Vortex Ring State in gyros?

WaspAir

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Not claiming to be an expert in this, but this part sounds incomplete and missing the critical step:

I go up to say 1500 ft then ease back on the throttle and gently dive to 1000ft at say 70kts ease the stick back to a hover thus increase G loading (gyro has more weight to lift) so blades increase speed and airflow goes downwards through the disk (would hear the blads slapping the air ?)

Increasing g-load and rpm isn't enough to reverse any flow per my understanding. The interesting stuff would happen when g-load is rapidly decreased immediately thereafter while a bit of extra rpm remains. I have seen it more often described in the context of promptly rolling out of a high-g turn (reducing g-load back toward normal while still oversped) rather than as a high-g pull up from a dive.
 

thomasant

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That is true, I don't think SandL was quite clear about the method.
I too was unaware of the phenomenon till I began to do some extensive searching regarding this "interesting stuff" after my crash. The only reason I keep posting about this is to highlight to others that there are some hidden gremlins waiting to catch the unwary, and it is not a bad thing to keep an open mind in trying to understand this better.
 

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On principle of rotor system behavior about which Chuck has not been able to convince me "he is the way, the truth, and the light..." is why he feels a gyro flies better with blades that have twist that makes tips have more pitch at the tips than at the root. I know with certainty, a blade that is twisted the other way more evenly distributes the aircraft weight along the span instead of making the tips lift all the weight. When the tips lift all the weight, the blades bend upward more and since gyro RPM may get low sometimes without any way for the pilot to quickly get RPM back, the tips may bend upward beyond the safe flying line.

In his early days, he used to fly with Hughes 269 blades mounted upside down and spun backwards to get the "wash-in" I mention above. I'll bet WaspAir's FAA Certified gyros didn't have wash-in. They most likely have washout so the weight supported doesn't bend the blades. I'll also bet he can autorotate vertically with those positively twisted blades.
It's well established that the optimum twist for autogyro blades is wash-out (higher pitch at tip compared to root) as Chuck has stated and apparently put into practice with Hughes blades in years past. For helicopters with powered rotors, it's the other way around (hence the inversion Chuck had to employ with the helicopter blades on his gyro).
 

Vance

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It's well established that the optimum twist for autogyro blades is wash-out (higher pitch at tip compared to root) as Chuck has stated and apparently put into practice with Hughes blades in years past. For helicopters with powered rotors, it's the other way around (hence the inversion Chuck had to employ with the helicopter blades on his gyro).
It is my observation that most rotor blades on currently produced gyroplanes have no twist.
I have flown blades with twist and did not notice a difference.
 

WaspAir

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It's well established that the optimum twist for autogyro blades is wash-out (higher pitch at tip compared to root)
I think you are using the "wash-out" term backwards from the usual usage. It generally describes a reduction in angle of incidence at outer span stations. That"s why Bryancobb was speaking of "wash-in" for tips with higher pitch than the root, looking for a descriptive term that would suggest the twist you have in mind.

The washout term goes back to early fixed wing designs with the intention of making the root stall first to maintain aileron effectiveness, with higher incidence at the root. In rotor blades for helicopters the spanwise lift distribution is a structural concern given the much higher airspeed seen by the tips, a problem not faced with fixed wings, but a similar approach helps in both contexts.
 

Resasi

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A pertinent observation.
 

Doug Riley

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Yes, the correct term for autogyro blade twist is "wash-in." More pitch at the tip. Cierva was inspired by traditional wood-and-cloth windmills, which have wash-in, and probably HAVE had it since the Middle Ages. It's not a novel idea.

In FW planes. wash-out is intended to delay the stalling of the wingtips, where the (rather important) ailerons are located. Still, the tactic has its limits in preserving roll control until the last minute; the very act of deploying down-aileron to lift a low wing increases the AOA of the wing and can stall it. That's why we're taught to use rudder to lift a low wing when near (or at!) stall. "Step on the high wing" is my little mnemonic.

OTOH, twist in gyro blades aims to operate the whole blade, from root to tip, at closer to its optimum AOA. The root portion tends to have a higher AOA than the tip, because the rotational component of the airflow that it sees is less. With untwisted blades, we find ourselves running the blades at a pitch that puts the tips at too little AOA to lessen draggy, power-hogging stall at the root. We use twist (=wash-in) to lessen ROOT stall -- just the opposite of FW practice. The downside (there's always one, right?) is that more of the blade will finally stall at once when stalling conditions develop -- e.g. flapping on takeoff can kick in more suddenly and aggressively with washed-in blades than with untwisted blades.
ll
 

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The windmill designers of the middle ages probably didn’t know very much about formal aerodynamics but from simple observation of the “sails” could tell when the angle at various stations was optimum. For example; whether the sails flutter or balloon out.

Dutch windmills have used “autogyro” twist for at least 500 years.

I’m surprised that some people can’t grasp that the angle of relative wind along the span of a blade is reversed between propeller and windmill; ie, steeper angle at the root end than at the tip end for a windmill, the reverse of a propeller.

Here’s an article that explains the twist of wind turbine blades:

 
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XXavier

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Windmills, either traditional or modern, may show washout, but they don't work with positive pitch, as our gyro rotors do. They work –as can be expected from them– in 'windmill state', not in autorotation.
 

C. Beaty

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Pitch relative to the hub is irrelevant; angle of attack is everything.

Of course pitch relative to the hub is very different between autorotation and a windmill extracting power but optimum angle of attack is about the same for both.

The increase in efficiency of twisted autogyro blades isn’t very much compared to untwisted blades but it can be measured via towline pull or by a measurement of rotor disc angle of attack.
 
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bryancobb

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I understand the variances in relative wind along the span of a spinning, twisted blade. What I do not understand about the idea of gyro blades having POSITIVE twist from root toward tip is...
* How do you folks reconcile the obvious condition that the outer most portions of the flimsy blades makes the majority of the lift?
* Flimsy rotor blades need two things to prevent them from folding-up in flight. First, the lift produced needs to be shared over as much of the
span as possible and not concentrated at the tips. Second, rigidity of a flimsy blade, and its ability to support a lot of weight is dependent on RPM.
Why are gyro folks comfortable flying a machine whose rotor RPM varies fairly significantly, making the flimsy blade's rigidity fluctuate, while at the
same time you are comfortable with blades twisted in a way that puts all lift very close to the tips?

Yes helicopters in autorotation and gyros have inner regions that make no lift but they pull the outer regions around the circle. However, because helicopter blades have negative twist, the lift needed during autorotation is produced along a much longer portion of the span and the RPM is held constant making the flimsy blades stiffer. Gyros that have positive twist have much lower RPM than helicopters because of super-high drag by the outer portion's high anggle of attack. The lift is along a much shorter portion of the blade and concentrated nearer the tips.

This looks like to me the low RPM will allow the blades to bend upward excessively, especially since all upward force is close to the tips. Seems very precarious to me.
 

XXavier

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RRPMs vary very little in flight. I use to fly an ELA, and the counter is showing these days 320rpm. Perhaps in the summer 330, but during flight, it stays practically constant...
 

WaspAir

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I'll talk about the J-2 because it is directly comparable to the 269 series helicopters for rotor construction. Flight rpm for my J-2 untwisted rotor at my typical light weights was 425, plus or minus no more than 5 percent with flight loads. That is not horribly slower than the helicopter counterpart with washout, and not widely varying, and was appropriate for the relatively lower gross weight (about 25 percent less than the 300C variant). Coning was pretty steady and rigidity not fluctuating.

Lift is produced in the driving regions as well as the driven (only stalled regions aren't making lift). Most of the disc contributes to lift in normal autorotative flight.

I would bet that most of spanwise lift distributions diagrams you have seen for helicopters are for powered flight and not for autorotation with flow from below the disc. If I can quickly find a suitable comparison of washout and washin lift distributions in auto, I'll post it. You might be overestimating the AOA at the tips.
 
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C. Beaty

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Here are calculated values of inflow for a Cierva C-30 rotor with straight blades and 3 different values of profile drag in vertical descent. With zero profile drag (impossible), the rotor tips would be operating at negative lift. Notice that the inner 1/3 of the rotor is stalled

Drawings from Principles of “Helicopter Engineering” by Jacob Shapiro but calculations were done by Prof. JAJ Bennett, Cierva chief engineer following Cierva’s death. Prof. Bennett was also chief engineer for the Fairey Rotordyne.
autorotation 1.png
 
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C. Beaty

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Optimum twist for vertical descent as depicted in the drawing above, is much too great for forward flight. Optimum twist for forward flight is only ~6 degrees or so; tips twisted up relative to root ends.

I spent many hours flying Hughes 269 and OH-6 blades flipped inverted and running backwards but the built in twist of 8 degrees was a bit too much. Still, better than running right side up with wrong way twist.
 

WaspAir

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I couldn't find the image I had in mind, but I did stumble across this from R.W. Prouty's Helicopter Aerodynamics, p. 94 (1985):

High twist that is good for hovering out of ground effect (OGE) will usually be too high for optimum hovering in ground effect (IGE) where the inflow distribution to the rotor is different. In addition, twist that reduces the chances of retreating blade stall in forward powered flight hurts the rotor performance in autorotation. (The optimum twist for an autogyro is with the pitch higher at the tip than at the root.)
 

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Each blade section of a rotary wing in forward flight sees its angle of attack vary greatly during a full revolution. By example from 2.5 to 9 degrees at 0.7R . To suppose that a very clear optimum exists in the midst of these variations doesn't make a lot of sense. So, flat or twisted, the difference of efficient is very low (*). What matters is the average Cl given to the blades by the pitch setting

(*) According "Fluid dynamic drag" of S. Hoerner, a twisted fixed wing has an additional drag coef = 4.10^-5 per square degree of twist between the middle and the tip.
So, if there are 3 degrees between the middle of a blade and the tip it gives 36.10^-5. So, when average Cd is 0.012, it's only 3% difference on the profile power.
 
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bryancobb

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Wasp's statement "I would bet that most of spanwise lift distributions diagrams you have seen for helicopters are for powered flight and not for autorotation with flow from below the disc" probably identifies the disconnect in my thinking. Seeing a diagram like this at 30, 40, 50, 60...100% would of a gyro at 70 kts would probably remind me of something I am not considering.
1583405153748.png1583405250205.png
 
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Hi All correct me if I am wrong. I also thought no way untill I stopped and really thought about it. to create lift there must be a downward flow of air. without a downward flow of air there can be no lift. in autorotation there is a downward flow of air creating lift otherwise we would just drop out of the air. Gyrocopters may not be as susceptable to VRS but I bielieve it is still possible. the upwards flow of air through the rotors drive the rotors to create lift. to create lift you need a downward flow of air.

Doug

Just my uneducated opinion
 

Jean Claude

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This simple sketch shows how the air flow through the rotor comes from below the disc to ensure the rotation, however it is deflected downward to ensure the lift
Sans titre.png
 
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