Prop efficiency

[DieselFan]

If prop
brand A says max rpm is 2500
and
brand B is 1500
now given that the gearbox ratios are the same could one just decrease pitch on B to give the 2500+ rpm? OR What stops a person buying A and increasing pitch so that max prop rpm is 1500 on a high ratio box?

Most manufactures ask about HP and direction. What about max prop rpm or gearbox ratio not to mention the engine rpm...

[smallfly]

Now this is a post i am going to follow closely !

IMHO horsepower has nothing to do with this equation ,but rather

TORQUE !!

[Miskiet]

Hi DF,

Prop max speed is governed by the tip speed (amongst other things). Bad news if your tip speed exceeds the speed of sound... Therefore the faster a prop spins the smaller the diameter gets in order to keep the tip speed below the speed of sound. A prop blade is also an airfoil and will stall just like your wing if you set the prop pitch to coarse - I haven't tried it yet but I think you can set a small prop (2500rpm like jabiru's etc) as coarse as you can and you will still over rev your engine with a 3.47 gearbox.

For our slow planes bigger diameter props that spin slower gives the best efficiency.

[Morph]

Firstly max RPM a prop can handle is governed by prop length. The longer it is the slower the prop is. So if you had a prop limited to 2500rpm and a length of say 60" then the 1500rpm prop would have a longer length at say 75"

The tip speed of the prop should not exceed .92Mach. You can calculate the prop speed vs Mach here

Over .92 mach the airflow begins to detach from the propeller which decreases efficiency and dramatically increases noise. Eventually the vibration will cause the prop to self destruct

There is nothing stopping you from increasing the pitch of the shorter prop. This will drop your static rpm, give you better cruise, increase the loading on the motor and thus the EGT temps will go up, and a decrease in climb ability.

The longer prop, if limited to 1500 rpm would/should not be decreased in pitch to increase rpm, etc because you stand the risk of pushing the tips beyond .92Mach resulting in destruction of the prop.

Generally speaking a prop is first selected on length (governed by the design of your plane, the Bush Baby uses a 72" and a Challenger a 60")

Next you decide how many blades. The efficiency of a prop is inversely proportional to the amount of blades. i.e. more blades, lower efficiency. All more blades do is give a smoother operation. A 2 blade has about 75% efficiency a 3 Blade 72% etc

Next you decide whether you want climb performance, cruise performance, or somewhere in between (adjustable props give you the ablity to play around.) This is used to determine the pitch required.

Here are two spreadsheets that you can play with

[Mogas]

Interesting Morph
Using the Ponk calculator, I would need to swing a 95" prop to get me into the "oh that's good" range, ISA conditions.
My 67" prop puts my tip speed at Mach .625 at 5800 engine RPM, 2.43:1 box, 20 degrees C.
Oh well, I guess low is better than too high.
Thanks for the links

[Dre'man]

Question? If a prop is more efficient with less blades, what is the benefit of fitting a 6 blade prop?

[Morph]

None except smoothness

[Andre]

I use to have a 4 blade Brolca composite prop on my 582. Take off RPM was 6800, cruise 50 knots @ 6200 RPM
Now running a 3 blade NC composite prop, 7100 rpm take off, 60 knots cruise bar neutral @ 6500 rpm at the highveld
My question is, I'm I cooking the engine by reving it higher, water temp still the same and EGT also still the same, fuel consumption seems to be little bit higher if not the same
The blade profiles however are totally different

[Henni]

Andre,

You should not exceed 6800 RPM on a 582, not even on take-off. Be careful - mine is placarded for operation above that. I was advised to pitch for 6400 RPM max static.

But I must say, an 10knot increase using a NC prop (similar to the Power Fin) is radical. I might just invest in a Power Fin prop yet!

Did you experience substancial shorter take-offs and better climbs too?

Henni

[Morph]

Now running a 3 blade NC composite prop, 7100 rpm take off, 60 knots cruise bar neutral @ 6500 rpm at the highveld
My question is, I'm I cooking the engine by reving it higher

You are definately running too high and you will cook your motor

You need a maximum static, i.e. tie the plane to your towbar and wack the throttle open, of 6400 to 6500. You would only ever run at the max of 6800 for 5 minutes. Re-pitch your prop to drop the rpms slightly.

[DieselFan]

Thanks for the xls's Morph

I understand the logic being large prop = slow, yet

...my prop is a 4 blade 1.66 diameter or 65inch max rpm is 1700 on the 582, altho I only get that around 95mph+ so mostly around 1200rpm.

yet if I'm using this right
my tip speed is around 480-490ft/s which is very low and the prop isn't exactly large for such a low rpm, my concern is now increasing rpm to 2500 or round abouts, if I had to change gear ratios. Would the prop shatter or be out of it's efficiency band - requiring me to buy a higher revving prop.

Admitably at engine idle about 1500rpm it can start moving although since I repitched, only without any persons onboard, altho at 2000rpm moves brisk with pax and pic.

Additionally how do steel tips affect prop efficiency?

[DieselFan]

Took another look at prop, the prop is short but alot wider than most, infact almost double as wide a the root tappering off to the edge. As the sound barrier should pose no problems, I should have no problems then installing another engine that runs at higher prop rpm due to lower gear ratio?

[Morph]

What gearbox ratio are you running?

[DieselFan]

What gearbox ratio are you running?

Currently a 4:1. Although it could change in the near future and push prop rpm to about 2000.

[Morph]

A 4:1 ratio box should be pushing a 72" to 74" prop. the 65" would be highly inefficient.

Check out Powerfin's recommendations here. I see they recommend a 4 blade 65 for the 3.47 ratio but then the pitch must be quite high. They do not recommend the 4:1 or 3:1 ratios as they have harmonic resonance problem