Ground Effect for gyroplanes

[Ga6riel]

Is there an increase in lift due to the proximity of the ground?
(this would involve being within 1/2 a rotor span of the ground)

and if there is, is there an alteration to the normal free space centre of lift?

[Doug Riley]

Gabriel, yes, a gyro rotor experiences a very distinct ground cushion.

I have not noticed a trim change in ground effect. Now that you mention it, however, there probably should be one. The back of the rotor is several feet closer to the ground than the front, especially when you are flaring for a landing. (in fact, a too-aggressive flare can result in a blade strike in the rear.) Therefore, the ground effect should be somewhat stronger for each blade as it passes through the aft quarter of the disk. If so, this would show up as a tendency to drift to the left (for a CCW rotor), because of the lag introduced by the teeter hinge.

[Ga6riel]

thanks Doug
without transposing to many ideas from FW IGE
where the downwash of a section is reduced variously to around a half, due to the inability for tip vortices to correctly form within the space remaining. Then the AoA flattens for the same lift found in free space.

Is it your feeling then, that some of the same things are occuring, and to do that, is it not the disc shape as a whole we are considering.

[Doug Riley]

Gabriel: I don't know. The early technical papers speak of the rotor as if it were simply a round wing -- the NACA engineers tested for lift coefficient, moment coefficients (both roll and pitch), L/D, and so on, of the disk as a whole. They seem not to have paid much attention to cyclic effects at first.

On a small Bensen-type gyro, however, the disk is over 97% empty space. Within this space, we have small wings flying in a circle. These wings have their own tip vortices, angles of attack, pitching moments, lift and drag, and mass. My personal model of a gyro rotor is a pair of small tailless sailplanes joined at one wingtip.

You might want to read the NACA reports on autogyros. They are posted at the NASA-larc web site. There is a list of them in the bibliography in the back of Gessow and Myers' Aerodynamics of the Helicopter.

[Rehan K.Janjua]

Hello all.

With 25 ft skywheels it feels great at T/O and Landings. Sort of ground cushion to surf on.
Greatest advantage is at short T/O and slow landings.
We also state it as take off roll performed in the air.
Getting airborne at slow speed transition to level flight speed and vise versa.
All this can be done unitizing the ground effect.

Regards.

Rehan Janjua
Air command. Pakistan

[Doug Riley]

I had a nice phone visit yesterday with one of the regular posters on this conference. He wondered how a gyro rotor could have a ground effect since the texts say that the airflow is UPWARD through the rotor (the opposite of a helicopter).

Well, some texts are a little careless about this upward-flow business. It's true that a gyro's rotor disk must be higher on its upstream side than on its downstream side if it's to keep spinning and lifting. IOW, the disk must have a positive angle of attack. It's also true that some air flows "up" against the bottoms of the blades, while other air travels farther "up" and curls over the tops of the blades.

However, the air flows "upward" around a fixed wing, too, yet a fixed wing generates lift.

Both the gyro rotor and the fixed wing "bend" the oncoming airstream in a very specific way. Both the air molecules that hit the bottom of a blade and those that curl over the top experience a change in their paths. In both cases, the blade forces the path to curve DOWN -- and that's the whole secret of making lift:

An airfoil manufactures a useful force (lift, thrust or other desired force) by deflecting the normally straight path of travel of air molecules in a direction opposite to the direction of the desired force. If you want lift or thrust UP (i.e. in an anti-gravity direction), the laws of physics say you must deflect air DOWN.* A gyro rotor does this just as a fixed wing does.

Most of the gyro's downwash is out near the blade tips. You can feel it as you approach the ground; the gyro's descent will get shallower and it'll almost flare itself as a result of ground effect, as long as you carry airspeed right down to the deck. You can see the wash's effects if you watch a gyro land in long grass or dust.

*The formal name for this activity is an "acceleration" of air molecules. In physics jargon, an acceleration can mean a change in EITHER speed OR direction (or both). The air that flows around a rotor blade or (other wing) experiences a downward acceleration because its path is deflected downward from what it would otherwise have been. Saying that air flows "upward" through the rotor disk is therefore an incomplete truth. After a gyro rotor goes by, there are more molecules lower in the atmosphere than there would have been if the rotor hadn't gone by -- IOW, there's a net downward displacement of air molecules.

[Heron]

Does air density and speed play a role on this effect?
Is there a minimum speed to create ground effect?
thanks
Heron

[Rotornut]

Doug, You never stop amasing me! I love to hear your Thoughts and Ideas.
You are a Wealth of Info, on Gyros, and I read all you Post.

Take Care till BD Days 2006 and we Meet Again. MJ :)

[Doug Riley]

MJ, I'm going to try to get to B-Days. I have to consult with She-Who-Must-Be-Obeyed, though. We know who holds the REAL power...

[Rotornut]

Yes Doug we Do. But Still Hoping that YOU hold the Persuaion part of it!

Give it your Best and, You will Be There I know IT in my Heart!

MJ :)

[SamL]

As air flows over an Aerofoil it produces changes in the pressures, both above and below, causing a suction drawing the wing up into the sky.
The Aerofoil also deflects a large amount of air downwards causing an opposing reaction, and also pushing the wing up into the sky.

Remember, a kite, a paper plane, and a balsa wood glider still do fly but all without camberd wings. The key here is the A of A and that famous 3rd law of motion. The camberd wings advantage is it is able to be moved to a greater A of A before the flow begins to seperate, or stall.

Most experts now beleive that the deflection theory is trully how an aircraft fly's.

Its the downwash caused by the blade deflection that becomes compressed with the ground and cusions us during landing, and takeoff , known as ground effect.

Regards Sam

[Ga6riel]

Just some small idulgences, if the rotor where a fixed wing, and in some aspects it can be considered so, and for ground effect in particular:

1 Low aspect ratio wings are greater affected. This is because the traverse flow that is associated with tip vortices is greatly slowed. On a normal wing, the outward flow from below, and the inward flow from above are disabled by the inability for the vortices to correctly form, being constrained by the narrowing air gap between the wing and the ground surface. This greatly reduces the induced drag and therefor the drag sum in total. It could almost be considered an Aspect Ratio boost.

2 The straighter flow increases the efficiency of the section, and is supported by the near stagnation of air beneath the wing. The lower the machine, the greater this effect.

3 The narrowing air gap at the trailing edge pushes the downwash vector up, and this is also visible as the total drag vector is radialy brought forward, there being less drag, and the downwash angle being perpendicular to the drag total is thus reflected. This has the effect of flattening the AoA.

4 In an ordinary section, the lift centre is pushed toward 50% of the chord on the bottom by the near stagnation of the flow beneath, while the upper surface is performing in the free stream, it maintains a near normal location, but for the straighter flow. The centre of pressure on the low side, is then variously affected depending on the height of the section above the surface.
***********

What interests me is this, that for ordinary wing, the lift centre is greatly affected, thus requiring the huge tails we see on ground effect machines, without which the machine will nose over due to the shift in the centre of pressure on the lower side of the section. Yet a disc described by a rotor is apparently not as affected, it already having a lift centre at 50% of the chord, or thereabouts, the forces are more or less in ballance. I mean, thats just wierd is it not?

[Doug Riley]

Gabriel, as I mentioned earlier, if there WERE a C.P. shift in ground effect, a (CCW) rotor with flap hinges ought to pull to the left. I'll pay more attention the next time I fly, but I don't believe there is a noticeable effect of this sort.

[Ga6riel]

right
sorta interesting
much like a tandem wing of narrow chord I guess

carefull you dont nose the thing over in the process Doug, just in case :)

[Udi]

...Most experts now beleive that the deflection theory is trully how an aircraft fly's.

Regards Sam
Huh? Which experts are you referring to Sam?

The real experts have known the answers for over 100 years. I think you are referring to the Bernoulli vs. Newton argument. This argument is taking place among amateurs, not experts. In fact, both Newton (the laws of motion) and Bernoulli (conservation of energy) can be used correctly to describe lift.

This is an excellent article about this "debate":
http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html

Udi