Mr Spaceman
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Small Flaps On Helicopter Rotor Blades - Why?

Sat Jun 21, 2003 3:05 am

Hi guys.

What are the small Flaps / Tabs shaped devices that are located on the trailing edges of the rotor blades in these photos used for?

Also, what are they called?

You can clearly see these rotor blade objects in these 4 shots of the strange looking (IMO) Kaman K-1200 K-max helicopter.


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Photo © Adrian Herzig
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Photo © Renato Burkhart



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Photo © DSAvit
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Photo © Markus Herzig



They're also attached to the rotor blades of the Kaman SH-2G Super Sea Sprite.


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Photo © Colin Hunter



From what I understand about helicopters, the angle of attack (AOA) of the rotor blades is normally controlled by a set of vertical linkages that extend upwards from the main rotor hub assembly. The AOA of the rotor blades control the amount of lift that they create.

The Kaman K-Max & Sea Sprite helicopters don't appear to have these vertical linkages. Instead they have small flap like devices attatched to the rotor blades that seem to have hinges.

Do these small devices controll the AOA of the rotor blades on these helicopters? (via internal pushrods in the blades). If the answer is yes .... are these devices unique to Kaman helicopters?

You can see the usual vertical linkages that control rotor blade AOA in these shots.


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Photo © John Olafson
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Photo © John Olafson



Thanks,


Chris  Smile

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NKP S2
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RE: Small Flaps On Helicopter Rotor Blades - Why?

Sat Jun 21, 2003 3:37 am

Just some peripheral knowledge. I used to work at Sikorsky A/C and we had a USN Kaman fly in and I immediately noticed the same thing. I was told they are 'Servo-Flaps'...and that rather than the usual swash plate arrangement for altering blade pitch in cyclic and collective modes, they act as small ailerons of a sort.

Not unlike the control tab system used by Douglas and others on control surfaces.

As for just how they are actuated/controlled...I know not.
 
Mr Spaceman
Topic Author
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RE: Small Flaps On Helicopter Rotor Blades - Why?

Sat Jun 21, 2003 5:00 am

Hello NKP S2.

OK, so these rotor blade devices are known as "Servo-Flaps" and act as small ailerons to change the pitch / AOA of the blades instead of the usual swash plate arrangement .....which I've heard of before.

I guess the vertical linkages I referred to (that can be seen in the Bell 206 Jet Ranger & Bell UH-1 Huey photos), are attached to the swash plate.

If the helicopter pilot wants to increase the AOA of the rotor blades to increase the lift they create (I think this is done in conjuction with increased blade rpm), then I suspect that these servo flaps's trailing edges move up, causing the leading edge of the rotor blades to move up, thus increasing AOA and lift.

I suspect this design is unique to Kaman helicopters.


Chris  Smile
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NKP S2
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RE: Small Flaps On Helicopter Rotor Blades - Why?

Sat Jun 21, 2003 11:45 am

Spaceman spake:
"....I guess the vertical linkages I referred to (that can be seen in the Bell 206 Jet Ranger & Bell UH-1 Huey photos), are attached to the swash plate."

I reply:
Those are pitch change rods. They are just more visible on Bell Helicopters because the rotor head is relatively higher from the top of the gearbox than most other designs.

"....If the helicopter pilot wants to increase the AOA of the rotor blades to increase the lift they create (I think this is done in conjuction with increased blade rpm), then I suspect that these servo flaps's trailing edges move up, causing the leading edge of the rotor blades to move up, thus increasing AOA and lift"

Irrespective of the method of changing blade pitch ( rods or servoflaps ), but speaking in general turbine-powered helicopter terms; Actually, it has very little to do with RPM. The increased pitch will provide blade lift, but the rpm doesn't really change...it just increases the load on the engine/gearbox ( Torque ). The engine(s) are automatically throttled up when more collective pitch is demanded to maintain rotor speed for a given load.

"....I suspect this design is unique to Kaman helicopters"

I suspect you may be correct. I have no Kaman experience, so again I cannot say just how the servo-flaps are actuated/linked/co-ordinated without guessing. Hope this helps.

Later



[Edited 2003-06-21 04:48:51]
 
Mr Spaceman
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RE: Small Flaps On Helicopter Rotor Blades - Why?

Sun Jun 22, 2003 3:06 am

Hello NKP S2.

Thanks for letting me know that the vertical rods that you can see on the Bell 206 & UH-1's that are attached to the rotor head are called Pitch Change Rods.

The increased pitch will provide blade lift, but the rpm doesn't really change...it just increases the load on the engine/gearbox (Torque). The engine(s) are automatically throttled up when more collective pitch is demanded to maintain rotor speed for a given load.

The above explanation was very good in clearing things up for me about the use of rpm's. Big grin


Chris  Smile

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FredT
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RE: Small Flaps On Helicopter Rotor Blades - Why?

Mon Jun 23, 2003 5:22 pm

As a sidenote, you'll find small fixed tabs on the blades of most helicopters. These are to adjust the blade tracking. You want each blade to generate the same amount of lift. This means each blade will follow the same path. Fit a small reflector to the end of each blade, spin it up and aim a strong light at the blade tips and you'll see if they follow the same path. Adjust these tabs until they do.

RPM is automagically governed in most expensive helos, while you still have manual throttle control to control RPM in many older/cheaper ones. Not sure if there's still some governing going on behind the scenes, but there is a throttle grip on the collective to pay attention to. And I agree with NKP, yet to see a turbine helo without automagical governing or a recip helo with (although I'm sure the latter exists). And yes, you generally want the RPM to stay in one place throughout the flight.

Cheers,
Fred
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