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Propeller
The engine needs to be relative powerful to get a proper climb. Propeller efficiency is about 35% at climb, so to get 2.5 m/s (500 fpm) at 330 kg TOW (727 lb.) you need,
Climb = 2.5 x 330 / 75 / 0.35 = 31.4 HP
Drag = (330 / 35 (L/D) @ 9.4 kg drag) 9.4 x 25 m/s / 75 / 0.35 = 9 HP
Power required = 31.4 + 9 @ 40.4 HP
The same problem but with a 350 kg heavy 13 meter WR. To get 2.5 m/s (500 fpm) at 772 lb. TOW you need,
Climb = 2.5 x 350 / 75 / 0.35 = 33.3 HP
Drag = (350 / 25 (L/D) @ 14 kg drag) 14 x 25 m/s / 75 / 0.35 = 13.3 HP
Power required = 33.3 + 13.3 @ 47 HP
Essential parts to get high propeller efficiency, is the engine cowling, the propeller it self and the right RPM on prop. And of course all three things together.
By Jan Carlsson
Wednesday 26 November 1997
|
HP @ 6500 RPM |
Fictive "Optimal" Propeller Diameter |
Recommended blade width on 32" Diameter Prop. At R .50 ( 8") |
Recommended blade width on 32" Diameter Prop. At R .75 (12") |
Pitch at R. .625 & .875 10" & 14" |
b @R. 4" Pitch 10.7" |
b @R. 10" |
b @R. 14 |
Prop. Effici-ency h % |
|
35 |
36" |
3.04" to 3.24" |
2.43" to 2.63" |
18.5" |
23 ° |
16.4 ° |
11.9 ° |
38 |
|
40 |
37" |
3.21" to 3.42" |
2.57" to 2.78" |
19" |
23 ° |
16.8 ° |
12.2 ° |
37 |
|
45 |
38" |
3.38" to 3.61" |
2.71" to 2.93" |
20" |
23 ° |
17.6 ° |
12.8 ° |
36 |
|
50 |
39" |
3.56" to 3.80" |
2.85" to 3.09" |
20.5" |
23 ° |
18.0 ° |
13.1 ° |
35 |
|
55 |
40" |
3.75" to 4.00" |
3.00" to 3.25" |
21" |
23 ° |
18.5 ° |
13.4 ° |
34 |
Following assumption is used in calculations.
HP = rated power at sea level, (the power is reduced by 3% per 1000 feet alt.) The optimal propeller diameter is calculated at 132 MPH and 6200 rpm, we can't use this optimal propeller diameter due to high Mach NR. and lack of space.
Blade area is based on the facts that the Equation for optimal diameter uses the blade width (chord) of 15% of propeller radii at R .50, and 12% at R .75. On our less then the optimal, 32" diameter propeller. We have to use wider blades to accumulate the power, the blade width above match the blade area on the optimal diameter propeller, using 15 - 16 % at R .50 and 12 - 13% at R .75. Many props use wider blades to give better climb performance.
Pitch at R .625 & R .875 is calculated at the best L/D speed = best climb speed, at this low speed the air that is sucked in (induced) to propeller disc is a significant factor, greater on higher disc load. The pitch is then chosen to give a alpha (
a) of 3° to the relative wind (induced + TAS)At R .25 (Sta. 4") the induced air speed is probably almost non in this pusher configuration, and this station is working in turbulent or even "dead air" so we can't use the true helical propeller layout as on tractor propeller's.
If we draw up the pitch versus radii as a diagram and draw a soft curve from the 4" sta. through sta. 10" and 14" we get the pitch at sta. 6, 8, 12, and 16". At sta. 12" (R .75) we will get a higher pitch, the induced air speed is at its highest here and is then reduced towards the tip and hub. The Blade width or chord at R .50 is used at sta. 4" and 6" as well, it will look narrower at sta. 4" due to the greater twist.
The Hp's we get at the propeller is not always matching the technical specs. It will change with the engine, the intake/ exhaust setup, and altitude.
A 1.5 to 1 propeller reduction and a 35" propeller (Or a light 4-stroke) is in my eyes the best setup; A 2 or 3 bladed folding propeller would then be nice. But if the rpm become to low, the blade will have a steep pitch, and reduced trust will be the result. Using a propeller speed reduction unit without increasing propeller diameter gives the same result. The 3 bladed will be slightly better at cruise, but the 2 bladed is better for takeoff and climb, and the main problem is to get off the ground and climb fast and safe.
