I can wrap my head around Juergen's explanation of ground effect....and it can stand firm with the ground effects I've seen and felt.I would like to propose this explanation for ground effect:
In naca 73254 ground effect for a rotor is modeled by introducing
a reduction factor for induced velocity of :
Kr = 1 -exp(z/D/G)
where z is distance from rotor hub to ground, D is rotor diameter
and G is some factor to match test data.
My interpretation goes like this (see attached drawing):
Out of ground effect the circulation needed to generate thrust to support
the weight of the aircraft induces velocity vi which inclines the flow
leaving the rotor downwards. To maintain sufficient inflow to keep up
autorotation the rotor disk has to be inclined backwards until it makes
the proper angle of attack with the resultant flow (airspeed plus induced
In ground effect the flow near the ground is parallel to the ground since the
air can not penetrate the surface. The streamlines near the rotor are forced
to be more flat since the flow changes gradually with height. The rotor can
thus be flown at a smaller backward inclination with the same angle of
attack as before needed to maintain inflow for autorotation. The reduction in
induced velocity leads to a very small overpressure below the rotor but the
decisive point is the difference in angle of attack of the rotor disk. The rotor
in ground effect has a better L/D and thus the aircraft as a whole has a better
This reduction does not depend on the way the induced flow is created,
whether by a rotor driven by an engine or a rotor in autorotation, therefor
ground effect is the same for helicopter or gyro and only depends on the
thrust coefficient/disk loading.
Early Helicopters like the S-58 were unable to hover in ground effect at maximum load, lacking the required power. These aircraft had wheels fitted which enabled them to accelerate on the ground since the forward speed would add to rotor inflow until the combined flow would lift the aircraft. What I wanted to point out is that a gyro with the same disk loading as the S-58 would benefit in exactly the same way from ground effect if it were flying at the forward speed the helicopter needed to break ground contact. Of course the ratio of rotor diameter to ground clearance and the inflow angles of the rotors would also have to be the same, but given these three factors mentioned previously there is no difference in ground effect whatsoever between helicopter and autogyro.therefor ground effect is the same for helicopter or gyro and only depends on the
thrust coefficient/disk loading.
Exactly correct. A gyroplane rotor is simply a wing, more like that on an airplane than a helicopter.Dennis, you use "except during landing" a couple of times in your post.
Do you remember any conclusions you came to during the flare and landing, that you came to after watching your yarn strings?
It sounds like your testing indicates no, or almost no benefit from ground effect in a low pass at speed.
An airplane making a low pass at speed does benefit from ground effect.
What is the difference in a rotorcraft and a fixed wing?
My guess is it has something to do with ratio of the chord of an airplane wing (in ft) to the airplane's speed (in ft/sec), as compared to a rotorcraft's ratio of the chord of a rotor blade (in ft) to the aircraft's speed (in ft/sec)??????
Economy Of Scale (of the airfoil) seems to have something to do with how much being close to the ground effects flow?
If an 8" chord rotor blade at 400 MPH takes 1/800 sec. to pass one spot on the ground,
and the 48" chord of an airplane wing at 150 MPH takes 1/55 sec. to pass the same spot, that seems to me why one could benefit from ground effect much more than the other?
A wings effect by ground effect is directly affected by its width ratio. The larger the wing, the more it can create a cushion of ground effect. But, it has to be near enough to the ground, and under a certain speed, or it will pass over the cushion before it can affect the wing.
So you guys are telling me that an ASG-29 sailplane, with a 59 foot span and a 30.4 : 1 aspect ratio (average chord less than two feet) will not experience ground effect on a fast pass at 30 feet (1/2 span above the ground), because it's more than 15 chords above the ground and/or your "cushion" can't keep up with you?A low wing airplane with a 48" chord, in a 5' high low pass is 1.25 chords above the ground.
A 8' tall gyro with an 8" chord, in a 5' high low pass is 19.5 chords above the ground.
There is no "step". But if you mean out of the landing flare, not behind the power curve, all you need to do to experience the "nonexistent" effect for yourself is to try to climb out from a field elevation near your ceiling.A gyro flying "up on the step" experiences ground effect like an airplane, and since the rotor is above, and the chord is so small, it's almost nonexistent.