Gyrocopter aerodynamics

[SumoSurfer]

Hi there;

I am new to the gyrocopter scene, a fledgling at best. I have been really fortunate to be trained by some great instructors. I have also had some great conversations, everyone seems really happy to help out the new kid on the block.

One area where I have found some interest is that of aerodynamics. That is:

• basic aerodynamics,
• rotorcraft aerodynamics, and
• gyrocopter aerodynamics.

My initial career was as an aeronautical engineer, this remains a topic of interest to me.

I am keen to know if there is interest in covering or discussing these topics in:

• an informal (forum) for this sort of topic, or
• occasional (or even regular) articles in Gyro News.

Some of the discussions that have prompted this post (and the offer to continue it) include:

• the basics, including forces (lift, drag, weight and thrust) acceleration,
• lift, and how it is produced,
• the drag (also called the power or power-required) curve, and
• differences between helicopter and gyrocopter aerodynamics, including autorotation.

Happy to clarify (expand on) these (and others) if I can, but certainly don't want to go wasting time, mine or others.

Again, thanks so far for all the encouragement and support.

cheers

D

[Tim]

You have brought up a very complex list of questions there, far more than can be covered simply. Perhaps an individual question at a time and a few might chime in with some information. Also I think the ASRA web site covers most of what you want to know, however there is no problem throwing it open for discussion. Many members have left the forum but a subject like this might draw some back.

[SumoSurfer]

Tim;

Thanks for the reply.

My intent in posting was to understand if the area of aerodynamics / flight dynamics is of interest to the community.

In starting to learn to fly gyrocopters I found differing views on the way gyrocopters behave.

Two issues I have found interesting are Ground Effect and Translational Lift.

Ground Effect.

Ground Effect applies (differently) to fixed wing aircraft and helicopters. Ground Effect in fixed-wing aircraft refers to a reduction in (lift induced) drag close to the ground. Glider pilots will be really familiar with this, as they come in to land, the plane will 'float' as they are close to touching down. For helicopters, Ground Effect is a a very different thing, it is the interaction between the downwash and the ground. This means that helicopters can hover In Ground Effect (IGE) more easily that when further away from the ground.

So, what about gyrocopters and Ground Effect?

I don't suspect that gyrocopters experience either form of Ground Effect. The rotor is (in flight) not producing a downwash. Helicopters (except when in autorotation) have air moving down through the rotor, gyrocopters (always in autorotation) rely on air moving up through the rotor. As for the other (typically fixed wing) type of Ground Effect, the rotor just isn't close enough to the ground.

Translational Lift

Translational Lift is (traditionally) associated with helicopters. Helicopters when at cruising speed are in a phase of flight called Effective Translational Lift (ETL). ETL is more efficient than hovering flight because:
- the rotor outflies (much of the) tip-vorticed
- air is not recirculating through the rotor
- the Angle of Attack is lower
- there is forward (translational) airflow to the rotor
This results in less power required. Also, the airframe and tail rotor are now more efficient as well. In practice, this means that helicopters transitioning to and from the hover (from cruising flight) are transitioning to and from ETL. Perhaps the best practical example of this is when helicopters roll (or slide) along the runway as they try to take-off without hovering. Why do this? It takes less power to take off this way than it does to hover.

So, what about gyrocopters and Translational Lift?

Translational Lift applies to helicopters in powered (not autorotative) flight. It is a term used to describe all the collective efficiency gains experiences as the rotor system accesses increasingly horizontal airflow. A helicopter in autorotative flight does not have any of the issues that a hovering helicopter does. SImply put, that air is not going down through the rotor, the air is not recirculating, it is outflying the tip-vortices.

As such, I don't suspect gyrocopters experience Translational Lift, they are always in autorotation.

D

[RickE]

Hi there;

I am new to the gyrocopter scene, a fledgling at best. I have been really fortunate to be trained by some great instructors. I have also had some great conversations, everyone seems really happy to help out the new kid on the block.

One area where I have found some interest is that of aerodynamics. That is:

• basic aerodynamics,
• rotorcraft aerodynamics, and
• gyrocopter aerodynamics.

My initial career was as an aeronautical engineer, this remains a topic of interest to me.

I am keen to know if there is interest in covering or discussing these topics in:

• an informal (forum) for this sort of topic, or
• occasional (or even regular) articles in Gyro News.

Some of the discussions that have prompted this post (and the offer to continue it) include:

• the basics, including forces (lift, drag, weight and thrust) acceleration,
• lift, and how it is produced,
• the drag (also called the power or power-required) curve, and
• differences between helicopter and gyrocopter aerodynamics, including autorotation.

Happy to clarify (expand on) these (and others) if I can, but certainly don't want to go wasting time, mine or others.

Again, thanks so far for all the encouragement and support.

cheers

D

One of the main purposes of this forum is to provide and share information amongst a like minded group of enthusiasts. As an aeronautical engineer I’m sure you have a lot to offer which will be of great interest to us all. The forum USE to have a massive knowledge base and was most likely the best resource of its type in the country. Legends of the sport such as Tim, Max, Chopper and many others have a lot to offer, and all we have to do is ask.

[SumoSurfer]

Rick;

I have access to a bunch or research papers. Happy to put them on the site so that they can be accessed by any insomniacs out there. Not sure how to do it, but certainly happy to learn.

D

[RossM]

I would offer a guess/suggestion that the rotors of a gyroplane do provide some ground effect. Yes, the rotors do not have the same wing loading as a plane, and that the air is up-moving through the rotors. However, there is a small amount of air hitting the rotors that would be bounced back. If there was not any, you would not get lift. (some rule about equal and opposite force).
When landing on a dirt runway, there is usually a pillow of dust blown up from the down wash of air from the rotors.

[SumoSurfer]

Ross;
Ross, thanks for taking the time to read the post, and to write back.

Wing Loading
I suspect that most gyroplanes have a much higher wing loading than equivalent aircraft. Why? the wing on a gyroplane is flying much faster than that on a fixed-wing aircraft flying at the same speed. As such, for the same AoA and airspeed a much smaller surface area is required. I think we experience the benefit of this in turbulence. The susceptibility of an aircraft to being buffeted in turbulence is a function of the wing loading. The higher the wing loading, the less susceptible to turbulence.

Lift
As for Newton's Third Law of motion, lift obeys this, but it doesn't explain lift. Lift is created (by an airfoil) when there is a lower pressure created on the upper surface when compared to the lower surface. It is the pressure differential that causes lift, not a downward acceleration of the air. The perfect illustration of this is wing-tip vortices. They are caused when the high pressure air on the lower surface of the wind migrates around the wing-tip to the upper surface.

Dust on landing
As for dust on landing, I can't claim to know, I just don't have enough flying experience. However, is you look at the accompanying video (https://www.tagaviation.com.au/video-library) of a gyrocopter taking off and landing on water and dirt I think it shows some things.
When landing on water, the first visible interaction between the air and the water is the thrust from the propeller. There is no visible downdraft from the rotor. I think you will observe the very same thing when the gyrocopter takes off. There is more interaction in the take-off, just because the propeller thrust is higher. The dirt landing / takeoff (to my eyes) looks similar. But, as I say, I don't have the experience to comment form first hand observation.

D

[RickE]

A couple of questions for Sumo Surfer;

I fly both a tandem and side x side gyro and on landing the side x side floats along the runway considerably further than the tandem machine. I had always put it down to the wider bodied machine creating a cushion of air or ground effect that caused this effect? If not ground effect, what do you believe would cause this?

If I was performing a vertical descent at say 5000rpm and then without adjusting power lowered the nose to gain airspeed the resultant effect would first of all be the arresting of the descent followed by lift created by an increase in airspeed. I understand that I am not transitioning from a hover to lift created by increased airspeed but I am creating lift by increased airspeed albeit in an autorotative state. What would you term this lift effect as?

I’m sure I’m not the only one that has interest in these type of topics.

[SumoSurfer]

Rick;

Thanks for the observations and the questions. I will do what I can to answer them.


Side by side versus tandem

On the side-by-side versus tandem and the different behaviour on landing. I don't suspect the fuselage is capturing a cushion of air underneath it, certainly not one sufficient to explain your observation. What we know (from your observations) is that the side-by-side machine is staying airborne longer (distance and I assume time) than the tandem. If this is happening, then the side-by-side is producing lift for longer, even as the driving force is reduced. I suspect (I am speculating here) that the side-by-side has rotors with more rotational inertia in them. This is of course assuming that weights, approach speeds and power settings are all the same.

Vertical to forward flight with the same power setting (a setting sufficient to fly straight and level)

Another interesting one.

Let's talk about going from straight and level, to descending vertically, and then back to straight and level. We will assume you don't touch the throttle.

Starting point: Straight and level with 5,000 rpm

This means that lift equals weight, and thrust equals drag. The aircraft is in equilibrium. Now, you want to transition to a vertical descent, lets assume a constant rate (say 500 fpm) vertical descent. This means you need to take the aircraft out of equilibrium (straight and level) and establish a new equilibrium (descending). You do this by moving the cyclic aft. What happens? Moving the stick aft increases the Angle of Attack (AoA), this increases the co-efficient of lift and the coefficient of (lift induced) drag. You end up on the back of the Drag Curve, the slower you go, the more (lift induced) drag you create. So, for a short period of time drag exceeds thrust, the aircraft slows to a stop (relative to the airflow). At the same time, the reduced airspeed (even with the increased AoA and therefore Coefficient of Lift) is unable to counter gravity, the aircraft starts to sink. After a while, the aircraft reestablishes equilibrium. You control the horizontal equilibrium by holding the cyclic aft. The rotor reaches a state where it controls vertical equilibrium. How? If you sink faster, the blade will speed up, this will then produce more lift - and more drag - the rotor will slow, less lift, faster sink, more upward airflow, more lift.

Mid point: Established in steady state vertical descent

Now, we want to return to straight and level flight, without changing the power setting. This part (power setting) is the key. We are on the back side of the Drag Curve, we need to generate more thrust to overcome drag, but we can't use more engine power. Once again, we need to upset the equilibrium by accelerating the aircraft to establish a new equilibrium. So, we release the aft cyclic a bit. This reduces the AoA and the (lift induced) drag, the aircraft accelerates forward. As it accelerates forward (and downward I suspect) the increased airflow over the blades makes up for the decreased AoA. We will then be in a new point of equilibrium where all the forces are in balance.

Of course, the reason that we don't keep accelerating is that as the Lift Induced Drag comes down the more we reduce AoA (push the cyclic forward), the more parasitic drag (fuselage, undercarriage, mast, etc.) takes over. The point where the dominant source of drag changes is the point of least drag, the low point in the drag curve.

So, the question, what is the effect? It is just an increase in lift with an increase in airspeed. The key is the intrinsic link between airspeed (over the blades) and lift induced drag.

[RossM]

Yes, the gyroplane rotors (airfoil) do produce lift, But do they at the speed that they rotate produce enough lift?
Image driving along in a car, and you put your hand out the window, palm level. Now slowly turn your hand so that wind pushes against the bottom of your hand. The wind will want to push your arm up.
Pause: Go for a drive and test this if you never did this as a child.

The gyro rotors are tilted back.
You cannot say that all the incoming air that is in front of the rotors is not going to hit some sort of resistance!
I did not watch the video landing on water, but is the prop at idle? (power off)

You do not need a dirt strip. Put some paper on the runway and land with power off, landing where you have put your newspaper (not a windy day that will blow away before you land)

I just watched the link,
Nice video. The landing is a bit quick when back at the home dirt strip. But if there was no roll-on while landing, you would see.

[SumoSurfer]

Ross;
Like I say, I can't comment on the dust/paper.
However, I can on the lift part. Sticking your hand into the airflow produces Flat Plat Lift. That is, lift not depending on an airfoil shape. Yes, some of the lift on a wing (rotor) is produced this way, but not nearly all of it. Importantly, the point where the airflow is deviates downward is at the trailing edge. Further, this deviation is relatively small and does not continue to the ground - unless the aircraft wing is very close to the ground.

To be clear, I am not suggesting there is no downward flow from a gyrocopter to the ground on landing. For a start, I am sure that the fuselage (like your demonstration of the hand) pushes some air down. What I am suggesting however is that the rotor is not experiencing Ground Effect. That is, there is no (appreciable) reduction in lift-induced drag caused by the interaction between the ground and (a) the wing-tip vortices, and (b) the deviated airflow (trailing edge downwash).

[Tim]

[RossM]

Hi Tim
You have not lost your artistic skill!

[SumoSurfer]

Very nice diagram

[Tim]

A rough schematic of airflow through rotors. It makes it a bit easier to see where the downwash comes from, though it is mostly rearward of the gyro. Also, the higher pressure under the rotor does create a “cushion” or ground effect if you are close to the ground.

[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

[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]

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

[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]

Ross;
Posted some pictures that might be useful.
SS

[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.

[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]

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]

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.

[SumoSurfer]

[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;

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