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DALLASROTOR (Structural) (OP)
24 Oct 05 18:17
HEllo all...my apologies for activating a new thread, but I didnt see my issue discussed...

Im curious how the community views the idea of affecting torque control from captured rotorwash?  

Notwithstanding the complications and variables associated with foward flight and considering a stationary hover only, is it theoretically possible to introduce pitchable surfaces into the rotorwash in order to generate adequate sideways counter directional thrust and retian yaw flight control?

Further, given the linear increase of rotor disc down flow field velocity as one moves out radially from the hub, would it be within the scope of imagination to construct a rotor that promotes and greater force near the hub by adding significant depth and twist to said rotor blade.

Therefore, by creating more downflow in closer to the fuselage, the passive torque control system could be more compact and efficient with respect to total drag.

This is my first post!
karlbamforth (Aerospace)
1 Nov 05 14:10
Its good to know that ppl are still thinking of alternate ways of overcoming aviation problems. A lot of companies have already tried the idea of control surfaces in the downwash. One used a control surface in place of the tailboom, it was adjustable and was basically a rudder in the downwash. The NOTAR uses coander effect to make the tailboom act like an aerofoil surface to offset torque. I'm not sure about increasing the downwash on the inner part of the disc, my immediate response would be that it would increase the power required to turn it and therefore increase the torque generated. On the surface it seems to be a backward step. I still believe that reducing torque with the use of tipjets is the way to go, doubt it would ever replace the medium and big choppers but should be great for 2 and 4 seat aircraft.
Helpful Member!  DALLASROTOR (Structural) (OP)
1 Nov 05 14:27
yes Im aware of the existing art for the most part, but with Notar they only scratched the surface of the Coanda effect potential and creating horizontal thrust from downwash.  The requirement for the vectorable thrust tip in NOTAR systems defeats the purpose of simplifing the equation away from the tail rotor's complexity and energy drain.  NOTAR is evolutionary, but not terminal.  Other art was on the right track as well conceptually, but never got it right either in my view as a to form truly fitting function of design.

Thanks so much for your advise, but you speak of aviation problems maybe as a foregone conclusion and yet there is so much to come.  With respect to tip jets, you are trading one complex active power system for another and its downfall in my opinion, but I mean that with respect.


  
karlbamforth (Aerospace)
6 Nov 05 4:17
Hi Dallasrotor,
Thanks for your comments but I think you may have misunderstood what I though I had said. Badly written on my part possibly. I do not consider aviation problems as a foregone conclusion. The first line on my post maybe should have read " Its good to know that I am not the only person thinking of alternate ways to overcome aviation problems."

You may be correct about trading one complex power system for another, but it does dispose of the requirement for a tail rotor to overcome torque. It is then quite easy to use an alternate and simple system (of which there are many)for yaw control.

Two of the most successful tip jet helicopters (Djinn and Fairey ultralight) suffered from the same restrictions, fuel consumption and blade life. Other than that they were reported by test pilots to be the easiest and most enjoyable helicopters to fly. Both of these helicopters were designed and built in the early 50s and I think the problems they suffered can easily be overcome with modern technology.

Just as a side note I was onboard a military helicopter that lost tailrotor control, a very scary experience. Two weeks later I saw a friend die in the wreckage of another helicopter which suffered an identical failure. These incidents were both caused by mechanical failure and after much research over the past 20 years I am convinced that avoiding torque is better than trying to overcome it.

I think that in essence we both have a similar aim, to "get rid of the complex and fragile tail rotor". I wish you all the best and hope we can bounce ideas of each other.

Karl.
Fixed wing is just a passing phase.
DALLASROTOR (Structural) (OP)
6 Nov 05 12:54
I re-read your first comment and it reads fine, thanks for the response and will reply in order...
Helpful Member!  Onemorechance (Aeronautics)
24 Jan 06 15:02
Hi Dallasrotor,

Once, I also played with the idea of using a wing-like tail-boom in the down wash to counter torque. My conclusion at the time was that the surface area needed to be too high to be practical and that the rotation in the down wash even increased the already high "induced drag" (pointing downward) thus requiring an even higher lift-force and consequently a higher torque.
To give you an idea:
When the power P required for hovering the helicopter is approximately (without counting the extra losses in rotational momentum etc… ):

(1)   P = Sqrt( W^3 / (0.5 * ? * Pi * d^2))

W = Helicopter weight
? = air density
d =  rotor disk diameter

When the rotation speed of the blades is Rot, then the torque moment M is:

(2)  M = P / Rot = Sqrt( W^3 / (0.5 * ? * Pi * d^2)) / Rot

This moment has to be countered by a moment produced by the lift on the tail-boom-wing. Considering this wing of length d/2 (only towards the back of the helicopter and in the down wash). Assuming for simplicity, a constant chord lenght and a constant cl-distribution, then:

(3)   M = 0.5 * ? * v^2 * cl * c * 0.5 * (d/2)^2

v = down wash air-speed close to the rotordisk:

(4) v = Sqrt(W / (0.25 * ? * Pi * d^2 ))

c = chord lenght

cl = lift coeficient

Substitution of (1) in (2), (4) in (3) and finally (2) = (3) you find:

(5) c = Sqrt(32 * Pi * W / ?) / (cl * d * Rot)

Now some numbers:
Say: W = 14000 N, d = 10 m , Rot = 45 rad / s (numbers representing a typical helicopter like the jet ranger for example), ? =1,225 kg/m^3, (cl = 1 for simplicity, which is even on the high side)

Then c = 2,4 m

Conclusion: with a wing of 5m in the down wash you need a chord of almost 2,5 m! (A surface of about 12 m^2) Of course by increasing wing-chord towards the back (blade tips) and reducing towards the front (blade root) you could reduce that surface a bit.

Regards,

OneMoreChance
Onemorechance (Aeronautics)
24 Jan 06 15:17
Whoops, its even worse because I made a mistake in equation (4).

should be v = Sqrt(W / (0.5 * ? * Pi * d^2)), so (5) becomes:

c = 2 * Sqrt(32 * Pi * W / ?) / (cl * d * Rot)

in the example the chord c would become almost (5m) the same as the span d/2 (5m) of the wing!
DALLASROTOR (Structural) (OP)
24 Jan 06 15:26
In consideration of your thoughtful and extrememly helpful reply and through your calculations, like us, you have made the correct determinations that the square footage of required surface area, in addition to the extra induced drag creates an improbable structural solution....why does everyone seem to leave the idea with a "a giant wing in the rotor wash?"....this is not the answer as you have so elegantly shown...

Our goal is to evolve away from any tail configuration whatsoever, passive or active, while at ther same time not trading it for coaxiality, tandem, intermesh, or "tip jets" and so one is left with creating the desired moment forces by applying the ENTIRE fuselage body as the "wing"...there is only one fuselage shape that will accomplish this...are you following

Onemorechance (Aeronautics)
25 Jan 06 1:22
Sorry for answering so late, I don't have too much time for this.

Do you mean to shape the fuselage as a vertical wing over the whole diameter of the rotor, from back to front?

Anyway, there are more options also. I am not kidding; one option could be for example to drive the rotors not by rotating them, but by flapping them. When moving the rotor blades up and down, they will create a lift and be pulled into rotation. In that way they work like a birds wing flapping up and down creating lift and thrust. The torque action – reaction is now directly between the air and the blades leaving the fuselage with only a very small torque due to the friction in the bearings. By moving some blades up while others move down, the overall up-down motion of the fuselage can be avoided. This is not a joke. Some people are already studying this possibility for future helicopters, (for example at the Delft University of Technology in the Netherlands).

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