Low Rotor RPM Ground Resonance

[Bristol]

Food for thought..

 

In a Schweizer with a fully-articulated rotor system developing ground resonance, the cure at flight RPM would be to lift off the ground so that the blades have the opportunity to rephase. But consider, if there were not enough RPM available to get off the ground, the standard is to shut down or lower pitch? Wouldn't it be correct to put as much pitch in the blades as possible to force the blades to all rephase accordingly. The resistance cause by the increased pitch, would cause the blades to lag (or lead) back into phase?

 

Your thoughts?

 

Bristol

Edited November 25, 2006 by Bristol

[AndrewT]

For ground resonance, you have two options, first is to pull collective and reload the disk as you lift off, OR you can drop the collective and roll down. Like you said, if you have the RPMs in the correct range, then lift the helicopter up, however if you don't have the RPM's, then lower the collective, roll down, and you can actually apply the rotor brake if possible.

 

I think that raising the collective when you don't have the RPMs might acutally cause more of a vibration as the blades stall.

[flyby_heli]

Pulling pitch as you roll off the throttle would cause the blades to flap more. The retreating blade wants to keep the amount of lift equal to that of the advancing blade to avoid dissymetry of lift. Therefore the retreating blade will flap down to increase the angle of attack, to make up for the lost RPM/velocity squared and the increased pitch angles(lift formula anyone? lift= coefficiant of lift x pressure x surface x velocity squared x one half)

Maximum downward deflection of the blade happens 90 degrees later in the direction of rotation, due to gyroscopic precession, in other words over the tailboom.....

In addition, as the blades slow down, they "fly lower" than they would if the RPM was in the normal operating range, because the centrifugal force is less.

The retreating blade might flap down enough to actually hit the tailboom. Specially in windy conditions this would be a big problem.

 

Therefore, do not pull collective to slow down the blades faster on shutdown, it could get really expensive.

 

As said above, if you get into ground ressonance, pick it off the ground if your rpm is in the green and you are light on the skids. If you are fully down and don't have the rpm, roll off throttle and lower collective and it will go away instantly.

 

I have tried both methods, and they do work!!!!

Edited November 25, 2006 by flyby_heli

[PhotoFlyer]

Pulling pitch would not work, except for the fact that it would lower the RPM, due to the increase in drag. As was said before, you also run the risk of taking off the tail.

 

Ground resonance is a resonance (duh!) between an out of phase blade, and a rocking motion of the helicopter that can only be experience while in contact with the ground. Please, keep in mind that this explanation is slightly simplified. Some force (wind, hard landing, etc) moves one blade out of phase, which results in an out of balance condition in the main rotor. If this out of balance condition rotates in sync with a rocking motion of the helicopter while in contact with the ground, the rocking will tend to force the blade further out of phase. As the main rotor gets further out of balance the rocking will intensify, which will push the blade further out of phase, hence, resonance. In order to fix the problem you must eliminate the rocking, or the resonance. Picking the helicopter up into the air eliminates the rocking. Changing the rotor speed may eliminate the resonance. The fastest way to change rotor speed, because it requires no additional power, is to roll off the throttle and let drag slow the rotor down. However, it is possible for the resonance to be so strong that changing the rotor speed will not eliminate said resonance, so it is more desirable to pick the helicopter up into a hover, if possible.

 

Slightly simplified, but should make it clearer...

 

maybe...

[deadstick]

I got into it one time in a TH55/269A. Kind of unsettling.

 

One question for others in the know is the strut condition. It turned out that one of the struts was weak and, apparently, this made the condition worse. Can anybody back this? I figured it was because the strut lost some of its dampening capability. Is this right (or)....I just assume it is since I always am

 

I wear a big watch and mirrored (on the inside) sunglasses, too.

[Heloplt]

I got into it one time in a TH55/269A. Kind of unsettling.

 

One question for others in the know is the strut condition. It turned out that one of the struts was weak and, apparently, this made the condition worse. Can anybody back this? I figured it was because the strut lost some of its dampening capability. Is this right (or)....I just assume it is since I always am

 

I wear a big watch and mirrored (on the inside) sunglasses, too.

 

Absolutely correct. Usually it is the damper that is attached to the strut that causes the problem. If the damper is weak, or the fluid has leaked out it is relatively easy to get into ground resonance. I had the onset occur in one of our OH-6's, I pulled pitch and it stopped. It turned out one of the dampers needed to be overhauled. Landing on a cart that has air filled tires can also cause it. Make sure someone checks the tire pressure regularly if you fly off a cart with those tires.

[deadstick]

Landing on a cart that has air filled tires can also cause it. Make sure someone checks the tire pressure regularly if you fly off a cart with those tires.

 

I never thought about it, but it works for me. All the dollies I've used have had solid wheels.

 

Does this happen in wheeled, fixed or retract type, helicopters, too? They have struts and pneumatic tires, too.

[flyby_heli]

Actually it happens easier in wheel type helicopters, because you put more weight on one single spot and depend more on that point. (tire pressure, dampers and struts)

 

Just like walking in snow with shoes vs skis.

Also easier to get it in skid equipped helicopters during slope landings and running landings, cause you initially put more weight on one skid than the other.

[Gomer Pylot]

I'm not sure I agree with your first statement. With skids, you actually have less surface area in contact with the ground, and many have springs on the skids, which exacerbate any tendency to bounce. The only aircraft I've ever had it happen in is the AS350, in which it happened often. We couldn't even land them on the practice offshore platform, because every one of them would go into it (Aerospatiale said ground resonance couldn't lhappen, but 'dynamic divergence' could). I've had a coupld diverge with powerful dynamics, mostly cured by picking it back up to a hover, but once after the throttle had been pulled back to idle, and there was no way I was going to put it back up. Pulling the throttle all the way off immediatelly and putting on the rotor brake stopped it with only a 45 degree turn on the pad. The springs on the skids were deadly. OTOH, I've flown wheeled helicopters for thousands of hours, and have never had one come even close to ground resonance. IMO, skids are the worst choice for a helicopter, and used only because they're cheap. Cheap and good aren't synonymous.

[Tenacious T]

Is it difficult to practice full down hover autos in a Schweizer or other helicopters with fully articulated rotor systems? I am training in the R22 and some times bounce around a little in my hover autos if I land heel/toe or if my attitude is a little off. That seems like a good opportunity to get into a ground resonance situation in helos with 3 or more bladed rtr systems. But then again throttle is rolled off when practicing hover autos. So maybe that keeps it from happening? Just curious.

[Wally]

The rule in responding to ground resonance is to STOP whatever you're doing. That's really all you can do, because if you don't the helicopter will beat itself to death, self destruct, in something like 3 revolutions of the main rotor once the cycle has developed fully. You can't land, or run-up through the issue. the guys I know who've tried regretted the attempt. One said that the helo was in pieces so quickly he couldn't have changed his mind even if he wanted to.

The good news is that if you're flying a 269, and your struts and dampers are in good shape and serviceable, they work pretty well. Pay close attention to the struts, and check for proper inflation- not to much or too little in the strut.

If you encounter ground resonance as you land, taking off again is the answer because you get a chance to do it better the next time, change the surface you're landing on, even a little sometimes helps- or let the stars realign themselves... If nothing works, then it's "hovering auto" time.

Hovering autos work well in the 269. The rotor has a reasonable amount of inertia, the struts have a fairly long stroke, you've got good vis, and the bird's really responsive. If ground resonance was the issue, then you'll be landing with a slower NR and ground resonance is a harmonic effect- the energy added by the rotor occurs in the phase with the airframe's natural frequency.

I may not have explained that all very well, but don't be lazy with the issue. It's important that you understand it, because the issue will be there every time you fly anything other than a teetering rotor.

Edited November 27, 2006 by Wally

[500E]

We were always told get it back in the air Quick ! if you had flying power. There will be no time to think about it.

Gomer P is not dynamic divergence another way to say ?? Hmmm

I have seen it from the the ground once, it was scary to watch just started to rock , the pilot picked it up real fast.

I always understood it was the real gentle lowering onto the dampers that could start it (well thats my excuse for real solid put downs) as well as poorly maintained dampers

[nsdqjr]

Pulling pitch as you roll off the throttle would cause the blades to flap more. The retreating blade wants to keep the amount of lift equal to that of the advancing blade to avoid dissymetry of lift. Therefore the retreating blade will flap down to increase the angle of attack, to make up for the lost RPM/velocity squared and the increased pitch angles(lift formula anyone? lift= coefficiant of lift x pressure x surface x velocity squared x one half)

Maximum downward deflection of the blade happens 90 degrees later in the direction of rotation, due to gyroscopic precession, in other words over the tailboom.....

In addition, as the blades slow down, they "fly lower" than they would if the RPM was in the normal operating range, because the centrifugal force is less.

The retreating blade might flap down enough to actually hit the tailboom. Specially in windy conditions this would be a big problem.

 

Therefore, do not pull collective to slow down the blades faster on shutdown, it could get really expensive.

 

As said above, if you get into ground ressonance, pick it off the ground if your rpm is in the green and you are light on the skids. If you are fully down and don't have the rpm, roll off throttle and lower collective and it will go away instantly.

 

I have tried both methods, and they do work!!!!

 

I'm still trying to figure out where you got dissymmetry of lift while thinking of this problem??? Dissymmetry of lift is not a product of pitch in the blades, it's a product of airspeed, which you should have very little of while setting the aircraft down, depending on the wind. Also, the maximum downward flapping of the retreating blade doesn't occur over the tail. In something like an R-22 (counterclockwise rotation) it would occur at about the 9 o clock position. It's easy to see, all you have to do is look at a helicopter in flight from the rear. Anyway, interesting answer...

[Wally]

500E-

Not to steal Gomer P's thunder, but dynamic divergent excitation is a term I first heard in Eurocopters. They say it can occur in any phase of operation. Another fun thought for those automatic-rough moments: Dis 'ting could beat itself to death, in flight, showering the darkness of the Smokies with French parts- in three turns of the main rotor. Merde!

I do wonder if Sikorsky allows the same possibility?

 

nsdqjr-

You assume that the bird's parked nose into the wind..

 

Wally

[nsdqjr]

500E-

Not to steal Gomer P's thunder, but dynamic divergent excitation is a term I first heard in Eurocopters. They say it can occur in any phase of operation. Another fun thought for those automatic-rough moments: Dis 'ting could beat itself to death, in flight, showering the darkness of the Smokies with French parts- in three turns of the main rotor. Merde!

I do wonder if Sikorsky allows the same possibility?

 

nsdqjr-

You assume that the bird's parked nose into the wind..

 

Wally

 

No, I wasn't assuming that the aircraft was parked into the wind, I was implying that dissymmetry of lift, in order to flap excessively enough to strikes the tail rotor, occurs during forward flight. There would have to be a SERIOUS wind in order for it to cause the retreating blade to strike the tail boom.

[500E]

So Wally

What you are saying is that we should not have to vivid an imagination if we fly the French egg beater they never told me that when we looked at one

[newbie]

I'm still trying to figure out where you got dissymmetry of lift while thinking of this problem??? Dissymmetry of lift is not a product of pitch in the blades, it's a product of airspeed, which you should have very little of while setting the aircraft down, depending on the wind. Also, the maximum downward flapping of the retreating blade doesn't occur over the tail. In something like an R-22 (counterclockwise rotation) it would occur at about the 9 o clock position. It's easy to see, all you have to do is look at a helicopter in flight from the rear. Anyway, interesting answer...

 

Aside from autos,

 

My instructor never lets me drop my rpm out of the green, atleast until every thing has settled, about 15 secs. This should solve any problem of not being able to lift back off to rephase.

 

I understand that there shouldnt be any airspeed, but if there was the gyroscopic procession would put the downflap over the tail rotor, or at the 6-oclock position.

Edited November 28, 2006 by newbie

[flyby_heli]

I'm still trying to figure out where you got dissymmetry of lift while thinking of this problem??? Dissymmetry of lift is not a product of pitch in the blades, it's a product of airspeed, which you should have very little of while setting the aircraft down, depending on the wind.

 

Dissymetry of lift is a product of speed AND pitch angles (since these are the only variables in the lift formula), but the rotorsystem itself can only affect the angle of attack. The higher pitch angle (AOA) you have, and the lower rpm (v squared) the less wind/airspeed it takes to create a relatively large dissymetry of lift. Therefore, if you pull full pitch on shutdown for instance it doesn't take a lot of wind (airspeed) for the blades to start flapping.

I was implying that dissymmetry of lift, in order to flap excessively enough to strikes the tail rotor, occurs during forward flight.

In forward flight you have sufficient centrifugal force on the blades that they will not flap down low enough, when your rpm is low however, the centrifugal force is almost non existant, so the blades fly a lot lower.

 

Also, the maximum downward flapping of the retreating blade doesn't occur over the tail. In something like an R-22 (counterclockwise rotation) it would occur at about the 9 o clock position. It's easy to see, all you have to do is look at a helicopter in flight from the rear.

 

In forward flight, moving through effective translational lift, what way do you need to put most force on the cyclic? Forward, right? This tells you that when the rotorsystem compensates for the increased airspeed by flapping up/down, the maximum deflection of the blades happen 90 degrees later than the flapping started, so over the nose (flap-back/blow-back). This again means the lowest point is over the tail.

 

Flyby

Edited November 29, 2006 by flyby_heli

[Wally]

To the second part of the oiginal question-

"Wouldn't it be correct to put as much pitch in the blades as possible to force the blades to all rephase accordingly."

My guess is that would not be a good idea. The idea behind lifting off with flight RPM is that that is the only thing you can do. It may rephase the blades, or it may just allow a chance to change something, anything, and land without further problem. But- If you don't lift off again as ground resonance, DDE, or whatever you call it starts- The helo will destroy itself, and quickly. It used to happen a couple times a year at Fort Wolters as students tried to run up or land through ground resonance.

If you're running up and ground resonance starts, roll the power off and leave it in flat pitch. Pulling pitch may "rephase" the blades, but it's more likely to aggravate the problem. You need three things, as I recall, for this to be an issue- a blade damper that allows a rotor imbalance; a strut that doesn't diminish harmonic vibration; and energy in the system. Pulling pitch would aggravate the first problem by allowing disproportianate "hunting" in the affected blade- probably lag- increasing the imbalance; it would do nothing for the other two problems- a strut over/under-inflated and/or just bottomed out and energy in the system. Even a rotor brake might temporarily aggravate the problem, but it should slow things down quickly enough to minimise damage and move the vibration out of phase with the airframe's harmonic frequency.

That's how I see the issue.

 

The state of the art seems to have advanced a lot since the 269's design. The Astar has 2 struts on the forward cross-tubes, one on each side, and 2 leaf springs on the heels, also 1 each- and various composite bits in the head. The struts are stiff enough that they give very little, and the heel springs keep the aft end of the skids off the pavement. But, they all change the natural frequency of the airframe sufficiently that there's very little ground resonance problem- unless you put the heel of the skid on something without the leaf spring taking the load- the struts are very reliable. Usually high spotting the heel or the whole skid isn't an issue, unless there are balance issues in the rotor.

Edited November 29, 2006 by Wally

[Gomer Pylot]

Here's my theory.

 

Resonance depends on a frequency. Change the frequency, and you change or stop the resonance. It's a lot like feedback from a microphone or guitar. In the case of helicopters, different parts move at different frequencies, and get two or more parts moving at the same frequency, or at harmonics of the same frequency, and the result can be feedback, which increases exponentially. Change the frequency of one or more of the contributors, and you may break the chain. Thus, lifting off the ground can stop it, and changing the rotor RPM can stop it. Neither is certain, but are worth a try. With the AS350, I always got it stopped in time by either lifting off, or by changing the RPM. If it tried to go wacko during runup, I cut the throttle. The worst time, when it started when the throttle was reduced to idle, I got it stopped by killing the engine and engaging the rotor brake, to stop the vibration as quickly as possible. I won't guarantee anything, but that has been my experience. I'm just glad I don't fly Astars or TH55s any more.

Edited November 29, 2006 by Gomer Pylot

[HH60Pilot]

Is it difficult to practice full down hover autos in a Schweizer or other helicopters with fully articulated rotor systems? I am training in the R22 and some times bounce around a little in my hover autos if I land heel/toe or if my attitude is a little off. That seems like a good opportunity to get into a ground resonance situation in helos with 3 or more bladed rtr systems. But then again throttle is rolled off when practicing hover autos. So maybe that keeps it from happening? Just curious.

 

Doing hovering autos in the 269/300 is a bit easier than in the R22 for a new student. There is more inertia in the rotor system and having struts on the skids can help take a bit of the bump out if you don't do it just right. However, if you land a Schweizer from a hovering auto on the skid toes or heels, you are going to find that you are probably going to get a bit of a bounce as well. If you land skids level, you should not have to worry about entering ground resonance as there is little chance of bumping one of the blades out of phase.

 

Doug

[Gomer Pylot]

Then again, sometimes you just have to ride things out. http://www.ntsb.gov/ntsb/brief.asp?ev_id=2...01713&key=1