Ground effect in a gyroplane?

[XXavier]

I agree with that. My spreadsheet gives a drag of 1000 N, IGE and 1100 N, OGE (when z/R = 0.6) for the same weight of 900 lbs and the same steady forward speed of 45 mph (*)
However, this is just an anecdote without practical interest.
(*) induced speed corrected according NACA TN D 234

In the case of a takeoff roll on a wet grass runway, it may be of practical –even vital– interest to get unstuck asap, and then accelerate in ground effect without wheel drag until you reach a safe airspeed...

 

[Jean Claude]

In my opinion I would need to have a very narrow view of autorotation to imagine the angle of the disk and collective do not change the physics of ground effect

When landing a helicopter in autorotation, it is not the greater ground effect that gives it the advantage over the gyrocopter. It is just because the variation of its collective pitch allows to recover the kinetic energy of the rotor with the decrease of the Rrpm and to maintain the lift during a short hover

Many flight instructors teach to remain in ground effect until reaching Vx because it avoids learners tendency to simply pull back to climb and it is what is in many pilot’s operating handbooks (POH).
I teach remaining in ground effect until reaching Vx if I am teaching in a gyroplane that has it in the POH.

If it is important to wait near the ground to reach a minimum speed before starting to climb, this is only to obtain a sufficient rate of climb. The ground effect of the gyro exists, but it remains an anecdote.

In the case of a takeoff roll on a wet grass runway, it may be of practical –even vital– interest to get unstuck asap, and then accelerate in ground effect without wheel drag until you reach a safe airspeed...

This is also true without any ground effect. Thus why mention it?

 

[XXavier]

(...)


This is also true without any ground effect. Thus why mention it?

Because in ground effect you need less airspeed to fly, and once unstuck you can accelerate faster, without rolling drag, that may be very high in some cases. In other words, it's easier for you to stay near the ground at the start of the takeoff.

 

[kolibri282]

Quote: Your description of ground effect reads to me like you feel there is a cushion of air beneath the helicopter and that is what causes ground effect /Quote
At no point in this thread did I mention any "cushion of air" causing ground effect. I rather stated in #70
Quote: Near the ground the induced velocity is slowed down by hitting the ground thus reducing the rate at which energy is lost /Quote
Now energy rate (energy/time) is power, which is force times velocity. In our case it is thrust times inflow rate, which, in hover, is equal to induced velocity, so my statement in #70 basically says that ground effect reduces inflow speed.

(Note that required thrust does not change IGE or OGE since aircraft mass is constant.)

This site:
https://mediawiki.ivao.aero/index.php?title=Ground_effect
explains at length that ground effect reduces rotor inflow speed, thus reducing power. My attached drawing shows why.

To sustain hover you need to generate a thrust in global Z direction equal to the vehicle weight (mass*acceleration) Treq. For this the rotor blades have to be inclined at the proper local AoA (here 15°) to generate that lift. Since the profile lift is always perpendicular to the resultant air speed VR you have to create a substantially greater resultant thrust Tres if your resultant flow speed is inclined more with respect to the global Z direction. In ground effect, with a lower inflow speed, you have to generate a lower resultant rotor thrust Tres, since it is less inclined and so you can even use a lower local AoA reducing blade drag even more.

This lengthy explanation was implied in my statement #70, which seems to have eluded you. That probably occurs if you are a really great pilot but have a rather limited understanding of flight physics.

 

[kolibri282]

Quote: When landing a helicopter in autorotation, it is not the greater ground effect that gives it the advantage over the gyrocopter. It is just because the variation of its collective pitch allows to recover the kinetic energy of the rotor with the decrease of the rrpm and to maintain the lift during a short hover /Quote
Great explanation, JC!

 

[WaspAir]

Quote: When landing a helicopter in autorotation, it is not the greater ground effect that gives it the advantage over the gyrocopter. It is just because the variation of its collective pitch allows to recover the kinetic energy of the rotor with the decrease of the rrpm and to maintain the lift during a short hover /Quote
Great explanation, JC!

In light helicopters it is fairly common to terminate an emergency autorotative approach with a "running" landing rather than slow it all the way to a hover. You may still be at or above effective translational lift speed when the skids or wheels touch down. One levels the aircraft, and then cushions the touch with collective, but if you lack wind you'll be sliding it onto the ground.

 

[Tyger]

 

[Vance]

An airfoil is an airfoil and looking at it through that narrow window ground effect is the same for a helicopter and a gyroplane.

Fortunately rotor aerodynamics and flying are more complex than that.

It is my observation that ground effect has little effect on a power off autorotation to the ground in either aircraft.

In my opinion trying to get into ground effect in an autorotation to the ground in either aircraft is a great way to bend the aircraft.

In an autorotation to the ground in a helicopter stored kinetic energy plays a major roll as does the collective.

Rotor rpm needs to be kept in a very narrow range in a helicopter and a gyroplane can be operated over a very large rotor rpm range.

For example according to the pilot’s operating handbook for a Robinson R44 power off maximum rotor rpm in is 108% or 432 rotor rpm and minimum is 90% or 360 rotor rpm. In powered flight maximum rotor rpm is 102% or 408rrpm and the minimum is 101% or 404rrpm.

Flying The Predator the maximum rotor rpm is 475 and the minimum is 275 rotor rpm.

Stored kinetic energy is much less important in a gyroplane in a power off landing.

Most helicopters have a collective and most gyroplanes do not.

Because a gyroplane can’t hover (zero indicated air speed); out of ground effect, in ground effect and translational lift are not relevant to a gyroplane pilot.

As Javier correctly pointed out a gyroplane will fly with less power in ground effect and that has value for a soft field takeoff. He did not mention that the ground effect is less pronounced over anything other than a smooth, hard surface.

 

[kolibri282]

Quote: In light helicopters it is fairly common to terminate an emergency autorotative approach with a "running" landing /Quote
Is this technique perhaps also used because the rotor blades of a light helicopter have a low moment of inertia and thus can not store enough energy to substantially slow down the aircraft? It would then make sense, IMO, to touch down with the forward speed near the lowest sink rate to minimize damage.

 

[Martin W.]

As someone who self-taught in the old Bensen way, I logged a lot of hours under power at under 10 ft. off the runway.

Adding to my scores of hours in ground effect was the fact that I started out with a 1500 cc VW, which would not fly out higher than ground effect at any airspeed! Yet it would fly all day IN ground effect (at under 10 ft.). I'd call that a "tangible" (noticeable) benefit, though not a very useful one.

Before Vermont's winter climate moderated in recent years, Lake Champlain would freeze over, all the way across to New York state most years. Buddy Bill Raub and I would layer up our clothing in February and go blasting about, just off the ice, for miles. This was a real treat. You could throttle back significantly and still maintain speed and altitude, just as Vance describes. The reduced noise from lower engine RPM was pleasant.

Some incoming air, of course, spills up through a gyro rotor disk; there is not a vacuum just above the disk. The physics of lift cannot work, however, without there being a net surplus of air molecules ending up below the level of the disk and a net shortage of them above it. Overall, the rotor deflects air down-and-back. The resulting downwash is quite visible (or, well, visualizable) behind a gyro flying low over grass or crops.

A rotor blade is a very ordinary wing. It makes lift the same way as any other wing. All wings create tip vortices, as some of the air pushed down by the wing squirts out and around the end of the wing. Making these little horizontal tornadoes soaks up power in an un-useful way. Partially blocking them adds to the efficiency of any wing, including a rotor blade.

Thank you Doug

I know this topic can get very technical and with your experience and knowledge you are capable of such high level discussions.

But your post is written for the average guy (like me) and is more informative than 10 pages of physics and mathematics.

Much appreciated.

.

 

[WaspAir]

Quote: In light helicopters it is fairly common to terminate an emergency autorotative approach with a "running" landing /Quote
Is this technique perhaps also used because the rotor blades of a light helicopter have a low moment of inertia and thus can not store enough energy to substantially slow down the aircraft? It would then make sense, IMO, to touch down with the forward speed near the lowest sink rate to minimize damage.

There are other reasons to touch with less forward speed, particularly the risk of a rollover for skid-equipped helicopters. A slight misalignment or surface irregularity can start you tipping over and make a mess of things very quickly. It is generally good to shed energy in the air with little left to dissipate after touchdown. As you lose rotor rpm you also lose tail rotor effectiveness making steering on the ground, and avoiding tip over, an extra challenge. Keeping skid shoes in good condition helps a little.

Some Bells have blades sufficiently massive to let you touch down, pick up, and go down again.

 

[Vance]

I found doing an autorotation to the ground in an R44 intimidating with my monocular vision and lack of depth perception. I found the noise disquieting.

It was a part of my decision to embrace gyroplanes rather than helicopters.

I find it concerning that an autorotation to the ground is no longer required for a private pilot, rotorcraft-helicopter certificate and the NTSB accident reports seem to validate that concern.

In my opinion an autorotation with a power recovery is missing some important skills and lessons.

 

[WaspAir]

In R-22 training, one does autos through the cyclic flare (like a quickstop) and recovers to a hover with skids level, but one also separately does running landings (with power available). The idea is that a real engine failure will require you simply to link the two tasks, with a collective cushioned running landing following the leveling of the skids (omitting the hover).

There is much luck involved in a flawless R-22 full down auto, and the likelihood of expensive damage is extremely high. The FAA thinks mastering the two tasks to be linked will be enough to make a real incident survivable, and they are not overly concerned about the resulting damage from a rare true emergency.

It has some parallels to the removal of spin training from the FW syllabus, as a cost/benefit analysis in light of the risks in training.

 

[500e]

"As Javier correctly pointed out a gyroplane will fly with less power in ground effect and that has value for a soft field takeoff. He did not mention that the ground effect is less pronounced over anything other than a smooth, hard surface."
Reminds me of the story about a Brantley ice cream cone, just managed take of over hedge !! to be confronted by a large field of corn.
All the little lift he had dissipated

 

[Abid]

...

I find it concerning that an autorotation to the ground is no longer required for a private pilot, rotorcraft-helicopter certificate and the NTSB accident reports seem to validate that concern.

....

I think if you first hand saw a reasonably new R44 smash up to bits and pieces in front of you in a private pilot check-ride auto down to the ground right at the airport, you may understand the reasoning

 

[Martin W.]

I think if you first hand saw a reasonably new R44 smash up to bits and pieces in front of you in a private pilot check-ride auto down to the ground right at the airport, you may understand the reasoning

.

True .... Frank designed a very light blade .... then incorporated coning hinges at the root to relieve bending stress ..... which allowed him to build an even lighter blade .... which does not retain inertia when you need it at the bottom of an auto.

.

 

[Jean Claude]

Because in ground effect you need less airspeed to fly, and once unstuck you can accelerate faster, without rolling drag, that may be very high in some cases. In other words, it's easier for you to stay near the ground at the start of the takeoff.

The level before take-off is only there to keep an unexpected passage "behind the power curve" away. It is not motivated by a search for climb performance, and the ground effect is just serendipitous.

For best takeoff performance (i.e., up to a height of 50 ft) the climb should begin immediately at VT-O , therefore without ground effect, as surprising as it may seem.

 

[Burrengyro]

Ground effect depends on the smoothness and hardness of the runway surface? On a grass runway, to what extent is the ground effect advantage reduced compared to hard runways?
If I stay in ground effect, I can reach best climb airspeed quicker and also depart the runway quicker during takeoff to make the runway available for other aircraft, if required.

 

[Abid]

The level before take-off is only there to keep an unexpected passage "behind the power curve" away. It is not motivated by a search for climb performance, and the ground effect is just serendipitous.

For best takeoff performance (i.e., up to a height of 50 ft) the climb should begin immediately at VT-O , therefore without ground effect, as surprising as it may seem.
View attachment 1153785

That’s true but I think what is being said is more about soft field takeoff. Unfortunately you really can’t truly combine soft and short field takeoffs and get the best of both of them. That doesn’t happen. So the guy with the 800 foot cow pasture is out of luck.

 

[Vance]

The level before take-off is only there to keep an unexpected passage "behind the power curve" away. It is not motivated by a search for climb performance, and the ground effect is just serendipitous.

For best takeoff performance (i.e., up to a height of 50 ft) the climb should begin immediately at VT-O , therefore without ground effect, as surprising as it may seem.
View attachment 1153786

I love your numbers and agree with your conclusions Jean Claude.

I don’t fly by your numbers and in my opinion there is no Vro in a gyroplane.

She will fly when she has an acceptable combination of indicated air speed and rotor rpm.

I feel forcing her off the ground at some specific indicated air speed is poor technique.

In a soft field takeoff I encourage her to lift off early to reduce the rolling resistance of the soft field. I do not think of it as rotating. Once clear of the ground at less than Vx I find staying in ground effect until reaching Vx gives me more options if things aren’t working out.

I have not flown a gyroplane that follows your curves on a consistent basis.

Most will lift off, hesitate and then begin their climb in earnest.

If staying in ground effect till Vx is in the pilot’s operating handbook that is what I teach.

On a gyroplane with lots of power I have found simply allowing the normal climb to take place is the best way to clear an obstacle.