# Question for the propeller experts among us

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#### Winginit

##### Well-Known Member
Is there a formula to compute or at least a rule of thumb to figure how much propeller efficiency is lost as rpms increase ? What I would like to quantify is as efficiency is lost
( in 100 rpms increments) how many additional horsepower would it take to reasonably offset the loss in efficiency ? My thought is that if a reasonably light weight auto engine can be built that gains more than the needed hp as rpms increase, I can get an idea of what hp/rpm level would be needed to maintain roughly similar efficiency.

Basically what I'm thinking is an rpm level that would make about 2/3 of a HP for each cubic inch. A 300 cu in engine making 200 HP and weighing 300 or fewer lbs. It would have to be direct drive to keep the weight down.

#### TFF

##### Well-Known Member
Are you talking about over spinning a propellor past its reasonable tip speed?

#### pictsidhe

##### Well-Known Member
If the prop goes supersonic, efficiency goes down the toilet and the noise will be horrendous. That will happen when the tips are moving relative to the air about mach 0.9. That's a max, less would be better.

#### Winginit

##### Well-Known Member
No, not overspeeding a prop. As rpms go up, prop diameter would be reduced. That results in a drop in the propellers efficiency which can be somewhat offset by the increased horsepower at the higher rpms. I'm trying to see if there is a definable ratio between the three variables. Sorry I didn't make that part clear.

God ! Sometimes I hate spell check&\$#@

#### TFF

##### Well-Known Member
How will you reduce diameter?

#### Jan Carlsson

##### Well-Known Member
The losses by increasing rpm/Power/reducing diameter is very small, the increase in hp wins.
the higher reving prop will have less pitch as speed don't go up linearly, that compensate for the smaller diameter at take off

#### Winginit

##### Well-Known Member
How will you reduce diameter?
Right now there is not an actual prop to reduce. I'm looking for correlation on how to decide what elevated rpm somewhere between 2700 and maybe 3600 rpms would provide the best overall performance. I would expect that for each 100 RPMs added up to the arbitrary 3600 will produce additional HP. It doesn't necessarily follow that the increase in rpms each time will also get an equal or better performance boost from the propeller. I'm not saying it will or won't, that's what I'm trying to find out. Then I can decide what prop diameter to purchase. Probably buy one longer than I think I need and then see what it does and work down from there.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
The prop is sized for a particular aircraft at a particular speed.

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#### TFF

##### Well-Known Member
All you can do is build a prop for 3600 rpm. Its the way propellors work. Back to the overspeeding, without a constant speed propellor that change change load, fixed pitch is either on the money or over/ under speed for the shape.

#### Winginit

##### Well-Known Member
I realize that diameter doesn't answer all the questions as far as rpm interaction is concerned. What I'm thinking is that an engine has to have sufficient power to overcome a chosen pitch. I can pitch for speed, for take off, or somewhere in between, but I have to have some idea as to how many HP it will take to turn the chosen pitch/diameter.I have always felt that one of the negative aspects of aero engines is that although HP is generated linearly, the amount of additional rpms available to them is very limited (for normal use). Thus the amount of additional HP available is limited and you can't easily overcome pitch changes. If on the other hand an engine is built that (with a reasonably comparative weight)that builds linearly but greater HP (due to better design efficiency), it could overcome pitch that the aero engine can't overcome. In other words, if engine C gains 20hp with a 500 rpm increase, and engine R gains 50 HP with a 600 rpm increase, the second engine would have more flexibility in operation. Not the prop flexing, but the responsiveness across the power and. I realize I'm treading into the unknown (for me), but that's how it appears to me.

#### Jan Carlsson

##### Well-Known Member
if we Think that Power is linear up to 3600, we have 33% more Power, the airspeed goes up with 10%
pitch goes "up" with 10%+ / 1.3333 = ,825%
it will take 2,37 times the Power to increase the rpm on the same propeller 33.333% so it will be smaller.
if we go with the same tip speed it will be 33% smaller
6´ x 2700 = 848 ft - sec
848 at 3600 = 4,5´ = 54"

200 hp @ 2700 and 200 mph = 72"x83" eff ~84% 2 blade
267 hp @ 3600 and 220 mph = 55,5"x68,5" eff ~84% 4 blade

#### larr

##### Well-Known Member
I am going to go off in a slightly different direction here.
The prop creates thrust by accelerating and compressing the local air as it spins. The result of this is that the air in front of the prop is below the local air density. The faster the prop spins the less time there is for the local air to return to local density.

#### HapHazard

##### Well-Known Member

This Scimitar prop has always intrigued me. Not easy to construct, but often wondered if it would help improve some of the inherent direct-drive VW problems by allowing for a longer effective blade for a smaller blade diameter, therefore lower corresponding tip speed? Maybe one way of getting better low speed climb performance from a VW? I only have a cursory understanding of propeller dynamics, but like the advantages of Scimitar profiles which are being seen more and more in commercial airliner design these days. One for you Jan Carlsson? I'd be intrigued to know your thoughts on the subject....