Rotor clearance

[F5 Tornado]

Does anyone know how much clearance I should have for a rotor to the ground, and how should I go about testing that? I think the cyclic should be tilted all the way back, and the gyro sitting on the tail wheel and maybe the rotor hub Teeter tilted all the way to one side. Would that be correct? Then how much clearance should the rotor be to the ground. The reason I ask is that I got a new rotor that is 24' in length and an 8" chord. They are also made out of extruded aluminum, so they're a little heavy. The Hub already seems to have a little bit of a cone angle machined into it.

 

[WaspAir]

Rotor blades can be more of a noodle under load than some realize, so allow for flexing of the blade, not just a passive position.

 

[F5 Tornado]

Rotor blades can be more of a noodle under load than some realize, so allow for flexing of the blade, not just a passive position.

So do you think maybe 1' clearance? I'm not sure how much the rotor flexes.

 

[Doug Riley]

Rotors vary in their flexibility. A foot clearance with (1) craft on its tail wheel (2) controls full aft and (3) rotor teetered all the way back.

As props got longer with the introduction of redrives in the 1980's, some designers started fudging on ground clearance. No doubt this was to keep the craft from being 20 feet tall. As a result, for example, the original Air Command had zero clearance under the three conditions I recited -- though, in practice, the rotor usually grazed the prop first when it flapped.

A very gentle rotor-prop hit, with a composite prop and rotor, made a little chirping noise and left a smear on the gelcoat.

 

[BrianInVa]

Rotors vary in their flexibility. A foot clearance with (1) craft on its tail wheel (2) controls full aft and (3) rotor teetered all the way back.

As props got longer with the introduction of redrives in the 1980's, some designers started fudging on ground clearance. No doubt this was to keep the craft from being 20 feet tall. As a result, for example, the original Air Command had zero clearance under the three conditions I recited -- though, in practice, the rotor usually grazed the prop first when it flapped.

A very gentle rotor-prop hit, with a composite prop and rotor, made a little chirping noise and left a smear on the gelcoat.

A Facebook friend sent me this illustration of just how much a rotor can bend with serious flapping, and said he’s seen this level of flapping and that this is not an exaggeration:


It’s hard to imagine a rotor can flex this much.

 

[Kevin_Richey]

I'd say it is exaggerated more than a bit, because I've seen it occur w/ McCutchen Skywheel (fiberglass) rotors that flapped almost to that degree. That black line example assumes the hub bar portion is fairly level (w/ a slight rearward tilt).

The hub bar angle (when I witnessed that example) was tilted as far rearward as possible, in order to get the blades up to speed.

That severe flap was when the operator of the taxiing gyro had the cyclic fully aft, attempting to spin up the rotor too quickly & @ too fast of ground speed, since he shoved the throttle forward, in spite of being cautioned to not do that!

No surprise to anyone that he was a long-term airplane pilot!

 

[Mike G]

With the GWS you won't flap your rotor ( unless you really want to) so noodle shaped rotors shouldn't be a problem.
Mike

 

[F5 Tornado]

A Facebook friend sent me this illustration of just how much a rotor can bend with serious flapping, and said he’s seen this level of flapping and that this is not an exaggeration:
View attachment 1163020

It’s hard to imagine a rotor can flex this much.

Geez, that's crazy. That looks like that would be to the absolute extreme to where you were crashing soon after if you were airborne. If just taxiing, your tail rudder would be history. I don't think the extruded aluminum rotor that I have wouldn't even come close to that.

 

[F5 Tornado]

With the GWS you won't flap your rotor ( unless you really want to) so noodle shaped rotors shouldn't be a problem.
Mike

Sorry, I'm still pretty new at this. What does GWS mean?

 

[Therapy]

Sorry, I'm still pretty new at this. What does GWS mean?

Thread 'Accident prevention with the Gyro Warning System (GWS)'

Nov 7, 2022

We’ve finished testing the GWS and the production version has been available to gyrocopter/autogyro manufacturers since August 2022. To help promote it, I put a video on YouTube of a presentation for manufacturers and professionals to help them get a complete picture of what the GWS is and how it works. I sent a link to most of the manufacturers and organisations that I thought might be interested. It’s probably too long (50 minutes) for the average YouTube viewer but my thinking was that professionals in our gyro community should be interested enough in reducing the number of gyro...

• Mike G
• Replies: 114
• Forum: Piloting Technique / Accident Discussions

• 
• 

 

[F5 Tornado]

Thread 'Accident prevention with the Gyro Warning System (GWS)'

Nov 7, 2022

We’ve finished testing the GWS and the production version has been available to gyrocopter/autogyro manufacturers since August 2022. To help promote it, I put a video on YouTube of a presentation for manufacturers and professionals to help them get a complete picture of what the GWS is and how it works. I sent a link to most of the manufacturers and organisations that I thought might be interested. It’s probably too long (50 minutes) for the average YouTube viewer but my thinking was that professionals in our gyro community should be interested enough in reducing the number of gyro...

• Mike G
• Replies: 114
• Forum: Piloting Technique / Accident Discussions

• 
• 

Good information.

 

[BarryK]

Geez, that's crazy. That looks like that would be to the absolute extreme to where you were crashing soon after if you were airborne. If just taxiing, your tail rudder would be history. I don't think the extruded aluminum rotor that I have wouldn't even come close to that.

Oh yes…..all rotors would look like that during a severe flap…..seen that before. It’s pretty amazing.
Spend lots of time learning rotor management and mechanics of rotor flight…..

 

[Abid]

A Facebook friend sent me this illustration of just how much a rotor can bend with serious flapping, and said he’s seen this level of flapping and that this is not an exaggeration:
View attachment 1163020

It’s hard to imagine a rotor can flex this much.

Well this is extreme but yes this can and does happen. Flapping is a retreating blade stall and when wings stall hard, they tend to fall out of the sky leading edge first. When that happens, the rotor RPM also starts to slow down very quickly and when it falls below 100 rotor RPM, the blade can start to take this form. Most of these flaps would be on takeoff run with really badly managed (or unmanaged) takeoff.

 

[Doug Riley]

That picture, of course, is artwork, not a record of something that actually happened.

The "flap" phenomenon in teetering autogyro rotors is troublesome because of the rigid rotor hub. As Abid says, one blade "stalls and falls." But it's not just that. The other blade (the advancing blade) continues lifting because it has greater airspeed. This aggravates the retreating blade's stall by forcing it down (increasing its angle of attack). Moreover, its 100+ pounds of lift, being now unbalanced, slams the stalled blade down very hard. Think of a seesaw, where one kid just jumped off. This can lead to the down-flexing that drops the stalled blade into the tail, prop or ground.

Certain rotor blade designs stall more completely, or abruptly, than others. unfortunately, higher-efficiency rotors tend to have worse retreating-blade stall behavior. There's no free lunch.

I've never flown an articulated rotor; those who have can comment on retreating-blade stalls in those rotors. You won't get the violent, unbalanced seesaw effect, but you'll certainly get a strong circular motion of the control stick and vibration, thanks to the unbalanced lift and unequal coning.

 

[WaspAir]

I've never flown an articulated rotor; those who have can comment on retreating-blade stalls in those rotors. You won't get the violent, unbalanced seesaw effect, but you'll certainly get a strong circular motion of the control stick and vibration, thanks to the unbalanced lift and unequal coning.

For articulated systems, only the high speed regime is of concern. You prespin well above flight rpm, so you won't have retreating blade stall issues on launch.

In the McCulloch J-2, you have to exceed Vne to get near retreating blade stall. By that time, the windshield has flexed inward so far that it touches the back of the glareshield-mounted compass, so you know to slow down.

In the A&S 18A, if you are very lightly loaded, the cruise rpm could be down in 210-215 range, and at speeds above 80, you can start to get a vibration as the first symptom of blade stall onset. Nothing happens rapidly or by surprise. It is not dangerous, but it is uncomfortable to sustain for long periods. Slowing slightly will stop it, as will using the collective trim to pump up the rpm a little. With more load the rpm will naturally be more like 240 and there is no issue. If you continue to push beyond onset vibration, it wants to pitch up, which is a proper corrective action anyway.

 

[Doug Riley]

WASP, I would imagine there's a low-speed R.B. stall regime out there, too, if two things were true: (1) the gyro has a weak prerotator, and relies on a ground run to build the latter half or more of flight RRPM, and (2) the rotor can be tilted back 30 deg. or more, to accomplish this.

I realize that the A&S and McC J-2 aren't built this way. Perhaps this is a thought experiment -- what if you replaced the teetering rotor with full hingeing, but retained the tilt-spindle head, had no collective, and relied on a ground run for RRPM? This more or less describes some of the pre-jump Cierva machines. Dunno if anyone has built one since.

The typical Bensen derivative has a teetering rotor, of course. It attracts sometimes scathing criticism for its ground-flap tendencies. How much more benign would the on-ground R.B. stall be if the only change were the substitution of separate flap and lag hinges for the seesaw variety? Presumably, the advancing blade would no longer force the retreating blade deeper into stall. How much would that matter?

 

[WaspAir]

Never saw an articulated tilting-head rotor. In the articulated models I know, a takeoff run causes rpm to decay, not build, so it isn't practical to use a ground run. The semi-rigid rotorcraft I have flown all eliminate the lead-lag but not the pitch change hinge, so this particular gedanken is unique.

I know of one articulated two-blade rotor of the Cierva era, but it suffered nasty ground resonance by all accounts and was quickly abandoned.

If an up-flap of the advancing blade does not force an increase in AOA for the retreating blade, that does offer the intriguing possibilty of a smaller stall region on that blade. My intuition says less extreme, later onset troubles but I have no quantifiable basis.

 

[Doug Riley]

Right. The A&S and McC J-2 don't have enough after-tilt available to taxi-start their rotors. IIR, they also both have symmetrical airfoils, which reportedly don't especially take to slow startups.

Very low initial RRPM was normal in the Cierva era -- and the Cierva-Pitcairns all had articulated blade suspensions. Heck, Cierva invented the articulated blade suspension. Some of those '30's gyros had offset gimbal, tilt-spindle heads. To keep control pressures reasonable, the flap hinge axes were VERY close to the rotor's spin axis. But I've never come across any report about how these critters behaved if they got into on-ground retreating-blade stall.

 

[WaspAir]

Yes, the airfoils are symmetrical. Center of pressure doesn't shift much chordwise with AOA, and that simplifies some design choices

My impression has always been that from about the PCA-1A on, Pitcairn had reasonable clutched power takeoff prerotators (after abandoning the scorpion tail approach) and flight rpm was pretty low, so the issue was not a big one. Running three or four independently flap hinged blades might have given just enough additional margin to avoid the problem.

 

[Doug Riley]

They got their prerotators right after a bit. One of their early ones was a rope wound around a drum on the rotor (lawnmower-starter principle). A crew pulled on the rope to get things moving. That system could not have yielded much initial RRPM.