# Ground effect in a gyroplane?

#### Vance

##### Gyroplane CFI
A recent post referenced this presentation of ground effect so I felt a new thread was appropriate.

https://tinyurl.com/yxm2b66y

In my opinion things fly level when they displace air equal to the weight of the aircraft downward.

According to the Rotorcraft Flying handbook: https://www.ronsgyros.com/Gyro_Handbook.pdf

“ROTOR FORCE As with any heavier than air aircraft, the four forces acting on the gyroplane in flight are lift, weight, thrust and drag. The gyroplane derives lift from the rotor and thrust directly from the engine through a propeller. The force produced by the gyroplane rotor may be divided into two components; rotor lift and rotor drag. The component of rotor force perpendicular to the flight path is rotor lift, and the component of rotor force parallel to the flight path is rotor drag. To derive the total aircraft drag reaction, you must also add the drag of the fuselage to that of the rotor.”

“ROTOR LIFT Rotor lift can most easily be visualized as the lift required to support the weight of the aircraft. When an airfoil produces lift, induced drag is produced.”

I use the Rotorcraft Flying Handbook to teach rotor aerodynamics because that is what the FAA wants.

Yesterday in The Predator I flew half the length of the eight thousand foot runway at less than two feet above the ground at fifty knots. The power required for level flight at fifty knots was reduced from approximately 2,150 to 1,720. The density altitude was just under 2,000 feet. She has a digital tachometer and it jumps around. It is always wind at SMX so the rpm numbers are only an approximation.

This appears to me to be in conflict with the presentation.

I was not looking at the rotor tachometer so I don’t know how the apparent ground effect affected rotor rpm.

I do not recommend this as I feel flying that close to the ground at fifty knots increases risk.

I often fly low down the runway with clients at around 30kts with similar results to demonstrate ground effect.

The Rotor of the Predator is 115” (9’7”) high and 30 feet in diameter so at two feet above the ground the rotor is less than half the rotor diameter above the ground.

In my opinion the ground effect reduces the rotor tip vortices and increases the efficiency of the rotor.

I am not an expert and do not pretend to be one.

I have a call in to Raul Salazar and I am hoping he will get back to me. We have talked at length in the past. I will share what he shares with me.

The picture is not from yesterday and is closer to three feet above the ground.

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#### Vance

##### Gyroplane CFI
I received a call from Raul and he is going to call back with Jerry Higman because Raul feels that Jerry will be better able to explain the math.

I told him of my demonstration in The Predator and he feels it is something other than ground effect.

As I interpret what Raul says he feel that an auto rotating rotor only has air going up through it and has drag like a world war II round parachute.

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#### Vance

##### Gyroplane CFI
A very patient Raul and Jerry spent a very long time (an hour and thirty eight minutes) on the phone sorting through our divergence of opinions.

I gave Jerry my examples and he suggested we compromise with; “there is no tangible benefit from ground effect in a gyroplane.”

I agree, flying low enough to experience ground effect in a gyroplane is dangerous.

In my opinion using ground effect to manage a takeoff is poor aviation decision making.

Jerry wanted to go a step further and say any benefit from ground effect is not from the rotor and we did not reach agreement.

Raul is doing some interesting work with the FAA that may cause some changes.

#### WaspAir

##### Supreme Allied Gyro CFI
They are completely wrong, Vance.

Their presentation suggests (see slide 19) that a gyroplane rotor is sucking air up from below and ahead, and blowing it upward and behind. If it did that, it would propel the aircraft right down into the ground and level flight in a gyroplane would be impossible. They have the physics very, very, very wrong.

Airflow encounters the disc from "below" only in the sense that the disc is tilted back and its underside is exposed to the relative wind from straight ahead when in level flight. The disc does not suck up air from below the aircraft and deflect all that flow upward into a rising wake like some perverse vacuum cleaner, or a demonic upside-down helicopter.

The claim that "air moves upward through the rotor" (slide 21) is naïvely wrong; upward only in the sense of passing from the underside of the blade toward the top side of the tilted blade, but not upward with respect to gravity.

Kind of scary to realize these guys claim to be gyro CFIs. They really don't understand how gyros fly.

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#### Sv.grainne

##### Active Member
Thanks for taking the time to deal with this, I for one appreciate it. Looking forward to more from the FAAST.

Bobby Ward

#### WaspAir

##### Supreme Allied Gyro CFI
P.S. Fixed wings encounter the airflow the very same way as a gyro rotor in level flight. With a positive angle of attack, the relative wind strikes the fixed wing from "below" (on the underside of the wing) and flows "upward" (around the leading or trailing edge of the wing toward the top side), but the flow quite obviously does NOT go shooting upward with respect to gravity.

#### thomasant

##### Member
Unfortunately, these are the experts working with the FAA to make some changes, from what I understand.

Also, I feel that the illustration with the windmills does not seem to apply for a gyroplane. The windmill is stationary and cannot be compared with the gyroplane rotors in forward flight.

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#### WaspAir

##### Supreme Allied Gyro CFI
Well, anybody can join the FAASTeam, with no vetting, so one can hope that they're not influencing anybody who matters.

#### hillberg

##### Super Member
in 1983 I was having some fun with a Bell 47 - Just finished an autorotation with a gentle flair and was level at 18" skid height
I kept a little aft cyclic no collective yet and let it 'float' for over 250 feet along a dirt field, I asked Kenny if he wanted a power recovery or a full down.

Hiller said 6" was OGE
Robinson a foot or so
Bell was 18"
Sikorsky was 60 feet.
Had a Brantly owner so big it took all it had to just stretch its legs...on a running take off
Don't think the manufactures ever got together on just what was OGE... Only it got off the ground at its own pleasure.

I do so enjoy the spray patterns as they worked the fields...

#### fara

##### AR-1 gyro manufacturer
Well the ground effect in a gyroplane is always to bring descent rate to zero . Simple.

Well we know that ground effect is reduced induced drag and in helicopters the ground acting like end plates for downwash induced flow in hover or very slow speed.
I do believe the ground effect in helis shows itself differently with this induced down flow and thus seen most in hover and much much less as it moves at a click.
Is there ground effect in gyroplanes in auto-rotation. I have to really think about it now that I have sat down and read that presentation. I missed attending it so did not get to participate in that meeting. However, some things in the presentation are correct like the pilot lingo of describing what is ground effect. Things like cushion of air, high pressure below etc. Those are definitely pilot lingo and may be they suffice for pilots but they are obviously not correct. The main thing being reduced induced drag near the ground so we can generate the same lift at lower angle of attack or if you would, create more lift with same angle of attack. In gyroplane case this angle of attack is the blade AoA.

These videos clearly show that there are tip vortices similar to an airplane in rotating wing however, the point is due to the way autorotation flow of wind occurs do these tip vortices then have a similar effect near the ground as a fixed wing. I agree the effect is in a different format in helicopter. May be they meant to say that gyroplane does not have the same kind of ground effect as a helicopter which has it most pronounced at IGE hover.

After thinking it over I think there is indeed ground effect very low (AGL) in auto-rotation and it reduces rate of descent a bit

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#### ventana7

##### Gold Member
In my opinion using ground effect to manage a takeoff is poor aviation decision making.

Their mistaken explanation on ground effect aside, I’m confused by your own comment.

I believe most of us flying heavier two place machines accelerate in ground effect prior to climbing out at Vy.

Rob

#### ventana7

##### Gold Member
They are completely wrong, Vance.

Their presentation suggests (see slide 19) that a gyroplane rotor is sucking air up from below and ahead, and blowing it upward and behind. If it did that, it would propel the aircraft right down into the ground and level flight in a gyroplane would be impossible. They have the physics very, very, very wrong.

Airflow encounters the disc from "below" only in the sense that the disc is tilted back and its underside is exposed to the relative wind from straight ahead when in level flight. The disc does not suck up air from below the aircraft and deflect all that flow upward into a rising wake like some perverse vacuum cleaner, or a demonic upside-down helicopter.

The claim that "air moves upward through the rotor" (slide 21) is naïvely wrong; upward only in the sense of passing from the underside of the blade toward the top side of the tilted blade, but not upward with respect to gravity.

Kind of scary to realize these guys claim to be gyro CFIs. They really don't understand how gyros fly.

Agree completely. They seem to think a rotor acts like a Fenestron or ducted fan shooting air upwards, rather than an airfoil generating lift just as an airplane wing does.

Rob

#### Jean Claude

##### Junior Member
Others errors:

Gliders benefit a lot from ground effect because
1- They can fly closer to the ground, relative to their wingspan. At 4 ft above the ground, the height is only 1/12 of its wingspan.
2- At best speed of glide, their induced drag is half of the total drag, like all fixed-wing aircraft.

The induced drag of an elliptical wing is far from zero. It is only slightly weaker than the other shapes: Cd ind = Cl²/π.A (A is the aspect ratio)

Totally wrong. As for all the wings, the lift produces an air deflection toward the ground.
Thus, the ground proximity decreases also the induced power.
The improvement is small however because the minimum flight height remains above 0.6 * span, and the downwash angle is never as large as the helicopter

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#### C. Beaty

##### Gold Supporter
Most of my gyro flying has been from grass strips, nearly all of which of which have been bumpy.

Standard procedure has always been to get the wheels off the ground as soon as the rotor would take the load. This occurs before the rotor is up to full speed and the gyro is still well on the backside of the power curve. Stay in ground effect as the rotor comes up to speed and as the machine accelerates. Only then leave ground effect.

I have always used analog rotor tachs made from 0-50 mph motor scooter speedometers so that I have a clear presentation of rotor behavior without having to interpret a bunch of blinking digits on a digital display.

Incidentally, I designed the first digital rotor tach using standard ICs and a quartz clock crystal from which the timebase was derived. This was before the Red Lion tach was available. I gave Ken Brock the schematic and he had a job shop design a proper circuit board and fit it all into a stock housing. He listed it in his catalog for a number of years.

#### Resasi

##### Gold Supporter
All in all it is my impression that there are a number of forum members who fly gyros either professionally, or who have done so for a long time, or who have flown various types of flying machines, or who are pretty good at aerodynamics, who are all of the opinion that there is a perceptible ground effect in a gyro, despite what has been said in this presentation, and advice to the FAA.

Soft field take off procedure would seem to presume there there is, and doing one would seem to confirm there is.

So Bobby, take your pick, webinar or Forum. Better still try for yourself and see what your behind tells you.

#### Sv.grainne

##### Active Member
So far, my opinion is there is no noticeable ground effect. But I'm always trying to learn so I'm pretty open to new ideas.

#### XXavier

##### Member
So far, my opinion is there is no noticeable ground effect. But I'm always trying to learn so I'm pretty open to new ideas.

I use to fly gyros, and feel the 'ground effect' every time I fly a low (0,5 m above ground) pass on the runway. Of course, it's not as noticeable as if you were flying a low-wing FW, but it's clear in gyros too. Very clear. You fly that low pass with less engine revs that you need to fly s/l out of ground effect...

#### Vance

##### Gyroplane CFI
Their mistaken explanation on ground effect aside, I’m confused by your own comment.

I believe most of us flying heavier two place machines accelerate in ground effect prior to climbing out at Vy.

Rob
I was trying to keep an open mind and not turn our conversation into a semantics debate staying clear of what they felt was the math.

This was in relation to a long talk about a “tangible benefit” from ground effect as I was describing the various ways to experience ground effect.

One way to experience ground effect is with a very marginally performing gyroplane using it to get airborne but not being able to climb out as JR has described.

In my opinion if I am stuck at the top of ground effect and not being able to climb out further it is time to abort the takeoff rather than continue.

In The Predator if the balancing is done correctly she lifts off close to Vx and Vy happens so quickly ground effect is not noticeable and might not be considered a tangible benefit.

With a client gets her nose too high she can lift off as slow as twenty knots and may be misaligned so with my long (8,000 feet) wide (150 feet) runway I usually have them straighten out, land and start over. At anything over 25kts lift off I usually just have the client climb out. We are often misaligned with the runway and they are over controlling so we do not linger in ground effect. It usually does not take long to get past this phase of leaning to take off.

In some gyroplanes if I lift off prematurely (before Vx) staying in ground effect is a good way to continue the takeoff and probably fits into “tangible benefits” although being low and fast over a paved runway is not what I recommend in most gyroplanes particularly in gusting winds.

One of the things I am required to teach is lift off at low airspeed and high angle of attack because if this situation is mishandled it may precipitate a tip over.

#### WaspAir

##### Supreme Allied Gyro CFI
Back in my McCulloch J-2 days, one could put a little rearward pulse on the stick at 45 IAS, and it would come off the ground crisply, but best climb was more like 70. It was routine to stay low for a just a bit and accelerate to that higher speed before attempting a climb-out. However, in that particular design, 45 was not struggling on the back side of the curve and control response was good. It didn't produce the low airspeed/high angle issue anticipated in the PTS and there was no wallowing about.

Staggering prematurely into the air and then attempting to gain speed isn't generally a good practice.

#### fara

##### AR-1 gyro manufacturer
I use to fly gyros, and feel the 'ground effect' every time I fly a low (0,5 m above ground) pass on the runway. Of course, it's not as noticeable as if you were flying a low-wing FW, but it's clear in gyros too. Very clear. You fly that low pass with less engine revs that you need to fly s/l out of ground effect...

Trust me. Raul feels that too. He is not unaware of this perceived effect near the ground and his soft field takeoff technique instruction is exactly using this same thing. I think Jerry (?) is the gentleman who is the aerodynamic expert that has come up with this postulation. The problem is I believe he's got the airflow really exaggerated and that is throwing off the setup to his problem. Just my opinion. I will have to talk to Raul to understand their setup of the problem. Wrong setup will lead to wrong solution.
By the way an experiment with some smoke machines or on a foggy day with good recording could clear up the setup of the problem and provide a good visualization. This can be done using a RC gyroplane model or in a simulation in a software like Onera

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