Prop Questions
 

Im trying to fully understand props and what causes them to be the way they are. I just finished reading the September article in the AMA magazine about this subject, and while the article was really good and informative, theres still a few things Im confused about. Im hoping maybe you can help me...here goes:

Lets compare a 10x6 and a 11 x 6 prop. The 10x6 is spinning at 11,000 RPM and the 11x6 at 10,000 (greatest rpm in both cases)

The 11x6 prop will have slighty greater peak forward airspeed than the 10x6 even though the 11x6 is rotating at a lower rpm. Is this because of the 11x6's increased disk (thrust producing)area?

However, the 11x6 CLIMB RATE is MUCH higher than the 10x6, even though it is turning slower and moving forward slower than the 10x6 in this climb. This implies that climb rate and forward airspeed are unrelated. Is this because its greater disk area is placing more air over the lifting surfaces of the airframe, and completely unrelated to the forward airspeed?

Now lets say we compare an 11x5 and a 10x6. Will the 11x5 have peak rpm lower but closer to the 10x6's 11,000 rpm and higher than the 11x6's 10,000 as stated above? Is this due to the fact its moving less air and has less load because the pitch is lower? In other words, Im gathering that the major limitation to the prop you can run with a specific engine (besides ground clearance) is a props pitch. Seems to me the higher the pitch, the more air you will move in each revolution, so theres more load to bog the engine down...Without regard to standard prop sizes and ground clearance, in theory if you wanted to maintain the same forward airspeed at the same engine RPM you could pitch up and reduce diameter OR pitch down and increase diameter. Am I on the right track here?

Now, would a lower pitch increase climb rate but decrease max forward airspeed? Are low pitch high diameter props pretty much whats used in the 3d world for this reason?

If this is the case, then a props max forward airspeed and that same props "pulling power" are totally unrelated, in other words you can have a slow prop that pulls like mad, correct? A low pitch high diameter prop would then be similar to 1st gear in a car, and a high pitch prop would be 5th gear.

The last bit of confusion is then in comparing two low pitch props of differing diameters...say an 11x4 and a 10x4. I would assume that the 11x4 would have far greater "pulling power" than the 10x4 ONLY because its disk is larger. Even though the 10x5 will spin faster because of decreased load, the effect of greater diameter far outweighs the rpm increase.

Im just trying to visualize it all....

Another question: why does the angle of attack on a propeller get closer to 0? I know its related to forward airspeed, but Im having trouble visualizing this concept.

I know this is way too long, but I thought Id ask. Thanks!

J

RE: Prop Questions
 

First off if the 11x6 is flying the model faster that's because the model has too much drag for the 10x6 to overcome. With a cleaner airframe the extra 1000 RPM would make the model fly faster even with the small prop.

The 11 will climb better because of the larger affected disc of air.

For your 11x5 and 10x6 the '5 will accelerate faster and climb steeper all else being equal but at shallower climb angles the '6 may out pull the '5 just because the higher pitch makes the model fly faster. Here again a lot has to do with airframe and weight but all else being equal the finer pitches will climb steeper but not fly as fast. And larger diameters will pull harder up until you get close to the pitch limited speed range at which point RPMs and pitch determines how fast the model can fly.

Exactly.

With very high pitches you also have problems with the blades being stalled at lower airspeeds. This is why models pitched for very high speeds are slow to accelerate but once up to speed you hear the engines pick up a lot of rpms and the model seems to hit a passing gear. Sometimes this can be masked by tuned exhaust related rpm rises but even with a plain muffler you get some of the same thing.

Comparing a model with various props like 10x4, 11x4 and 10x5 is hard to do. The various sizes will also affect the engine's rpms and that means the engine is not always operating at it's peak power rpm. To get the best out of the model you need to let the engine operate at it's peak power rpm at the type of flying you want the best performance. For a 3D model this means you should be able to hit the peak power rpm at 0 airspeed. For a pylon racer the engine needs to hit the peak at speed. This is why you play with props and pitches. To tune the prop to both the engine's best rpms and the type of flying performance you require.

As the airspeed of the model closes in on the pitch/rpm limited speed of the prop the angle of attack of the prop approaches 0. In fact in a dive with a clean airframe the angle of attack goes negative and the prop is actually holding BACK the speed of the model. This, and to some extent the Doppler shift thing, is why the engine seems to scream at an RPM higher than your ground setting. It's actually being OVERdriven by the airspeed of the dive.

RE: Prop Questions
 

Propeller aerodynamics are extremely complicated, but there are some overall aspects that you can accept as fact without actually becoming an expert in props. First off the velocity of the blade is very low at the root, and very high at the tip. The maximum RPM that a prop can turn efficiently is limited by the velocity at the tip. The tip velocity cannot exceed roughly .9 mach without creating a bunch of drag, and a bunch of noise. The propeller is twisted to optimise it for a certain airspeed. As airspeed increases thrust decreases. A prop is just a wing, and if you look at one station (diameter) of a prop you could calculate the lift (thrust) using conventional math. But, the volocity and angle of attack will be different at every other station, so you would have to do dozens of calculations to come up with the total lift (thrust) at a given airspeed. Want to know the thrust at a different airspeed, do it all over again. It's best to let the computer do all of that for you and use a prop calculator... This one seems pretty good:

http://www.gylesaero.com/_frames/f_propcalc.shtml

Ty

RE: Prop Questions
 

Thanks for your replies, folks. Im gonig to study your replies further once I get home and probably ask some more questions shortly :D . Thanks again.

J

RE: Prop Questions
 

jwalsh1

Climb rate and foward airspeed are closely related. The thrust x forward Velocity will give you the power being generated. If you know the glideslope you can figure the sinkrate of a given plane sinkrate=Velocity/glideslope. If you take the sinkrate*weight this well give you the power to maintain level flight. The (available power)-(power need for level flight) will give you the excess power. The excess power/weight will give you the climb rate.

Let look at a few examples:

Lets say we have a prop (A)that generates 4 pounds of thrust at 30 fps
the power being delevered is 4x30 or 120 fp/s

Lets say the glideslope at this speed is 5 to1 and the plane weighs 4 pounds.
30/5=6 fps sinkrate
6*4=24 fp/s to maintain level flight

120-24=96 excess power
96/4= 24 feet per sec climbrate.

Same plane same speed different prop(B)...only generating 3 pound of thrust at 30 fps
3*30=90 fp/s
90-24=66 excess power
66/4=16.5 feet per sec climbrate.

Now lets look at 50 fps forward velocity:

both props are generating 2.5 pound of thrust so
2.5*50= 125 fp/s
lets say the glideslope is still about 5 to 1 so:
50/5=10 sinkrate
10*4=40 fp/s power needed for level flight

125-40=85 fp/s excess power
85/4=21.25 climbrate.

Now lets look at the props at 80 feet per sec.

Prop (A) is only generating 1 pound of thrust.
1*80=80 fp/s avialable power
lets say that the glideslope has dropped to 4 to 1
80/4=20 sinkrate
20*4=80 power needed for level flight
80-80=0 so there is no power available to climb at this speed.

Prop (B) is generating 1.75 pounds of thrust at 80fps
1.75*80=140 fp/s
so
140-80=60 excess power
60/4=12.5 fps climb rate.

Prop (A) is a lower pitch "Climb" prop and Prop (B) is more of a "Speed" prop.

Hope that helps some.

Happy Flying!!!
Steve

RE: Prop Questions
 

There are multiple issues in determining the ideal prop, and the right prop for a given flight envelope won't be right for another. Here are three major contributors:

Engine:
At what rpm does the engine produce it's peak power? You want your engine prop combination to reach that peak during the flight parameter in question. It's different for climbing, maximum speed or takeoff. That's why most full scale prop planes have propellors with variable pitch.
airframe:
At what angle does the airfoil produce maximum lift. To get your maximum climb you want to fly at this angle. Actually, it's more complicated than this, but you get the idea.
Flight envelope:
As mentioned by previous posters the pitch/diameter selection is different depending on how fast you want to fly.

The best performance will be achieved were these curves intersect, but it is much simpler in the case of models to go to the field with a bunch of props and experiment. Remember, too, that different brand props will perform differently. An APC prop won't be the same as a Master Airscrew.

Here is a rule of thumb to help, though. If you're flying slow and you want better acceleration or climb go with more diameter and less pitch. That's why you see .46 sized 3D models with 12x3.5 props on them. If you are looking for maximum speed decrease the diameter and go with more pitch. More speed has the effect of flattening the prop pitch because the relative wind vector that the prop "sees" is angled more toward the front of the blade (combination of the forward velocity vector of the plane and the perpendicular velocity vector from the prop's rotation).

I may have oversimplified here at the expense of technical correctness, but all you gearheads out there remember that I'm trying to keep this short!

RE: Prop Questions
 

Man you guys are great. I need to digest this all, but its making sense. Thanks a lot for the responses.

RE: Prop Questions
 

jwalsh1

RE: Prop Questions
 

heres a prop question?

I have my new Geobat out today and trying to get it off the ground for the first time. She will not lift off. Now I have seen other questions such as this before, all controlling surfaces are working and going the proper way. On the last try, before I lost the prop, I increased the elevator travel to 1.5" of movement in the up range.
When she travels down the runway she is pretty strait the tail lifts up off the ground but she will not lift off. This is a pusher (see geobat.com) My question is about the small slot that the prop fits into, there is an opened area of .75" by 11". behind the prop there is another aileron set. Could this be causing a Prop-wash which hits the elevator and keeps it on the ground?

Any help please

Lawrence

RE: Prop Questions
 

You have the green arrow in the third diagram pointed the wrong way it shoud be pointed toward the prop blade. The prop is only interested in the airspeed it sees (green arrow) which is the combination of the rotational vector and the forward airspeed vector. As the airspeed increases the blue vector gets larger bending the green arrow more effectively flattening out the prop