. . . yes, a gyro rotor experiences a very distinct ground cushion.
A gyro certainly has ground effect. You can feel it more distinctly by coming in with a little extra airspeed and no power.
Stan, that dovetails with what I tried to impress upon students about the Zen of gyro landings.
Many of them will over-rotate as they come through 10 feet, and then either zoom or pancake in (or both). I tried to get them to feel the effect you are mentioning -- let the ground cushion do some of the work of arresting your descent. Use less back stick than you think you need, and don't bring the nose any higher than level as you pass through ten feet. Work with the machine's natural tendency to level out.
Save the rest of your back-stick until you're a foot off and suitably slowed down.
My own experiences over nearly 40 years of gyo flying is that there certainly is a ground effect -- most apparent in lightly-loaded machines. This effect helps the pilot "round out" at the bottom of a descent. In fact, it will fool a student into popping back up again if he has adequate airspeed on approach and does not anticipate the cushion. My advice to students to overcome this problem has been to use less back stick than you think you'll need to level out.
I never completely understood the explanations that I've seen for ground effect. Some say the ground is reducing wing-tip vortices, thus improving wing efficiency. Some say the ground is creating a "high pressure cushion", again improving the wing efficiency. In any case, my own experience is that ground effect is noticeable in gyros. I feel it mostly during roundout and flare in a lightly loaded gyro (both single and tandem) using a Dragon Wings rotor. I feel a noticeable change in the descent attitude in the last 10-15 feet - more than what I would have expected for the amount of back stick used for the roundout. To me it feels as if I am hitting a cushion of air, and I have always attributed that feel to ground effect.
Udi 12-13-2012, 09:41 PM
Two phenomena are involved when a wing approaches the ground. Ground effect is one name for both effects which is sometimes confusing. These two phenomena are sometimes referred to as span dominated and chord dominated ground effect. The former results in a reduction of induced drag (D) and the latter in an increase of lift (L). The designations span dominated and chord dominated are related to the the fact that the main parameter in span dominated ground effect is h/b (height/span), whereas in chord dominated ground effect it is h/c (height/chord).
Chord dominated ground effect
As described above, ground effect increases lift. The air cushion is created by high pressure that builds up under the wing when the ground is approached. This is sometimes refered to as ram effect or ram pressure. When the ground distance becomes very small the air can even stagnate under the wing, giving the highest possible pressure, pressure coefficient unity.
The high pressure air cushion can clearly be seen in the illustrations. The pressure around an airfoil has been calculated with and without ground effect, both at a five degree angle of attack. In free air the (2D) lift coefficient was 0.8 and at a ground clearance of 0.05 times the chord it was 1.1. [Kolibri note: This is an increase of 37.5%.] The high pressure at the bottom of the airfoil in ground effect is caused by the ram effect.
The combined result of the two phenomena described above is an overall increase of the ratio between the lift and the drag (L/D). The lift increases when the ground is approached and because of the increasing lift the induced drag may not even decrease in absolute numbers, but even a slight increase still leads to an increased L/D ratio.
Figure 3. shows a computer simulation of a conventional wing profile both in ground effect and free flight.
By comparing the total Cl (Co-efficient of lift) of both, it can be seen that the same wing in Ground Effect has an increase in lift of approximately 75%.
You can also see that the pressure below the wing has increased dramatically, this is called the dynamic air cushion.
There are two aerodynamic changes associated with the ground effect: (i) a reduction of induced drag and (ii) the presence of an effective air cushion. When an aircraft is flying close to the ground surface within a distance of one wingspan, the induced drag experienced by the aircraft is reduced because the vertical component of the airflow around the wing tip is limited, and the trailing wing tip vortices are disrupted by the ground (see Figure 3b). The downwash intensity is therefore reduced leading to a beneficial effect on lift and drag. If the aircraft is flying extremely close to the ground, within roughly 1/4 of the wingspan, the air flow between the wing and the ground is compressed to form an air cushion. The pressure on the lower surface of the wing is increased creating additional lift. Both of these effects lead to an increase in the lift-to-drag ratio (Figure 4).
No, EdL, I don't read it that way because the paper didn't claim that increased lift is zero between 25-100% of wing span altitude.but the actual increase in lift per se is negligible at best for most gyros - and not present above, say, 18 inches at absolute best for the Magni?
Do you read this the same way?
Where does it exclude gyros?As your article noted, there is indeed a ground effect but as it also effectively notes, for gyros it is, for all intents and purposes,
because of reduced drag causing an improved RATIO of lift to drag (factor 1) and not increased lift.
1. Ground effect does not provide a "cushion" to a gyroplane. It provides a reduction in induced drag which enhances performance when close to the surface. It is not an upward force to overcome downdrafts, and in no way acts like a spring to resist descent. The benefit you seek from it isn't real. It will not absorb any kinetic energy from dropping. Those who talk about a "cushion" of air don't understand the physics; ignore any sources that put it in such terms. There will simply be no advantage in your procedure from this phenomenon.
I didn't say, and that paper didn't claim, that lift occurs only within 25% of wingspan.Yours clearly said the LIFT part from the air cushioning happens within 1/4 of the wingspan and I showed how, on a Magni...
No, because multiple aeronautical papers clearly state that when there is ground effect, it's due to two concurrent factors: span-dominated (less induced drag) and chord-dominated (increased lift).Would you now agree your citations support the idea that Ground Effect, while present, is indeed due to reduced drag and not to increased lift for gyros?
[FONT=arial,helvetica]An additional bonus of ground effect that becomes more significant as speed increases is called ram pressure. As the distance between the wing and ground decreases, the incoming air is "rammed" in between the two surfaces and becomes more compressed. This effect increases the pressure on the lower surface of the wing to create additional lift. [/FONT]
If the aircraft is flying extremely close to the ground, within roughly 1/4 of the wingspan, the air flow between the wing and the ground is compressed to form an air cushion.
The pressure on the lower surface of the wing is increased creating additional lift. Both of these effects lead to an increase in the lift-to-drag ratio (Figure 4).
This entire subtopic of increased lift is inherent to my discussion of rounding out low in turbulence to better handle energetic air.over 164MEGABYTES worth, on this topic - to apparently try to prove you're right and NOT to actually add meaningfully to gyro flying.
EdL, naturally I'm aware of that. I used that graph to place my RAF's rotorspan at 0-3' AGL within a percentage context.Look CLOSELY at your graph above. It specifically says decrease in induced drag, NOT increase in lift.
Uh, no:And you realize we're talking about a gyro, where the rotor is always in autorotation from air going UP through the rotor
and not a helicopter with air going DOWN from the rotor (and therefore has its own, different contributing factors for ground effect), right?
If there is no down wash from a gyroplane rotor, what holds us up?
Vance 11-01-2008, 05:38 AM
Rotor downwash velocity is identical between gyro and helicopter if AUW, rotor diameter and airspeed are the same.
C.Beaty 08-12-2015, 08:37 AM
Kolibri;n1142597 said:This entire subtopic of increased lift is inherent to my discussion of rounding out low in turbulence to better handle energetic air.
So, yes, I am trying to add meaningfully to gyro flying.
...but the elephant is still in the room...
EdL, naturally I'm aware of that. I used that graph to place my RAF's rotorspan at 0-3' AGL within a percentage context.
And, as the graph suggests, a reduction of induced drag by only 15-20% isn't enough all by itself to explain the sensation of gyros in ground effect.
Regarding the chord-dominated ground effect, you're applying a gyro's meager 8" chord to the formula as if it were a single fixed-wing.
It's not, as each blade creates lift 5-6 times per second (300-360 rrpm). Obviously, a different formula is required to measured a rotor's increased lift.
Goodness grief. Yes, the rotor disk is continuously creating lift . . . but because of blade rotation of 5-6x/second."...each blade creates lift 5-6 times per second..." is, frankly, a stunning example of not understanding what's going on.
It's creating lift continuously while spinning but that lift varies 5-6 times a second as the blades rotate
You don't believe that a gyro's rotor exhibits a "net positive downflow of air" and only a helicopter's rotor does?But my point was a helicopter's rotor is POWERED, so there is indeed a net positive downflow of air,
which is created by a different mechanism than a gyro's rotor's effect and is subject to ground effect but in ways which I suspect differ aerodynamically from a gyro's.
Kolibri;n1142607 said:You don't believe that a gyro's rotor exhibits a "net positive downflow of air" and only a helicopter's rotor does?
Actually, that is solely your mistaken inference.Your response implies you believe a gyro can generate enough lift without forward motion to enable it to hover like a helicopter.