1235

Item 1235

DESIGN: Dragonfly ~ Control - Flight - Directional - by Tip Brake

Drawing:

Rigid. Lower one longer.

Yaw Control:

Consider using blade tip drag as the only means of yaw control. I.e. Pedal to blade tip brakes without any interaction with the other flight controls

Reasons;

It will provide yaw control in regular flight and in autorotation.

There appears to be a passageway from the control stick base to the blade tips.
The tip drag, of all six blades at the same time, can assist (in conjunction with high collective) as a Rotor Brake:.

Tip Whistle: can be incorporated in the brakes as a safety device.
This, plus a limitation on the amount of lateral cyclic may result in a small angle between the masts.
Some of the tip flap's weight will be offset by the elimination of the conventional tip weight.

Concerns;

The mast is at an angle to the side from the vertical and the rotor is turning inside forward (breast-stroke). Therefor the nose will climb as the craft yaws. In addition, the lateral flow of air against the inverted 'V' tail may add to this problem.

Some potential solutions are;

Have the application of yaw put a little forward longitudinal cyclic into that rotor.
A reduction in the 'V' angle between the masts.
Have a high vertical fin.

_________________________________

Rotor Brake:

For yaw, only the tip brakes on the three blades of one rotor will be actuated. When there is the need for the rotor brake the tip brakes on all six blades will be actuated.

The thinking, at this time is to have relatively large tip brakes. These brakes will only be partially deployed for yaw during hover and the deployment will be even less during forward flight. They wull only be fully deployed when serving as a rotor brake. The only need for a rotor brake is on the ground after landing. Additional braking can be achieved by applying collective and/or an additional mechanical brake.

Vibration Reduction:

If the two halves, can act together to increase the pitch for a very short duration this device might also serve to reduce rotor-to-rotor aerodynamic interaction?

Reduced Tip Vortex:

If designed as a sub-wing tip, it might reduce the BVI.

Tip Whistle:

The thinking is that the blade tip whistles only work when the tip brakes are fully deployed. This way, immediately upon landing, the tip brakes and whistles are activated.

During takeoff, the tip brakes and whistles could be active. When RRPM has been achieved and the pilot is about to takeoff, he retracts the tip brakes and their associated whistles.

Weight Saving:

The brake and the whistles might qualify as safety devices and thereby be exempt from the 254 lb. limitation.

Cross-coupling:

The mast is at a lateral angle from the vertical. The tip brakes on a single rotor will not provide additional thrust and thereby cause the craft to roll slightly to the opposite side, but the additional torque will cause the nose to climb slightly as the craft yaws. A reduced 'V' angle between the masts will help to minimizes this cross-coupling.

Thrust:

The application of the brake may reduce the lift of the rotor slightly therefore the brake might be required to increase the lift slightly.

Default Position:

The default position should be with the brake not deployed. In addition, have centrifugal force cause this. This will mean that the control train will only operate in tension, and cane possibly be done by cable and pulleys.

Notes:

From Flight Safety on PPRuNe, June 7, 2003 discussing Drag Brake

You could use a small clamshell type Gurney flap on the trailing edge of each rotor blade. The "clamshell" could have a capacity for infinitely variable deployment from 0 to 90 degrees, and when fully extended, the 2 Gurney flaps (making up the "clamshell") would be deployed 90 degrees equally above and below the trailing edge.

This would have the effect of increasing or decreasing the drag of the rotor blade, with the variable deployment providing direct control over the amount of increase in drag. If the flaps (and mechanism) were small and light enough, they could even be cycled opened and closed during each rotorblade revolution to apply drag (and torque) where desired.

I can think of two possible means of inducing yaw with this idea. In one means, if we use the K-Max as an example aircraft, you could simply open the clamshells on both blades of a single rotorhead (without any rotational variance) to increase the drag on that rotorhead, to induce yaw. I think this would work well under power, but perhaps not too well during autorotation.

Another means would be to vary the opening of the clamshell so that only an advancing rotor blade on one rotorhead experienced increased drag. Say the clamshell started to open at 45 degrees after the advancing rotorblade passed over the rear of the fuselage, and closed again 45 degrees before passing over the nose of the fuselage. This would have the effect of increasing the drag on one side of the rotor disk, thus causing the aircraft to yaw towards the dragging side. I noticed on the K-Max that the advancing blade of each rotorhead passes over the fuselage, which would reduce the moment arm of an arrangment like this, but it might still work. I also think this arrangement might work better during autorotation.

Outside Web Pages:

Picture and information on the Gyrodyne Tip brakes; Gyrodyne Helicopters

____________________________________

Quote from above web page;

"The Model 2C used Movable vertical surfaces (rudders) and differential collective in the rotors for yaw control. The results of the instrumented flight test indicated that the coaxial rotor configuration possessed excellent flying qualities in all regimes of flight except for the low speed autorotation where the yaw control means proved inadequate. In order to overcome this difficulty, Gyrodyne continued its research work toward improving the directional control characteristics. In March of 1953, the idea of using tip brakes on the tips of the rotor blades was conceived. Flight tests of this concept proved that the problem of effective yaw control in autorotation for a coaxial helicopter had been solved. This was a major breakthrough for the coaxial configuration[. The Company applied for and was granted Patent No. 2,835,331 on October 24, 1954."

Additional information on Gyrodyne Tip Brakes; [Source ~ HDDM p.67]

Idea re Control:

Have both pedal-rotors independent from each other. Have the tip brake designed so that centrifugal force wants to keep the brake inoperative. Application of a pedal overcomes the centrifugal force by drawing the rod (or carbon threads) in the blade toward to mast and thereby activates the brake, and, with enough pedal, the whistle.

Note that there must be a swivel at the base of the rotor stick so that the upper 'threads' can rotate with the rotor.

Cancellation of Yaw-to-Pitch Cross-coupling:

The brakes also provide some additional lift to compensate for any loss of lift due to the application of the brakes.

There is some fixed cyclic in the brakes. Therefore, as they are activated, the brakes at the aft of the craft will open first. This means that at any amount of brake application, the brakes will be imparting a nose down tip on the one disk and this will offset the tendency for the nose to climb.

Introduction Page | SynchroLite Home Page | UniCopter Home Page | Nemesis Home Page

Last Revised: October 6, 2007