High Speed Gyro Flight-Chuck or Others

Quick 8

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Hey guys,
I have a question (and maybe an answer). Can someone confirm my interpretation? :

Assuming a gyro has more than enough power to reach a VERY high forward speed, what would happen to the rotor behavior?

I am GUESSING, from reading Gessow & Meyers and NACA 716 by Bailey, that one of two things would happen first. Either:

1. You would reach your speed limit when you experience too much retreating blade stall and the rotor flaps against its teeter stops.

-OR-

2. You would reach your speed limit when your forward speed added to your advancing tip speed starts to approach around .75 times the speed of sound and compressibility effects start to make rotor drag skyrocket.

Which one of these would happen first depends on rotor diameter and blade loading. Higher blade loading makes a faster turning rotor and you would reach limiting condition #2 first. A lower blade loading makes the rotor turn slower and you would reach limiting condition #1 first.

ASSUMING ALL OF THE ABOVE IS CORRECT, what would happen in the real world if you reached either one of these limiting factors? Whould you just reach a point where you could not go any faster? Or, is there somthing that would happen that would make you crash and burn?

Any opinions are welcome.

Thanks,
~JIM
 

C. Beaty

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A good many years ago, Jim, I did a what if investigation. Not at high speed; quite low speed in fact: ~20 mph.

I wanted to find out how pitch could be cranked into Bensen metal blades and still fly.

We eventually cranked them up to the limit of adjustment range and they could still be started and flown. I don’t recall whether I had a rotor tach or not but if I did, it would have a kludged up affair made from a permanent magnet motor and a meter.

The gyro simply would not fly faster than ~20 mph with the stick hard against the forward stop; more power, and it climbed, still at 20 mph.

So much of the retreating blade was stalled that it acted like the friction governor of a wind-up phonograph. The flapping angle would have put the blades near the flapping stops but I never touched them.

I’ve never investigated the upper speed limit with rotor blades at normal incidence setting but expect the same thing would occur. It simply won’t go any faster after the stick reaches the forward stop and additional power causes it to climb.

Helicopter rotors behave quite differently at high speed. Gyro rotors stall at the inboard ends and the stall gradually moves outward as speed increases. Helicopter rotors stall at the retreating tips and stuff changes in a hurry.
 

Quick 8

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Chuck: I would assume that if you reached condition #2 first, and compressibility started to come into play, that regardless of engine hp, (unless you had a truly rediculous amount of hp), that the added drag on the rotor would quickly stop you from going any faster.

But if you hit condition #1 first and had excessive retreating blade stall, do you see any reason why the flapping angle would be more exaggerated than in your slow speed test?

Resasi: I'm sure he really knows his stuff, and may well know the answers to some of the issues of high speed flight in a pure (non-winged) gyro. However, the winged and non-wing gyros are VERY different. At high speed the the wings tend to do most of the lifting and take a lot of the load off the rotor. This allows it to slow down so you don't have to deal with the compressibility issues of a high speed rotor or the retreating blade stall issues of a rotor that is trying to carry the load of the entire gyro.
 

ferranrosello

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Jim, you are right about your assumptions about rotors and high forward speed. But there is missing a very important thing: the blades collective angle. The bigger the collective angle the lower the rotor rpm. So in the event of big collective angles the rotor speed can be dangerously low, and the forward speed at which retreating blade stall can be encountered is lower.

Chuck, I appreciate your comment about this experiment. But I don’t think that you encountered retreating blade stall. The way in which this phenomenon occurs is the same in gyros and helicopters. The only difference is that helicopters are working with much bigger collective angles and therefore much bigger AOA’s than gyros. So helo’s are much more prone to retreating stall than gyro’s, but the consequences are the same.

Another question is that teetering rotors have a better behaviour in case of retreating blade stall than other kind of rotor systems.

The explanation of why the rotor head of your experiment was not able to fly faster than 20 mph I think is more related to twisting effects due to a positive pitching moment when working at big AOA’s than other. Additionally I know (by experience) that when an autorotating rotor works with bigger AOA’s (which means that it works with lower rpm) it will need a shallower rotor disc attitude and the pilot will have to set a more forward stick position.

About the maximum speed that can be reached by a gyro… I can’t say. I believe that the potential is much higher than our current speeds and that average helicopter speeds. But this is just a feeling, by now I don’t know. What I’m sure is that in our gyros we will reach the stop forward stick position prior to reach any speed approaching retreating blade stall or transonic air speed in the advancing blade. This is because in order to set a higher speed you need to move the stick forward and more forward still in order to compensate flap back effect.

Ferran
 

C. Beaty

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Ferran, the NACA mounted a movie camera on the rotorhead of a Kellett KD-1 Autogiro (military YG-1B) and measured the regions of retreating blade stall by observations of yarn tufts attached to the upper surface of a rotorblade.

Each circle on the plot equals 10% of rotor radius. Forward speed is given as a fraction of rotor tip speed. At a forward speed equal to 35% of tip speed, nearly 70% of the retreating blade is stalled.

The plots, ranging from mu = 0.15 to 0.35, would represent airspeeds of ~42 mph through ~100 mph. The rotor was 40 ft. in diameter and turned a little over 200 rpm.
 

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ferranrosello

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That’s right, Chuck. All rotors have a reverse flow region in the retreating blade which increases with air speed and with lower rotor rpm.

When we start to increase forward speed is like a race between the advancing blade and the retreating one. The higher forward speed the shorter length of retreating blade which can produce useful lift, and the bigger the AOA at which these blade sections are working. In some point the working section of the retreating blade is so short than it would require too big AOA’s and the blade stalls.

Now some interesting things:

1.- What is stalled in a retreating blade stall is the blade section close to the tip. The remaining blade section is working in reverse flow and it is in autorrotative mode.
2.- To reach a real retreating blade stall situation it is necessary that the retreating blade state has spread around an azimuth sector close to the retreating blade position, not just only this position.
3.- A teetering rotor cannot develop a fully retreating blade stall because of the rigid attachment between the two blades prevents from it. What it develops is a different thing which we call bad flapping.

I think what you are talking about it is µ (relation between forward speed and tip speed), no retreating blade stall. Nowadays helicopters are working at µ about 50 %, and is accepted that the maximum acceptable to avoid retreating blade stall is 75 to 80%. In gyros the µ can be greater because its lower collective pitch. Besides it, the increasing rpm with airspeed is always maintaining the µ in a low value in our gyros.

Regards. Ferran
 

Rotor Rooter

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Jim

The following is in reply to your question.
"Assuming a gyro has more than enough power to reach a VERY high forward speed, what would happen to the rotor behavior?"
The Sikorsky ~ S-69 (XH-59) ABC was designed to operate at very high forward speeds. In addition, it does not have wings and it operates as a gyrocopter during cruise since the rotors are in autorotation.

It requires two rotors to provide lateral symmetry of lift since the retreating blades are significantly unloaded and the reverse airflow regions on the retreating side of the rotors are detrimental to lift.

The continuing portion of the Sikorsky X2 thread on PPRuNe may answer your question, or alternatively create more questions. :)


Dave
 

ferranrosello

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Thank you very much for this paper, Chuck. By now I've only read the summary, and I'm going to study it deeply. But what the paper is talking about in the summary is not about retreating blade stall. It is talking about stalled regions in the retreating blade sector and the way in which this stalled region increases with airspeed.

There is a big difference between stalled region in the rotor disc and retreating blade stall. From 1939 to now the knowledge and understanding of rotors has developed very much, and nowadays we know more about these questions.

Firstly, the rotor blade stalled regions on the graphs are not accurate. There has to be a stalled region in the advancing blade sector too.

Secondly in the 30’s they had a problem with autogiros flying above 100 mph. Several autogiros were lost because of unexplained dives when flying at high speeds. So experiments were conducted by NACA to determine the cause of this problem. They calculated the stalled region and invented a way to measure it in order to compare the experiment results with theory. And the two things didn’t match.

What was discovered is that the blades were twisted because of aerodynamic forces which were increasing with airspeed. This blade twist was responsible of the increased stalled region in the rotor disc and a negative pitching moment in the advancing blade sector. So we cannot conclude any thing of this experiment because of the inherent fail in the blades design.

Anyway, the question is that this is not retreating blade stall.

Retreating blade stall occurs when the µ (relation between forward speed and tip speed) is so high that the remaining blade section not working in the stalled area doesn’t exist any more… In other words, the antirrotative area (the lifting area) of the retreating blade disappears. And as you can see in “Even more helicopter aerodynamics”, later experiments have demonstrated that any well designed and manufactured rotor is able to fly at airspeeds higher than the calculated in theory without encounter retreating blade stall. Blade stall occurs in reality at higher speeds that it should be if our rotorcraft theory was correct.

I’ll tell something more after reading the entire paper.

Ferran
 

Rotor Rooter

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Chuck;
The second paragraph in your posting #5 needs the following small correction; 'Each circle on the plot equals 5% of rotor radius. Forward speed is given as a fraction of rotor tip speed. At a forward speed equal to 35% of tip speed, nearly 35% of the retreating blade is stalled.'

For those who want some elaboration; Tip Speed Ratio
__________________

IMHO the subjects of the 'Stall Region' and that of the 'Reverse Velocity Region' is a discussion of two separate regions. Another region of relevance is that of Negative Thrust'

Fig_2-86.gif


The above sketch is from; Paul Cantrell's Aerodynamics of Autorotation. It shows the 'stall' and the 'reverse velocity' as two distinct regions.

I would guesstimate that regions are as follows;
  • Driven region (providing thrust) ~ Angle of attack = 0 to +9 degrees
  • Driving region ~ Angle of attack = +9 to +19 degrees
  • Stall region ~ Angle of attack = +19 to +90 degrees
  • Reverse velocity region ~ Angle of attack = +90 to +180 degrees
  • Negative thrust region (located at tip of advancing blade) ~ Angle of attack = -1 to -2 degrees

The 'reverse velocity region' is contributing slightly to the rotation of the rotor, however it is also creating a negative thrust.

The 'Negative thrust region' on the advancing side is an unwanted feature. It comes from extreme forward cyclic during fast forward flight and it is necessary to give lateral symmetry of lift.

Dave
 
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C. Beaty

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Here is Ray Prouty’s calculation of angle of attack contours for an autorotating rotor in “Practical Helicopter Aerodynamics” at μ = 0.3.

Observe that the 14º angle of attack region of the retreating blade extends to 65% of radius. Most plain airfoils begin to stall at 14º or less, depending on Reynolds number.

Mr. Prouty’s calculations are in substantial agreement with the measurements reported in NACA #741.

The reverse flow region is indicated by the solid inner circle.
 

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Rotor Rooter

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Chuck

I was wrong. You are correct.

The second paragraph in your posting #5 is referring to the region of stall, not that of reverse velocity. :sorry:


Dave
 

Mechteacher

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To all of you,
With your years of knowlege to my limited information of gyros.
I'll ask the thing that comes to my mind. As the forward airspeed increases we also have a decrease in the relative wind to the rotor disc. Then at what point would we lose airflow up in our disc to turn the rotor?
If I am out of order, please tell me. I like to learn.
Bob
 

C. Beaty

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You’ll never lose airflow up through the rotor disc as long as it’s supporting a load.

The problem is stall of the retreating blade. But the stall of an autorotating rotor is gradual, beginning inboard and gradually spreading outward toward the tip as airspeed increases. As this happens, the stick moves forward as the result both of the flattening of the rotor disc angle of attack and the increase of cyclic flapping angle required to equalize lift between advancing and retreating sides of the rotor.

I believe the stick will be against the forward stop and the machine will refuse to go any faster well before there’s danger of anything catastrophic happening.

Some of the old Autogiros of the 1930s were flown as fast as 50% of rotor peripheral speed without difficulty.
 
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WaspAir

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issues at 35%

issues at 35%

I've found that when flown solo at minimum pilot weight, an A&S 18A will operate at the bottom of the acceptable rotor rpm range (say, 210 rpm, with the green arc from 200-320), and that level flight at full sea-level power is simply more thrust than it can handle at that rpm. The aircraft becomes very, very rough and hard to control, and either slowing down while maintaining power (producing a climb) or reducing power (slowing in level flight) will cure it immediately. Using the Farrington STC electric rotor collective pitch trim, you can bump up the rpm to around 240, and that permits smooth controllable flight of 10-15 mph faster than can be reached with the lower rotor rotation rate. It seems to show the hallmarks of a mu / retreating blade issue.
 

All_In

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What a site!!!
More valuable info I didn't know and thanks for that link you are the best!!!
 

Quick 8

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Chuck,

I agree 100% with everything you've said.

Just for fun, I looked up the specifications for the Kellet YG-1B (KD-1A) used in the NACA 741 test of "STALLED FLOW on the blades of an autogiro rotor".
I got most of the specs from "Autogiro: The Story of the Windmill Plane" by George Townson.

With a little bit of sifting and cross referencing, you can determine that at the speed of mu=.35, the Cl for the disc area is about Cl=.063, and the solidity for the rotor was sigma=.0477.

Using this info, and plugging it into the graph for rotor profile drag developed in NACA 716 by Bailey, and later shown in Gessow and Myers (page 224; fig. 9-2), you find that their operating point is deep into the area of flow, where the average angle of attack of the of the average blade element is about 15 degrees.

So, its no wonder that such a large portion of the rotor was stalled. If they would have de-pitched their rotor to 3.5 - 4 degrees they would have had a lot less stall. (Their rotor was pitched to about 4.5 degrees.) With a rotor with less pitch, and less stalled area, they should have been able to fly much faster.

One of the nice things about this test though, is that it agrees fairly well with the rotor perfomance graph listed in the NACA 716 report. There is some variation, but again, the gyro was operating in an area of the curve that Bailey himself said that increasing errors would creep into his calculations. One would expect the correlation to be even closer when the rotor is operated above the 12 degree angle of attack line.
 

C. Beaty

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Autogiro development, at its peak, attracted a nucleus of extremely competent engineers who derived the core principles of rotorcraft flight, Jim. Gyroplane development was centered around Philadelphia and the NACA at Langley so it is not surprising that early helicopter development was an East Coast phenomenon.

If you’ll look through some of the wind tunnel testing of rotors at Langley, you’ll find that best L/D always occurs near μ = 0.35, the best tradeoff between retreating blade stall and high advancing tip speeds. When the pioneers set solidity, which controls tip speed, to give a value of μ = ~0.35 at the upper cruising speed, they weren’t simply whistling in the dark.

Tip speed will be ~ 66*(blade loading)^0.5 with optimum pitch adjustment. Most Autogiros ran a blade loading of ~35 lb/ft² which gives a tip speed of ~390 FPS.
 

All_In

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Chuck I marvel at your knowledge!
Thank you so much for sharing your wisdom.
I really hope I get to work on any project you are involved in!
I love this site!!!
 
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