Ground effect

[Rowdyflyer1903!]

Here is a question. When you are dealing with ground affect in a fixed wing airplane, the additional efficiency of the airplane wing is due to the disruption of tip vortices when the aircraft is with in a certain wing length of the ground. How does ground affect in gyro relate?

I swear I searched and could'nt find a answer. No doubt the forum has debated this before. If so point me toward those words of wisdom.

Bill C

 

[Vance]

Here is a question. When you are dealing with ground affect in a fixed wing airplane, the additional efficiency of the airplane wing is due to the disruption of tip vortices when the aircraft is with in a certain wing length of the ground. How does ground affect in gyro relate?

I swear I searched and could'nt find a answer. No doubt the forum has debated this before. If so point me toward those words of wisdom.

Bill C

In my opinion ground effect works the same in a gyroplane. It is not as noticeable because the rotor is so high above the ground.

Anyone who has flown both a low wing and a high wing airplane has felt this difference.

Typical level flight in the Predator at 50kts takes 2,100 engine rpm. If I get within a foot of the ground (rotor is still 12 feet above the surface) around
1,700 engine rpm will allow me to fly the length of the runway a foot off the ground.

If I am not paying attention I can go right through ground effect without noticing it.

A gyroplane with a shorter mast like a Cavalon tends to have significant ground effect around four or five feet above the ground.

Most pilot’s operating handbooks I have read advocate remaining in ground effect after an early slow lift off until the climb speed is reached.

 

[Jean Claude]

The rotor disc behaves like a "porous" wing, partially but surely opposing his crossing by the airflow.
When the proximity of the ground hinders the downwash, it allows the same lift with less angle of attack, resulting in reduced drag
But only 10% less with Z/R = .6

 

[Rowdyflyer1903!]

There is something called the "Phantom wing effect." This occurs behind the wing and is related to the Magnus effect which causes a base ball to curve. This is simply the air being influenced by the wing far beyond the confines of the wing. Are you saying the air moving over the disk is disturbing the air behind the disk and the inturuption of this airflow by the ground is the cause increased efficency of the system when near the ground?

 

[Jean Claude]

A rotor disk of gyroplane, like a fixed wing, leans on the air thanks to its angle of attack. But as the air escapes at the same time down, it takes a bigger angle to compensate.
When the ground is close, the air escapes with difficulty and thus the same lift is obtained with less angle of attack. Hence a reduced drag

 

[chrisk]

Not sure of the technical reasons why, but Ive seen noticeable "float" when close to the runway and fast. Similar ground effect is seen with soft field landings.

 

[Ed L]

Here is a question. When you are dealing with ground affect in a fixed wing airplane, the additional efficiency of the airplane wing is due to the disruption of tip vortices when the aircraft is with in a certain wing length of the ground. How does ground affect in gyro relate?

I swear I searched and could'nt find a answer. No doubt the forum has debated this before. If so point me toward those words of wisdom.

Bill C

Extensive “debate” of this under a mishap thread, “AR-1 N923DJ Texas 15-12-18” starting around post 130 or so. The upshot: two factors at play for ground effect in aircraft of all types: decreased induced drag plus increased lift. Gyros benefit from the first but, because the second is chord-based and our chords (of our “wings” - the rotors) are on the order of 8” or so and 9ish feet off the ground, the lift part mathematically doesn’t factor in.

 

[SportCopter]

Extensive “debate” of this under a mishap thread, “AR-1 N923DJ Texas 15-12-18” starting around post 130 or so.

The discussion of gyros in ground effect seems to have begun earlier, in post #99:

We once asked a professional aerodynamicist about rotors in ground effect. His reply to us:

Yes, there are lots of “interpretations” of the physics of ground effect, some of which are not strictly correct or at least would be debated in an academic setting. However, the classic “textbook” explanation (e.g. McCormick) comes about from changes of local velocity vectors at the wing section or blade element, which results in a less backward tilt of the local lift vectors.

So, based on that interpretation, there is a reduction in drag (hence a reduction in net rearward force) and a small increase in the vertical component of lift that overcomes weight. The “air cushion” interpretation for both wings and rotors is rooted in the more detailed velocity vector physics that I have just explained, so it all manifests as an increase in effective L/D on the vehicle.

As I have said, any transient reduction in induced torque on a rotor in autorotation near the ground (e.g., landing) most likely is going to be accompanied by some transient increase in rotor rpm, and so perhaps some small transient increase in lift until equilibrium conditions in autorotation are once again obtained.

The upshot of this is the marked “cushion” effect felt during landing by both airplane and rotary wing pilots.

While the IGE increased lift is less than that of a low-wing airplane, it is not zero, either. Some gyro pilots believe in a "cushion" while others do not, so the debate on this may continue regardless of what any aerodynamicist says.

Merry Christmas to all from Sport Copter!