A reduction in lift.

[Vance]

In an accident thread on the Rotary Wing Forum a CFI asks Antony to explain what Doug, David and Antony wrote about a situation where lift is reduced by descending into turbulent rotor downwash with an over speeded rotor

I am going to make an effort to clarify my opinion on this phenomenon staying inside what I feel is accepted rotor aerodynamics.

Here is a quick review of the most basic gyroplane rotor theory from the FAA publication; The Rotorcraft Flying Handbook.

“ROTOR DISC REGIONS As with any airfoil, the lift that is created by rotor blades is perpendicular to the relative wind. Because the relative wind on rotor blades in autorotation shifts from a high angle of attack inboard to a lower angle of attack outboard, the lift generated has a higher forward component closer to the hub and a higher vertical component toward the blade tips. This creates distinct regions of the rotor disc that create the forces necessary for flight in autorotation. The autorotative region, or driving region, creates a total aerodynamic force with a forward component that exceeds all rearward drag forces and keeps the blades spinning. The propeller region, or driven region, generates a total aerodynamic force with a higher vertical component that allows the gyroplane to remain aloft. Near the center of the rotor disc is a stall region where the rotational component of the relative wind is so low that the resulting angle of attack is beyond the stall limit of the airfoil. The stall region creates drag against the direction of rotation that must be overcome by the forward acting forces generated by the driving region.”

I feel it is fairly well accepted that in order for a gyroplane to fly it has to accelerate air downward.

In my opinion with an over speeded rotor the driving region shrinks and the driven region grows as energy of the over speeded rotor is dissipated.

Most people I know have experience the extra float they get with an aggressive flare on landing. It does not take long for the rotor to return to normal flight rpm. The heavier the rotor and the faster it is turning the more energy is available.

In my opinion an aggressive turn as part of a steep descent near the ground could create a situation where some of the rotor is descending into turbulent rotor down wash.

Under certain conditions I feel this could cause a short term loss of lift.

It appears to me Antony feels this played a part in his gyroplane landing mishap.

 

[Gyro28866]

A maneuver that used to be quite common at a flyin.
The gyro's and Pilot will approach the point where he wants to go vertical at a higher rate of indicated airspeed. When he gets to the position he wants, he quickly reduces throttle to idle, and pulls back on the stick. The gyro will climb/ascend vertically 50' to 100'.
Just like a gyro performing a jump takeoff.
So, ask yourself how?
The rotors potential energy is stored in the rotational mass of the rotor. I will give an example with real numbers on my gyro. At 85 mph and 720 pounds AUW, my rotor is at approx. 325 rpm. At 10 mph IAS my rotor is at 295 rpm. That is 30 rpm difference. Both rpm's are producing 720 pounds of lift, at a different rpm and angle of incidence. So, if you approach at 85 mph and chop the throttle and pull back on the stick, The rotors rpm at the increased angle of incidence is higher than what is needed and the lift increases. The higher total lift is converted into altitude as the rotors rpm slows. Just like a gyro performing a jump takeoff. During the vertical climbing portion of this maneuver I don't think there is any upward flow of air through the disk, only downward.

 

[WaspAir]

I think I know what you intended but I am uncomfortable with over-reliance on the jump take-off analogy for a couple of reasons. First, there is essentially no airflow at all through the disc in any direction just before the jump begins (all rotor blades in flat pitch). The jump is initiated by a rapid and large change in collective pitch to establish downward flow (albeit temporarily). I think it pushes the analogy too far to say that it is just like a cyclic-induced g-loading event begun while the rotor is in already in autorotation and supporting the aircraft weight in forward flight. (There is, of course, a flow reversal at the top of the jump, to establish autorotation, but that's a reversed reversal from what is under discussion.)

I understood the discussion on the other thread to be questioning whether any flow reversal actually happens. Presumably, autorotative flow is what leads to the increased rpm as energy from airspeed is converted to faster rotation. Does that increased rpm somehow cause the flow to change direction?

 

[Vance]

A maneuver that used to be quite common at a flyin.
The gyro's and Pilot will approach the point where he wants to go vertical at a higher rate of indicated airspeed. When he gets to the position he wants, he quickly reduces throttle to idle, and pulls back on the stick. The gyro will climb/ascend vertically 50' to 100'.
Just like a gyro performing a jump takeoff.
So, ask yourself how?
The rotors potential energy is stored in the rotational mass of the rotor. I will give an example with real numbers on my gyro. At 85 mph and 720 pounds AUW, my rotor is at approx. 325 rpm. At 10 mph IAS my rotor is at 295 rpm. That is 30 rpm difference. Both rpm's are producing 720 pounds of lift, at a different rpm and angle of incidence. So, if you approach at 85 mph and chop the throttle and pull back on the stick, The rotors rpm at the increased angle of incidence is higher than what is needed and the lift increases. The higher total lift is converted into altitude as the rotors rpm slows. Just like a gyro performing a jump takeoff. During the vertical climbing portion of this maneuver I don't think there is any upward flow of air through the disk, only downward.

In my opinion some of the flow in a gyroplane rotor is always downward and with the rotor above flight rpm there is more.

In other words the driving region shrinks and the driven region grows.

In The Predator I have seen rotor rpm over 460 on a day the 60kt straight and level rotor rpm is 330.

It is my observation I can use some of this stored energy to slow my descent.

Upward flow does not have to be eliminated to have this effect.

In my opinion under these rotor conditions and specific wind conditions in a steep descent it is possible to have the lift disrupted by the turbulence from the rotor down wash making recovery to normal autorotation more difficult.

I have not personally experienced this disruption.

 

[Vance]

I think I know what you intended but I am uncomfortable with over-reliance on the jump take-off analogy for a couple of reasons. First, there is essentially no airflow at all through the disc in any direction just before the jump begins (all rotor blades in flat pitch). The jump is initiated by a rapid and large change in collective pitch to establish downward flow (albeit temporarily). I think it pushes the analogy too far to say that it is just like a cyclic-induced g-loading event begun while the rotor is in already in autorotation and supporting the aircraft weight in forward flight. (There is, of course, a flow reversal at the top of the jump, to establish autorotation, but that's a reversed reversal from what is under discussion.)

I understood the discussion on the other thread to be questioning whether any flow reversal actually happens. Presumably, autorotative flow is what leads to the increased rpm as energy from airspeed is converted to faster rotation. Does that increased rpm somehow cause the flow to change direction?

I am imagining that the driving region shrinks and the driven region grows as the energy in the over sped rotor is used.

Because the flow of the driven region is already downward I don’t like to describe as reversal.

I avoided the jump takeoff comparison above because I felt the pitch change confused things.

It was originally mentioned to help people understand that energy can be extracted from an over sped rotor.

Doug Riley wrote: “A gyro can work like a helicopter for short periods of time. If RRPM is increased above cruising level, the stored energy in the rotor substitutes temporarily for an engine drive. This means that the gyro can stand still in the air, drawing air down through its rotor in a way that is inconsistent with autorotation.

But this can't last long. As RRPM decays, the gyro will settle. Specifically, it will settle into its own downwash. That is, the rotor settles into (1) disturbed air that (2) is already travelling downward.

Just like a helicopter in a similar situation, the gyro will descend vertically very fast and can get buffeted on the way down by the turbulent air of its own downwash.

If you have altitude, you'll recover from this predicament by nosing down and flying out of it.

If you are at, say, 20 feet, though, things will not go well for you. You will pancake in at greater than normal vertical-descent speed. Splat.”

Antony was making a steep turn near the ground with a steep descent to land engine at idle. He has done this hundreds of times and had already practiced it several times that day.

Something went wrong and he landed much harder than would normally be expected from a poorly executed landing.

There were variable gusting winds.

My original thought was his head wind turned into a tail wind.

After seeing the damage and much discussion my explanation seemed inadequate to me.

 

[WaspAir]

I think it is conceptually helpful to distinguish between (1) a high-g pull-up (that most any heavier than air flying machine can do), in which forward speed is lost, altitude may be gained, and in which one can appear to hesitate at the top before descending again, and (2) some sort of vertical levitation with a flat disc and without forward speed, or perhaps even slightly backward flight with the rotor disc tilted back (consistent with a high-g pull) in which excess rotor rpm is causing the disc to screw itself upward through the air like a helicopter. (1) definitely happens along with a spike in rotor rpm; as to (2), I'm not yet convinced that's really what one experiences. Too bad we can't see the air.

There will always be downward flowing air (thank you, Mr. Newton) when one is airborne, but the issue for me is does it come from "below"the front of a backward tilted disc, or from above it.

Either way, with an unpowered rotor you have to sink eventually, and that's where the problem arises.

 

[C. Beaty]

What most likely confuses the issue is gyroplane rotor blades with incorrectly located CG.

Tail heavy blades, those with CG to the rear of the aerodynamic center, twist nose up with increased load and produce a momentary lift increase, followed by a drop as the rotor rpm decreases. This behavior is frequently misinterpreted as “high inertia.”

Torsionally flexible blades are are affected more strongly than are torsionally stiff blades.

 

[C. Beaty]

What started this thread appears to have been a misapplication of vortex ring state; https://en.wikipedia.org/wiki/Vortex_ring_state .

Autorotating rotors can’t generate that condition.

 

[Jean Claude]

Tail heavy blades, those with CG to the rear of the aerodynamic center, twist nose up with increased load and produce a momentary lift increase, followed by a drop as the rotor rpm decreases. This behavior is frequently misinterpreted as “high inertia.”

Chuck,
The slowdown during a usual flare is quite low: From 75 mph to 25 mph in 7 seconds(*) , that's -3 m/s2
Since this deceleration combines perpendicular to gravity, that gives only 1.05 G on the rotor. How this could significantly increase the blade twist or the rpm to benefit from inertia?
(*) Based on unreleased recordings by Mike G

 

[C. Beaty]

The angle of attack instability resulting from tail heaviness can become cumulative and feed on itself.

For example, Skywheels rotors with CG at ~35% of chord frequently execute a midair flare when installed on a heavy gyro. This behavior is often triggered by a gust, causing the gyro to abruptly pitch noseup.

Skywheels rotors installed on light gyros are notable for extending the “float” during a normal landing. This is often misinterpreted as “high inertia” but as you know, a high inertia rotor has just the opposite behavior.

 

[Jean Claude]

Is not such rotor permanently unstable?

 

[C. Beaty]

Yes, of course it was permanently unstable, JC but it was very popular at one time because users misinterpreted its behavior.

On a light gyro, the noseup pitching with an upward gust was fairly mild and this was believed to be an indication of “high lift.”

It also produced a very light control feel similar to self energizing drum brakes (leading shoe) in an age before disc brakes with vacuum servos.

 

[Gyro28866]

Back in the mid 90's while flying with Ed Alderfer in his Tandem Air Command with ( I think ) 28' McCutchen Skywheels. We were doing pattern work at Shelbyville Illinios, during the fall flyin. At about mid field on the downwind leg. The Gyro pitched extremely high nose up attitude, probably to about 60-80 degrees up attitude. I instantly chopped the throttle and rolled into a hard right turn to try and keep the rotor loaded. I rolled out 90 degrees heading change and back in a level attitude. I added power and finished with a 270 turn and reentered the pattern a bit farter out from the runway. I was thinking we hit a strong thermal and it pitched nose up from that. Upon landing, we stopped for a looooong lunch.
Seems like the old long heavy loaded Skywheels had this issue. When it happens, it will get your attention!!!

 

[C. Beaty]

Misconceptions are ever present about rotorblades.

In more recent times, the heavy stick resulting from the nose heaviness of Magni rotorblades was blamed on control pivot friction; the factory even issued a service bulletin with the title in English “fictionalizing the controls”.

I didn’t pay much attention to the Magni rotor blade claims until Australian David Bird (Birdy) flew a Magni and called it a “hard mouthed ying yang” which I think is Australian for a hard to handle horse. When informed by a Magni rep the heavy stick was caused by control pivot friction, Birdy said “where does the friction go when it’s sitting on the ground with the rotor stopped?”

At that point I suggested the heavy stick was most likely caused by nose heaviness which started a long argument until Averso published a cross section cutaway of a Magni rotor showing the tapered spar.

 

[anthom]

Skywheels rotors installed on light gyros are notable for extending the “float” during a normal landing. This is often misinterpreted as “high inertia” but as you know, a high inertia rotor has just the opposite behavior.

Can you please explain in a little more detail why that happens. I mean the opposite behavior. Thanks.

 

[C. Beaty]

Can you please explain in a little more detail why that happens. I mean the opposite behavior. Thanks.

With a fixed pitch rotor, increased lift during the landing flare or any other pullup can only be generated by an increase of rotor rpm.

A high inertia rotor accelerates more slowly during the flare so will drop the gyro unless the flare is more extended to allow extra time.

Back during the Vietnamese war, I bought a truckload of runout Hughes 269 (military TH-55) and OH-6 rotorblades from the Army helicopter training center at Ft. Rucker for scrap prices and resold most of them to Sunstate members for my cost. They flew great but had to be flipped over and run in reverse because the helicopter twist was detrimental in autorotation.

Anyhow, the Hughes blades had 5 lb tip weights, equivalent to 15 lbs spread over the length of a blade for increased inertia. And drop you they would with a normal landing flare.

Most people sawed off the outer 6” of the blade tips and resealed which removed the brass tip weights.

 

[anthom]

So in a situation where heavy rotors (Averso Stella Rotors) increase in RPM in an aggressive flare, what causes the sudden and rapid drop of the gyro as Doug Riley describes? He mentions the disturbed airflow and "Just like a helicopter in a similar situation, the gyro will descend vertically very fast and can get buffeted on the way down by the turbulent air of its own downwash". I agree that it is not a vortex ring like in a helicopter where the rotors are constantly powered. But is it possible that there is considerable loss of lift which in effect simply makes the weight of the gyro slam it downwards with practically no rotor thrust.

 

[C. Beaty]

Sorry, I don’t know enough about Stella Averso rotors to offer constructive comment.

Do they drop you where other rotors don’t?

Hughes blades with tip weights could be rolled into a tight turn immediately before landing and play helicopter with the stored energy for a few of seconds. They could be brought to a stationary hover a couple of feet above the surface and slowly descend as rotor speed decayed.

 

[anthom]

That is very interesting. If I recall, Birdy describes it similarly. But then he mentions that there is a wobble after a brief hover, when the controls become unresponsive, and that's the time he powers out of it, as there is no other recovery action.

Now I'm getting a better idea, in that blade design has a lot to do with this phenomenon. Thanks a lot Chuck.

 

[C. Beaty]

Magni used to say that “harmony” accounted for the stability of their gyros.

I was going through some old DVDs recently and came across a bit of the science behind that harmony.

The photo is a cross section of a Magni rotor showing both root and tip ends. The tapered spar makes the rotor very nose heavy near the tip.

An upward gust for example, appears as an impulse since at 300 rpm rotor speed the blades spend 0.1 second on each side of the disc, advancing Vs retreating which twists each blade nose down since the center of lift is at 26% of chord for the NACA 8H12 airfoil, well aft of the CG.

However, since the airspeed is greater on the advancing side, the advancing blade twists nosedown more than the retreating side, the net effect being a built in nosedown cyclic pitch input. This keeps the gyro headed into the relative wind, just the opposite of the tail heavy blades discussed earlier on this thread.

This noseheavy behavior also accounts for the heavy stick on a Magni since cyclic input is also resisted for the same reason; the rotor lags farther behind a cyclic input than a rotor with CG on the aerodynamic center. Friction of cyclic pitch linkage pivots has nothing to do with it.