Gyrocopter aerodynamics

[SumoSurfer]

Very nice diagram

[Tim]

A couple of points worth mentioning. All the parameters depend entirely on the efficiency of the design. Given that both side x side and tandem are well designed, then the AUW of a gyro is not dependent on the configuration. The weight of a s x s can well be lighter because the extra length and strength required for a tandem, plus the extra rudder volume required to offset the extended nose. Another consideration is that most tandems have steel main frame which is of course heavier than Aluminium.

The little extra drag from a wider fuselage is comparable to the extra drag from the longer profile drag from the surface area of the tandem. The profile bottom surface area of both are similar, so the tendency to “float” on landing would be similar. The main problem with a s x s is that many owners want to carry the ‘kitchen sink” with them which of course makes them heavier. Heavier equals higher rrpm which takes longer to decay on flaring for landing, giving the illusion of floating, where in fact it is just simply still flying until the rrpm decays enough such as it can no longer support the aircraft, and it gently touches down. This is a correct gyro landing, sinking it on and not flying it on.

[SumoSurfer]

Ross;
Posted some pictures that might be useful.
SS

[SumoSurfer]

Rick;

I think the issue is the weight of the aircraft. I am assuming that you use the same approach speed?

If they have the same (fuselage) air speed, then we know at this speed, the rotor must produce more lift (lift = weight).

If the rotor is identical, then we can assume the airfoil is the same. In this case, the variable that is (likely to be different) is the rotational speed (RPM) of the rotor.

If this is the case, I am speculating here, then the rotor (on landing) will have greater momentum (momentum = mass * velocity). This greater momentum would mean that the rotor speed decays more slowly than the (same rotor system on the) tandem (lighter) gyrocopter.

Like I say, speculation. I will do some more looking. I am very certain it isn't ground-effect from the fuselage.

D

[RickE]

In answer to your question SS, the side x side I fly is a bit heavier than the tandem but both have an identical rotor system. Most pilots that fly the same brand side x side as myself agree that they tend to float considerably more than other styles.
Surprisingly this particular side x side is one of the quicker gyros available.

[SumoSurfer]

Rick;
Sorry, didn't answer part of your question. Yes, the airflow over the fuselage is the same as over the rotor. But, the rotor is going much faster, and is shaped to produce lift. Given the velocity squared part of the lift equation, the rotor produces much more lift, and lift-induced drag. The latter is why the rotor acts like a brake when you pull the stick back (increase the AoA) after landing.
D

[SumoSurfer]

Ross;
Just posted some that might be useful. Hopefully they show that GE (unless in a Lippisch Design) is not a cushion of air. Rather, it is the interference (and weakening) of the tip vortices (lift-induced drag).
Hope this makes sense.
D

[SumoSurfer]

[SumoSurfer]

Rick;
Thanks for this.
The body will do a tiny bit, but isn't (hopefully) going fast enough to create a cushion of air. Do your tandem and side-by-side have rotors of the same mass/rotating momentum?
As for the ArrowCopter, I think (like the CarterCopter) the claim (reasonable I suspect) is that at speed, the undercarriage (with an airfoil profile and appropriate AoA) produced lift. If this was the case, the rotor would not need to carry all the weight of the aircraft. If the rotor needed to produce less lift, it would (using the lift equation) also produce less lift-induced drag. This, I suspect, is where the extra speed (with the same powerplant) comes in.
The remaining issue of course, is parasitic drag. This increases with the square of velocity (of the fuselage). I suspect this is why at higher speeds, people do things like fair the rotor head, use wheel spats etc.

D

[SumoSurfer]

Tim;

Ground effect is, as you state, something that occurs about 1/2 of a span off the ground (surface). The benefits increase the closer the wing is to the ground. The advantage of ground effect (most notably exploited by the Russians and their Ekranoplan series of aircraft, is the modification of wing-tip vortices. This is different to helicopters that hover in ground effect. Hovering in ground effect provides an advantage by providing a hard surface (the ground) against which the thrust of the rotor can react.

As for gyroplanes, the reason that we can take off behind the drag curve is not actually because of Ground Effect. Rather, it is (as you not) just because of the Drag Curve. The Drag Curve is produced by the addition of two curves. The Lift-Induced drag curve, and the Parasitic (there are other names, as it is a few types drag, like profile and form) Drag Curve. The one that concerns us (taking off and not being able to climb away, is the Lift-Induced curve.

The Lift Induced Drag Curve is a plot of the drag generated at straight and level flight at different airspeeds. As speed increases, the amount of lift required remains the same. As such (using the lift equation), the AoA must decrease. The AoA if (for any speed) the determinant of the coefficients of Drag and Lift. (the lift and lift-induces drag equations are the same, except for the coefficients).

So what? What has this got to do with being airborne without the benefit of Ground Effect?
What happens is this. We pre-rotate, then start rolling. As we roll, the rotor speed decay and then builds. So, rotor speed is coming up. If we go back to the lift equation:

Lift = 0.5 * air density * rotor air velocity squared * surface area * coefficient of lift

At a certain speed, getting close to where we can (for the power we have currently applied) we will reach a point where we can get close to taking off.

If at that point we increase the AoA (that is, we pull the stick back a bit, or even, counter intuitively, pull the power back) then we increase the coefficient of lift, we now have sufficient lift to leave the ground. But, we have also increase the coefficient of drag and sent ourselves 'backward' on the drag curve. The only way we can sustain straight an d level flight is to increase speed. If we have insufficient power to overcome the drag, then we must lower the AoA and therefore the coefficient of drag. This results in sink.

Another way to think of it is this. If ground effect is really the cause, then we would (after sinking a bit) maintain a distance above the ground - rather than mushing in.

[RickE]

Wouldn’t the airflow over the body of a gyro in a flare attitude create a similar effect to that of airflow over a wing, rotating or fixed. As this is the attitude for landing and takeoffs, would this contribute to apparent ‘ground effect’?
If this is correct it would support my theory that side x side (wider bodied) gyros tend to float further than single or tandem style gyros, either that or I need more practice!
I also recall one of the selling points of the infamous “Arrowcopter” was the wide ‘winglet’ style undercarriage which was promoted as providing extra lift, which may also have had an effect when close to the ground.

[RossM]

Be interesting to see a similar type of photo of an aeroplane taking off, or just above the runway in ground effect!

[SumoSurfer]

Tim;


Attached are two screenshots from Stalesski (2018)+ that show the flow around a gyrocoptr taking off (the one on the ground) and in forward flight (just after taking off). You will see (hopefully) that the flow is up through the blades. There is some accelerated flow (red) around the fuselage. This is the low feeding the propeller. (shown as the

+ Stalewski, W., 2018. Simulation and Optimization of Control of Selected Phases of Gyroplane Flight †. Comput 6, 16. https://doi.org/10.3390/computation6010016

[RossM]

Hi Tim
You have not lost your artistic skill!

[Waddles]

A good demonstration of ground effect in a gyro is the classic case where a gyro tries to get airborne at a speed which is below minimum inflight airspeed; the classic "behind the power curve". If there was no ground effect, the gyro would not become airborne at all, and continue through the proverbial fence whilst still on the ground. instead, it does become airborne but cannot accelerate or climb. If there is no ground effect, what keeps the gyro airborne at a speed that is less than minimum inflight speed?

As a matter interest, ground effect in a fixed wing occurs at a height that approximates 1/2 the wingspan. This is general as there are several variables that may be involved. Not sure with a gyro as heights are very difficult to judge.

Waddles