When is a rotor too large for the ship?

Brian: Sounds like you want to know what the "doomsday" situation looks like. What happens if your RRPM goes too low in flight?

Well, I'm still here, so the simple answer is I don't know. We can predict with some confidence, based on what many, many observers have seen in the seconds before a crash, as well as what the aggressive yank-and-bank people, and the curious experimenters, report.

First of all, we've all experienced divergent retreating-blade stall on the ground. You pile on too much airspeed before RRPM is up (some call it "crowding" the rotor) and so much of the retreating blade stalls that the rotor rocks violently around the teeter hinge and hits the teeter stops and/or parts of the aircraft. We call this "flapping" but specifically it's DIVERGENT flapping (as opposed to the normal kind, which happens all the time in flight). On typical light gyros, on-the-ground divergent flapping can happen at up to 200 RPM or more, if you really crowd the rotor hard.

Why "divergent"? Consider the familiar normal operation of the teeter hinge in forward flight. The retreating blade descends below the plane that's perpendicular to the rotor spindle. This descent means that the blade "sees" the air approaching it more from beneath than it otherwise would -- IOW, the blade sees a larger angle of attack (AOA). This larger AOA causes the blade to make more lift than it would if it stayed in a higher orbit. Voila! The retreating blade's relatively low airspeed is compensated for by higher AOA, so it makes the SAME lift as the fast advancing blade.

All this happens automatically. The retreating blade sails down until its AOA is high enough to balance out the lift across the disk. Trouble is, airfoils stall when their AOA gets a too high (around 12 deg.). Our mindless automatic mechanism will keep dropping the retreating blade lower and lower as long as lift isn't balanced between the two blades. In low-RPM cases. this mechanism will drop the blade so fast that, before the retreating blade can make enough lift to balance, large portions of the retreating blade will reach their stalling AOA -- and stall. That's divergent flapping.

At a steady one G, the gyro either will take off and fly (with the teeter mechanism working normally) or it'll never get off the ground. There's bound to be a rotor size and pitch that will just flap endlessly if you try to take off with it. I've never heard of that happening -- you're talking a truly humungous rotor. Chuck Beaty got close by cranking some 6 deg. of pitch into a Hughes rotor one time. His RRPM was in the low 200's.

But a more realistic possibility is that your big rotor will take off and fly -- but, if a monetary low-G** maneuver or turbulence drops the RRPM by X%, the rotor will go into divergent flapping once the load is reapplied.

I don't have a mathematical model to offer you to determine X. It will vary with airspeed, blade pitch, airfoil section, chord and rotor diameter.

It would be ideal if all rotors were tested on a motor vehicle, to discover the "point of no return" RRPM at various airspeeds and angles of attack. Short of that, we're left with a random bunch of data points (aka hangar- flying tales). From these, we know that

(1) a rotor can lose enough RPM in flight that the teeter mechanism stalls the retreating blade down into the tail or prop this seems to have happened in some PPO/porpoising accidents);
(2) divergent flapping is more likely to occur at higher airspeeds; it won't happen at all in a vertical descent.
(3) normal* rotors on Bensen-style gyros at Bensen-style speeds seem well able to tolerate a drop of 10+% of normal RRPM without any issues -- below that, when you re-load the rotor, you will likely to experience stick hammering at the very least, and maybe much worse.

A corollary to #2 is that a low-powered featherweight gyro can get away with a very lightly-loaded rotor, with less chance of divergent flapping than a more powerful gyro of the same weight that can fly faster.
__________________________
* By "normal," I mean disk loading at 1.2 or more, mu ratios at or below 0.2 - 0.25.

** The use of "G" in describing a rotor's operating conditions can be confusing. Of course, gravity always exerts a force of one G on the gyro. However, if we suddenly reduce the disk angle of attack of the rotor, the rotor will quit making an opposing lift force. The gyro will accelerate downward, just as the Vomit Comet does. From the rotor;'s viewpoint, "zero G really means "low disk AOA". A by-product of low disk AOA is a fairly rapid loss of RRPM.
 
Brian Jackson;n1138504 said:
Doesn't the "vomit comet" fly a parabola to go zero G at the top? I would think a gyro is the same after a high speed climb.

Not only 'at the top'. Along all the parabolic trajectory, the plane is in free fall, and the effects of gravity are absent from the start of the parabola until the end of that trajectory, carefully followed by the pilots, who use the turbojets a little, just enough to approximate a free fall in vacuum.
 
Please, review the history. Most of the Cierva autogiro turned at 180 rpm. So low rpm is not bad, in fact, it is the opposite, low rpm rotors performs better than high rpm rotors.
 
Arco;n1138568 said:
Please, review the history. Most of the Cierva autogiro turned at 180 rpm. So low rpm is not bad, in fact, it is the opposite, low rpm rotors performs better than high rpm rotors.

Rotor RPM is irrelevant; rotor tip speed is everything. Rotor tip speed depends upon blade loading (not disc loading) and is approximately equal to 66 x square root of blade loading.

Cierva settled on a blade loading of ~ 35 lb/ft² which results in a tip speed of 390 fps, good enough for a top speed of ~ 93 mph.
 
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Fully right.
But could be how I wrote my sentence in English. For me rrpm is a consequence of other key factors, is not a driver factor. But this “number” is more easy to understand than µ.
In most of the conversations with my colleagues they are very concerned about rrpm because they feel that low rrpm means a very risky situation and as higher rrpm will be the best for safety. They think that the rotors below certain rpm will stop to turn, and this “minimum” rpm will be in the range of 300 rpm. This concept is not true. It is not a magic number for minimum rrpm
The rrpm range in which each rotor is safe ( it has the capability to recover from a low rrpm) depends of the its aerodynamic and inertia properties and each rotor design has its own rrpm autorotation range.
The current rotor design criteria, rotors turning at high rpm, 350 to 400 rpm is not the best choice for me. I think design rotors turning at low rppm are as safe than high rrpm rotors but with better performances.
 
Brian,
.... No spreadsheet and no engineer is able to calculate rotor rpm with such precision because there are dozens of relationships concerning rotor rpm. The matter is mostly aerodynamics but each detail and each change will cause different rpm level...
Best regards
Agnieszka


The autorotation rpm can be calculated with good precision provided known that the collective pitch angle that the blades take in flight under the effect of the load.
But because of the lack of twisting stifness, this data is often uncertain
In add the manufacturers are not sure of zero lift direction of the "modified" airfoils they use. So, they poorly know their actual pitch setting

The rpm depends little on the airfoils chosen, when the aerodynamic collective pitch is unchanged
 
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Arco;n1138568 said:
Please, review the history. Most of the Cierva autogiro turned at 180 rpm. So low rpm is not bad, in fact, it is the opposite, low rpm rotors performs better than high rpm rotors.
I have often flown an Air & Space 18A solo in steady cruise flight at 220-230 rpm, but with a 35 foot diameter rotor the tip speed is still above 400 feet per second,. It doesn't present safety or handling problems but does limit top speed. With a passenger on board the rpm goes up and so does practical top speed. There is a collective pitch trim device (fitted by STC) on many 18As to reduce collective pitch and bump up the rpm when lightly loaded; Using it hurts climb rate but allows comfortable flight at higher airspeed.
 
Arco;n1138588 said:
...I think design rotors turning at low rppm are as safe than high rrpm rotors but with better performances.
max L/D of slow rotor such PCA 2 is about 7
max L/D of fast rotor such Magni is about 11
 
Hi Jean Claude,
from where did you get those numbers?
from calculation or from wind tunnel data?
I guess PCA data are from wind tunnel, but what about Magni?
I have my own implementation of the NACA 487 and NACA 716 and I did several times the calculation of my rotor ( a ELA one that is the same than Magni from aerodynamic point of view), and yes the L/D of the Magni/ELA should be in that range, but I don't believe in my calculations.
For me they are too optimistic. A L/D of 11 is like a "standard" fix wing. I never did the PCA rotor calculation, so no way to compare apples with apples using my program.
Also, I did many "sensitivity" analysis about the impact of each parameter ( diameter, chord, profil,....) in the overall autogiro performances. My conclusion was that to be back close to the C30 performances we have to increase the solidity of our rotors.
Sorry, I am not able to show you the graphs, my Mathcad program is not working anymore in W10.
Do you also use the NACA 487and 716 as reference for your calculator?
 
A simple way of measuring the L/D ratio of a rotor is to measure the rotor disc angle of attack by using something like the method shown in the sketch. Use an electronic inclinometer to measure the angle of the straight edge. The primary source of error is the inaccuracy the typical airspeed indicator.
I measured the L/D ratios of several rotors a number of years ago with the gyros flying past at 50 mph and from memory, results were:
DW rotor..………………..…8:1
SkyWheels rotor..……….7:1
Rotordyne rotor…….…….6:1
Bensen rotor………….…….5:1

L-D.JPG
The Skywheels rotor is a good rendition of the NACA 8H12 airfoil and should be representative of Magni and ELA.
Results were published in a Sunstate newsletter but I don’t have a copy.
 
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Juan Manuel,
These numbers comes from my spread sheet, which uses only the laws of aerodynamics and uses no rotary wing results.
Giving it the exact PCA 2 rotor dimensions and twisting angles from NACA report 515, it results agree well with measurements obtained in flight from the NACA 475 (*).
So, I believe that the results obtained for modern rotors are credible.
L/D increases as the forward speed increases. Thus, max L/D is reaches for Mu about 0.35. It is rarely allowed with pushers today because airframe poorly streamlined, but not because a poor rotor.

In my opinion, the significant differences in L/D that Chuck has found come more from the collective pitch settings than from the airfoils.
To facilitate the launch by hand, it requires a small collective pitch, but it gives a higher rpm causing greater power losses by friction.
The high collective pitch of the DW rotor gives a lower rpm that improves L/D. But it also gives a lower forward speed limit.
Regarding the "solidity" factor σ, my conclusion is that it is better to reduce it as much as the torsional / flexural rigidity allows it.
It is because the power benefit induced by the lightening compensates more than the power lost in friction by the rpm increase

(*) Results of my spreadsheet for the data corresponding to the gliding flight n° 3 at μ= 0.324 (table 3 NACA report 475)
The calculed rotor speed is 144.7 rpm, while the measurement gives 142.2 rpm
The calculed angle of attack of the disc is 7.6°, while the measurement gives 4.5° (shaft) + 3.1° (a1) = 7.6°
The calculed longitudinal flapping α1 is 3.3° while the measurement gives 3.1°
The calculed lateral flapping b1 is 3.0° while the measurement gives 3.5°
 
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Thanks Jean Claude,

I hope in the short future to have my Mathcad running again to share data with you and others

My idea of an ideal rotor is:
- A three blade rotor with the same aspect ratio each blade of the existing two blade rotors. So higher solidity.
-With continuos collective control for flight optimization to the appropriate angle.
-Also with takeoff and landing capabilities in vertical.
-Hingeless with all the articulations built in the composite fibers, like some modern helicopter.

I know is quite challenging but this is my dream.

Today I am working on that, and I have some pieces waiting to be tested under static stress and fatigue,
the first test, a feasibility for manufacturability, was ok.
​​​​​​the pieces are done in RTM thechnology, so I need to validate my manufacturing process.

Thanks again to you and to Chuck for your contributions, I read all your comments very carefully



​​​​
 
I'm not sure I understand. Are you planning a gyrocopter or a helicopter?
Three hinged blades sometimes create ground resonances that are difficult to avoid.
 
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An autogiro, but with cyclic and collective
Yes, the lag shock absorbers is the more challenging task and the reason that three blade rotors are not popular our field
To avoid them the rotor head will be Elastomeric, so next level of complexity, so plenty of fun
 
For the benefit of the newbies here --
some L/D numbers have been cited above. DO NOT assume that you will achieve those values as a glide ratio in your gyro in the event of an engine failure. Plan for something considerably steeper.

Underestimating a glide ratio is usually not a problem, because losing extra altitude is easy to do. Overestimating a glide ratio can be a disaster, putting you into trees, fences, etc.
 
An excellent point J.R.

One of the features of my Garmin 196 is the glide ratio.

I have seen numbers over five to one in The Predator and she glides better with the prop stopped.

I use three to one to estimate my landing zone so from a thousand feet above the ground I need to find a place within three thousand feet of where I am.

I try to always be aware of which way the wind blows because an engine out downwind landing in a gyroplane is very challenging.

If the engine is not running with a seven knot tail wind my rudder is not very effective from about fifteen knots of ground speed down in The Predator.

Your results may vary.
 
I don't doubt your words, Vance, since you have an extensive experience in gyros, but the effectiveness of the rudder is independent from the wind intensity or direction, since those are ground-referenced magnitudes. When in flight, you move within the mass of air, and if that mass is moving with respect to the ground, that has nothing to do with the response of your rudder up there...
 
Thanks Chuck for your suggestion, but how works the flapping?
are free to flap? because I see links that looks block the movement.

About your implementation several years ago, you was so kind to send me a CD to my home with your videos. they are very impressive. thanks again
 
WaspAir;n1138645 said:
For the benefit of the newbies here --
some L/D numbers have been cited above. DO NOT assume that you will achieve those values as a glide ratio in your gyro in the event of an engine failure.
You are right to attract the attention of newbies. The L/D numbers quoted are for the separate rotor. By adding the drag of the "fuselage", max L/D barely exceeds 4.1 (at about 110 km/h) for the usual pushers. Just 5 for the Cierva C30 (at about 140 km/h)
View attachment Finesse max gyro.xls
 
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