl Froehlich
Well Known Member
You are mixing a bunch of stuff together. Assuming tip speeds all well subsonic, then some thumb rules follow:
- Lower RPM is more efficient than high RPM as you are reduction engine pumping losses (same engine power output - like driving in 2nd gear vs overdrive)
- Two blades are more efficient than three (efficient defined as speed or fuel use).
- The higher the RPM, the more power from the engine. This drives the compromise with FP props. You want high RPM for takeoff, but full power (or anything close to that) at take off will mean you have to throttle back in cruise to not exceed RPM limitations.
I'm told but have not seen data to support a 74" prop is better than a 72" prop.
I did not want to go off into the weeds on this, was just looking for some general guidelines.
It was always my understanding that a slower turning prop, taking big "bites" of air, was more efficient in cruise.
A 2.21-1 gear reduction will be used. Constant speed prop. 4 blades.
It was always my understanding that a slower turning prop, taking big "bites" of air, was more efficient in cruise.
A 2.21-1 gear reduction will be used. Constant speed prop. 4 blades.
scsmith
Well Known Member
Some big generalizations embedded here, but that's kinda what you asked for. There are a bunch of competing effects that mostly balance out. I started trying to write out all the pluses and minuses but as I said, the effects mostly cancel out.
Good design propellers have very flat efficiency curves up to the point where the blade tip speed gets high enough that transonic drag starts to occur.
No reason not to run props up to tip mach no. of 0.75 or so. Beyond that, the trade-off becomes, for a direct drive engine, whether the increased shaft power at higher RPM exceeds the efficiency loss to drive the prop at the higher RPM. GA airplanes pretty routinely get up around M=0.8 tip speed to get max thrust.
Assuming you do not compare to a propeller with nearly sonic tip speeds, then NO, slowing the prop down does not necessarily increase efficiency.
I think we talked about this before. You were trying to select a cam to move the torque curve to lower RPM so you could turn your prop slower and I kept questioning this. If you have an engine wear/reliability concern that says that you want to run the engine slower, and you can't change your gear reduction, then sure, go ahead and design a prop for the RPM that you are comfortable running the engine. You are making the choice to de-rate the power available for reliability reasons. But there is no gain to slowing down any more than that. The efficiency will stay about the same, and the thrust will just go down as the engine power goes down. Or, if you do change the gear reduction so that the engine RPM stays the same and just turn the prop slower, the prop efficiency will stay about the same, the power and thrust will stay about the same.
Good design propellers have very flat efficiency curves up to the point where the blade tip speed gets high enough that transonic drag starts to occur.
No reason not to run props up to tip mach no. of 0.75 or so. Beyond that, the trade-off becomes, for a direct drive engine, whether the increased shaft power at higher RPM exceeds the efficiency loss to drive the prop at the higher RPM. GA airplanes pretty routinely get up around M=0.8 tip speed to get max thrust.
Assuming you do not compare to a propeller with nearly sonic tip speeds, then NO, slowing the prop down does not necessarily increase efficiency.
I think we talked about this before. You were trying to select a cam to move the torque curve to lower RPM so you could turn your prop slower and I kept questioning this. If you have an engine wear/reliability concern that says that you want to run the engine slower, and you can't change your gear reduction, then sure, go ahead and design a prop for the RPM that you are comfortable running the engine. You are making the choice to de-rate the power available for reliability reasons. But there is no gain to slowing down any more than that. The efficiency will stay about the same, and the thrust will just go down as the engine power goes down. Or, if you do change the gear reduction so that the engine RPM stays the same and just turn the prop slower, the prop efficiency will stay about the same, the power and thrust will stay about the same.
Last edited:
More Questions...
In the following example, which will give the greater thrust:
#1
3100 engine rpm
212 hp (sea level)
Prop rpm 1630 - 466 mph/.60 Mach
#2
3100 engine rpm
260 hp (sea level)
Prop rpm 1402 - 401 mph/.52 Mach
The engines are not quite the same in the above. #2 has the GM Hot Cam.
The gear reduction is:
#1 - 1.9-1
#2 - 2.21-1
In the following example, which will give the greater thrust:
#1
3100 engine rpm
212 hp (sea level)
Prop rpm 1630 - 466 mph/.60 Mach
#2
3100 engine rpm
260 hp (sea level)
Prop rpm 1402 - 401 mph/.52 Mach
The engines are not quite the same in the above. #2 has the GM Hot Cam.
The gear reduction is:
#1 - 1.9-1
#2 - 2.21-1
rv7charlie
Well Known Member
I can't give you hard numbers, but I suspect that you're going to have 'issues' with turning the prop that slow, on a small airframe where you can't swing a large diameter prop.
Back into it. If a 200 HP engine on an RV-x (keeping this RV related) swings a 74" diameter prop efficiently at 2700 rpm but is somewhat noisy, why doesn't anyone run a gear reduction and turn the same diameter prop at, say, 1600 rpm? Ignoring the extra complexity/weight of the gearbox, the answer is that the prop can't absorb 200 HP at 1600 rpm. It can't flow enough air mass (at our typical speeds) to convert all that HP to thrust. Up the prop diameter to around 90-100", and it'll be a monster on takeoff (more low speed mass flow) without much, if any penalty in cruise. But you'll need 10-15" longer gear legs.
Can you swing a much larger diameter prop? Say, 80-90"? Or are you willing to go to 4 or 5 blades?
Have you tried playing with some prop design programs? If you run multiple iterations with various diameters, blade counts, etc, you can begin to see patterns that will drive you in the right direction.
http://www.mh-aerotools.de/airfoils/javaprop.htm
http://webpages.charter.net/bateseng/
http://www.jcpropellerdesign.com/
http://mail2600.com/PropDesign/index.html
Charlie
Back into it. If a 200 HP engine on an RV-x (keeping this RV related) swings a 74" diameter prop efficiently at 2700 rpm but is somewhat noisy, why doesn't anyone run a gear reduction and turn the same diameter prop at, say, 1600 rpm? Ignoring the extra complexity/weight of the gearbox, the answer is that the prop can't absorb 200 HP at 1600 rpm. It can't flow enough air mass (at our typical speeds) to convert all that HP to thrust. Up the prop diameter to around 90-100", and it'll be a monster on takeoff (more low speed mass flow) without much, if any penalty in cruise. But you'll need 10-15" longer gear legs.
Can you swing a much larger diameter prop? Say, 80-90"? Or are you willing to go to 4 or 5 blades?
Have you tried playing with some prop design programs? If you run multiple iterations with various diameters, blade counts, etc, you can begin to see patterns that will drive you in the right direction.
http://www.mh-aerotools.de/airfoils/javaprop.htm
http://webpages.charter.net/bateseng/
http://www.jcpropellerdesign.com/
http://mail2600.com/PropDesign/index.html
Charlie
Jordan1976
Well Known Member
#2. There's no way any reasonable prop will change efficiency enough to make up for the fact that your engine power has changed by 23%. Your real problem will be absorbing 260 HP at 1400 RPM.
scsmith
Well Known Member
Assuming both props are same diameter here:
Given the big increase in shaft power, #2 would make more thrust. But to absorb that much power, it is going to need many very wide blades.
If I were to suggest #3, which is the engine from #2 with the reduction gear and prop from #1, it would make similar thrust, perhaps slightly more, but would require somewhat less blade area.
Both of your cases are going to require rather unusual propellers with four very wide-chord blades. If you were to adjust the reduction gear so you could turn the prop at Mach 0.75 tip speed, it might start looking like a normal propeller.
You will be able to decide when you start running prop codes and seeing how much power you can absorb for a prop that looks like you want it to look for your airplane.
Here is a question, and I admit to being a bit ignorant in this area...
What is the most critical and/or limiting factor for a propeller during cruise flight? Is it tip speed as a critical Mach value, or is it speed in ft. per second, or a mph value?
That is, 700 ft/sec., or Mach .80?
While this is not necessarily RV specific, it can be useful to any RV owner contemplating the best/most efficient prop design.
The propeller in question is a 4 blade, wide chord, 94" diameter design.
What is the most critical and/or limiting factor for a propeller during cruise flight? Is it tip speed as a critical Mach value, or is it speed in ft. per second, or a mph value?
That is, 700 ft/sec., or Mach .80?
While this is not necessarily RV specific, it can be useful to any RV owner contemplating the best/most efficient prop design.
The propeller in question is a 4 blade, wide chord, 94" diameter design.
