Aerodynamic controlled blade twist/pitch

skyguynca

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When we look at Charles Kaman design of using servo controlled rotor blades we can see lots of benefits.
However for small ultralight and light helicopter design there are far more drawbacks such as complexity, parts and weights of two complete rotor systems.

I am looking at is borrowing some of the key design points for a ultralight helicopter. There are so many studies now that show we can move away from the drag inducing servo flap and move to a trailing edge flap for controlling blade twist. There still exists the possibility of staying with a external servo flap if some issues can not be overcome due to complexity.

Now I am trying to work on the advantage of either a teetering two blade system or a 3 blade system using a head that will tilt in-plane either by CVJ or elastic mounting (similar to C Beaty early work).

The biggest advantage of the two blade teetering system is fewer parts, no need for lead/lag dampners as used in Kaman's design because there will be no need of a teeter pin with such a large Delta 3 angle, we don't care about how much the system teeters, there is no other rotor to avoid. So it will be lighter and less complex. However we will need rotor blades of 10ft to achieve adequate blade loading to get the lift for hp required. Blade loading is a bit lighter than the Mosquito Ultralight.

With a 3 blade system there are more parts, 3 blades however same cord but only need 7.5 ft blades to achieve the blade loading for the lower hp required. The weight increase is just 3.5 lbs more which will still be below the 254 lb empty weight. I believe there will a increase in control response vs the two blade teetering system. Also since the CG is directly under the main rotor, and the pilots seat tank is directly on the cg, so the pilots weight nor the fuel weight will not affect the cg so I believe coupling between gyroscopic effects will be negligible.

Going with servo flap blades lowers parts count, and complexity since we will be going with one main rotor. The Sea Sprite is also a servo controlled rotor system by Kaman, all weather operations and now in its 30+ years of service around the world.

I do believe this is a good direction for the experimental and ultralight helicopter builders.

Opinions and discussions?

David M.
San Jose,CA
 

Jean - Claude

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skyguynca;n1126249 said:
With a 3 blade system there are more parts, 3 blades however same cord but only need 7.5 ft blades to achieve the blade loading for the lower hp required.
David
It seems to me you forget the increasing of induced speed . If you reduce the length of the blades to keep them the same blade loading, then you need more hp, because of a higher disk loading.
 

skyguynca

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first, I spoke incorrectly the blade loading is actually lighter in the 3 blade system.

Yes, disc loading increases but the blade area increases also , so the hp should balance out with the same required hp for both.

One thing left out in each also is that the 2 Blade rotor head adds 8 in to rotor diameter and the 3 blade head adds 12 inches to rotor diameter

2 blade teetering rotor 20.7 ft diameter is 34.7 lbs/ft blade loading and the disc loading is 21.74 lbs/ft Operational RPM 446
3 blade rigid rotor 16 ft diameter is 30.8 lbs/ft blade loading and the disc loading is 28.125 lbs/ft Operational RPM 568

I believe the hp required will be pretty close and the lift and control repose to be better with the 3 blade rigid rotor with a slight increase in weight.




David M.
San Jose, CA
 
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Jean - Claude

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About 500 h (FW + ultra light)
David
What matters are the lbs / sq.ft. The reduction in diameter dramatically increases the power required. This is not compensated by the reduction of the blade load. In hover your two bladed rotor will need 31.5 hp while your three bladed rotor will need 41.3 hp
 

skyguynca

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Thanks for the information, would you show me how you are calculating the hp required?

When I calculated lift I used:
PL [hp/ft^2]=power/A
TL[lb/hp]
=8.6859*PL^(-0.3107)
Lift = TL * power(lb)= (lb/hp)*(hp)

2 Blade 20ft disc, 3 blade 16ft disc, engine hp 48 available but figure 45hp for calculations, small reserve

2 Blade HP=35(subtracted 22% or 10hp for tail rotor. Higher than estimated hp of 10% or 4.5 hp) A=314.16, PL=0.112, TL=17.14 gives approx 599 lbs of theoretical lift
3 blade HP=35(subtracted 22% or 10hp for tail rotor. Higher than estimated hp of 10% or 4.5 hp A=201.07, PL=0.1741, TL=14.95 gives approx 510 lbs of theoretical lift.

My gross weight is approximately 500 lbs including fuel. Empty weight (modeled in solidworks) is approximately 220 lbs, calculations done at 254 lbs empty weight.

Can you please check my math on lift generated? I think I am fine though with both rotor systems with the engine selected.

David M.
San Jose, Ca
 
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