C

Captain Avgas

Well Known Member
There have been some very lengthy threads dedicated to the RV nosegear and ongoing failures thereof. Some people believe the gear has an intrinsically and fatally flawed design.

Other people maintain that there is nothing wrong with the nosegear and that incorrect aircraft handling is the sole root cause of all the failures.

But anyway I think that most people have now gravitated to the middle ground and have concluded that the Vans nosegear (on the two seaters) is a somewhat fragile device that is intolerant of poor landing technique.

At any rate the video below has been around for a while but is fascinating viewing if you have not seen it. Turn on your sound for the speed reports.

It's the nose gear of an RV6A.

It gives plenty of incentive for RV pilots to keep the nosewheel off the ground during both landing AND take-off at any speed over about 15-20 knots.

I wonder if Dick Van G has ever seen it.

www.eaa538.org/azwing/pictures/nosegear.wmv
 
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Please forgive my ignorance, having never flown an RV, but are you saying the stick was full aft at 42 mph and the nose wheel is planted to the pavement?
 
szicree said:
Please forgive my ignorance, having never flown an RV, but are you saying the stick was full aft at 42 mph and the nose wheel is planted to the pavement?
Click to expand...

Steve, on 9/28/06 Tom Velvick of Peoria, AZ, reported on Van Airforce that this video was of his friend's RV6A and that it was filmed while the stick was "aft". But, like you I have some reservations about that. At any rate the viewing is compelling, aft stick or not.

He accelerates to "41 miles per hour". I don't know if that's statute miles or nautical miles. If it's the latter his max speed is 35 knots.
 
I wonder how much the following could affect the shimmy we see on the video;
Runway surface, Tire inflation, Nose Wheel bearing torque, weight on nose
gear.

Perhaps the shimmy would Not be there if there was more pressure on the
nose gear at those low speeds, ie with the stick neutral. Just guessing. I
always keep the stick back on the rollout and subsequent taxi and never noticed
a nose gear shimmy.

It appears the video was taken specifically to document an issue the pilot/owner
knew about. Was the issue resolved? Who took the video?
 
That is an interesting video. What is the max speed before the nose-gear leg overstresses the frame, or breaks something? I can't help but think that there's a tolerable limit to the flexing and it's actually a desired trait. You wouldn't want it to be so rigid as to let the airframe absorb the shock, would you?
 
walter said:
I wonder how much the following could affect the shimmy we see on the video;
Runway surface, Tire inflation, Nose Wheel bearing torque, weight on nose
gear.
Click to expand...

Hi Walter,
I could be wrong, but I don't see any "shimmy" in the video. I'm defining shimmy as the left & right swing of the tire & fork. What the video is showing appears to be a dampening effect from a "non-smooth" runway (I'm guessing there are very few, if any perfectly smooth runways). Very similar to what the suspension on a car would do on a freeway.

The only problem on the RV is that you have the spring to absorb the bumps, but nothing to dampen it. Ever see a car driving down the freeway with bad shocks and the tire is bouncing along under the car?
 
lostpilot28 said:
Hi Walter,
I could be wrong, but I don't see any "shimmy" in the video. I'm defining shimmy as the left & right swing of the tire & fork.
Click to expand...
Ok maybe my definition of shimmy is incorrect. But it "looks" like a fore
to aft shimmy. The reason you don't see a car tire "shimmy" for and aft
is because it can't, by design, or you would be all over the road.

If you look at the shimmys the mains have it's generally fore and aft as well,
primarily. I haven't seen too many side to side shimmys.

Since it's just a straight piece of steel, it can move in all perpendicular
directions to the shaft direction, not just left and right. It's interesting
though. This may end up being one of "those" threads :)
 
Hmmmm... I can plant the stick full aft and be riding on the mains with 50 feet, even at gross weight. We've seen this type of movement in our RV6A, but it was before we added a wood stiffener to the nose gear and changed our take off procedure. I heard from Van's that the nose wheel is only for taxiing not driving down the run way.
 
Now the question to ask is....

Is the dampening effect (or shimmy...whatever we want to call it) exists in every RV nosewheeler? If it exists in some and not others, then obviously there is some difference between them and we should look for that difference.

Would a wooden stiffener on the nosegear help at all?

I have seen a couple of nosewheelers landing and I noticed this effect but it didn't seem that severe. I've also seen others landing and it didn't seem to happen at all.

Given the forces involved here my non-engineering mind tells me it's the 'rolling' resistance of the tire being too high.

Let's please keep this thread limited to facts and troubleshooting and abstain from peanut gallery comments. :D

I wonder if others wouldn't be willing to try this out to see what the result would be. You can easily do this with small cameras you can get from Fry's...around 40 bucks and it'll plug into a regular camcorder.
 
Jamie said:
Let's please keep this thread limited to facts and troubleshooting and abstain from peanut gallery comments. :D
Click to expand...


Jamie, a top suggestion.

If Tom Velvick is tuned in to this thread perhaps he can provide an update on whether the owner of the relevant RV6A has been able to eradicate this oscillation...and how he achieved it.
 
Folks,
For a data point:

I copied the pins to stop the nose wheel
bushings from turning from Roberta Hegy.
This allowed me to unload the nose wheel
bearings to spin freely. (That info is somewhere
on this site.)

An experienced 7A builder mentioned installing a
small stiffener under the nose leg fairing only.
The plans say to add the main gear stiffeners
only if needed. I didn't need them.
(I used the main gear plans scaled down to fit
the nose fairing.)

I copied both of these mods in construction and
have not felt any shake. I also asked others to
observe the nose wheel and they haven't seen any.
I should add that coming from 1000 hours in a Luscombe
I doubt if my nose wheel has any weight on it at very
high speeds.
Tom
 
Cool! classic under-damped system!

This is a great shot of an under-damped (for it's application) spring system. The nose gear is behaving like what it is--a nearly perfect spring with little damping to absorb the energy. In other words, some combination of events (rough pavement combined with the magic combination of tire interaction, etc), is putting energy into the spring (the nose gear) and the spring is doing what springs do best; Absorbing the energy and then giving the energy back. This cycle is repeating over and over.

Assuming other measures have been tried first (like checking breakout force and varying tire pressure), a damper is in order here. The purpose of a damper is to turn some of the energy released from the spring into heat. Just like the shock absorbers in your car.

The standard wood damper may just do the trick here. Although the wood does behave like a spring itself, it is not nearly so efficient as the spring in the gear leg and therefore is a damper too. If you consider wood as a bundle of fibers, all of the fibers rub together when you bend it. This takes a lot of energy. In this case, the wood stiffener would stiffen the gear leg first, (thus raising its natural frequency and reducing the amplitude of the oscillation), and secondarily it would absorb some of the energy that the spring is giving back to the system.

You definately want to use a very straight-grained piece of wood for the stiffener. This provides longer fibers to rub against each other and will absorb more energy. The grain of the wood should be parallel to gear leg.

This is all just my opinion. However I am a mechanical engineer and former furniture maker--I like to think my opinion is based in some knowledge.

Guy
 
walter said:
I wonder how much the following could affect the shimmy we see on the video;
Runway surface, Tire inflation, Nose Wheel bearing torque, weight on nose
gear.
Click to expand...

My thoughts exactly. I would imagine with the stick being full aft, we would begin seeing oscillations at airspeeds just high enough that put significant force on the elevators. I'd like to see another video of a neutral stick run-up and I'd expect to see a slower, progressive deformation in the nose gear rather than this wild shimming.

Of course this is from a guy whose never even flown IN an RV - let alone had time to experiment with nose gear. :D
 
it might be an under-damped spring-mass system, but it needs to be excited by something. The speed-dependency is interesting and given that any possible excitation from the pavement is going to be random, I'm thinking it is the radial force variation, or "out-of-roundness" in the front tyre.

All tyres tend to have some level of RFV, due to the manufacturing methods - even if the tyre looks and measures perfectly round when inflated and unloaded and the whole assembly has no imbalance.

Damping is probably the only way forward, since increasing the stiffness is probably undesirable. Damping will reduce the amplitude, but it won't change the frequency by much, so it will have to be fairly high to be effective.

A
 
This test is clearly invalid. First off, you would never taxi at more than 10 mph. This test was not done at full throttle, so even at 42 mph you probably would not get the nose gear off the ground with the absense of the full throttle prop blast. The taxiing was most likely done just off idle, hence the slow acceleration. Full throttle launches would cetainly lift the nose gear long before 42 mph, probably closer to 15-20 mph. I wouldn't expect any nose gear to react any differently, under those circumstances. It looked like simple undulation due to terrain variation amplified by speed, and possibly tire imbalance. You can watch your nosegear do quite a bit of that just pulling it out of the hangar.


Roberta
 
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7A, 8A, 9A or 10

This poor quality video was of a 6A. Have changes been made in the designs/sizes/materials/structures of the 7A, 9A or 10 to address poor landing/taxing techniques/practices? It has been my observation that Vans usually designs most systems/parts to exceed the needed strength. I might add that my recollection of similar problems with certain Grumman aircraft and several Wichita spam cans.
 
Since the nose gear should come off the ground at relativeyl low speeds on the take off roll, any problems this video suggests would most often apply when braking during landing, which, of course, seems to be when nose gear collapses occur.

Even if you hold full stick back, if you have to brake you put additional force on the nose wheel as the brakes try to twist the plane nose down. Anyone who has braked hard in a taildragger has an idea of the pressures that can be put on a nosewheel during hard braking as you can lift the tail off the ground with stick back, and even flip some planes (well, ya gotta have brakes are actually effective, which some old planes didn't have).

Don't really know what all this means as informed folks seem to have different opinions about whether it represents a problem, but nobody addressed the situation of landing roll and braking forces.

Richard Scott
RV9A Wings
1941 Interstate Cadet
 
If you look carefully between "6 miles an hour" and "8 miles an hours" you will see the same oscillation at the same frequency, just much smaller.

Being that there's very little information to go on here (and I'm not even building an RV anymore), I'm not going to offer an opinion on what's happening other than to say it's pretty clear, I think, that the oscillation is a natural mode of the landing gear and has nothing to do with following the pavement or anything else. It would probably do the same thing at the same frequency if you jacked it up and smacked the gear with a hammer (not exactly the same....you have to account for the added weight/drag when the thing is supported the engine).

An interesting question, though, is what happens to the gear when it's in the air but the mains are riding on a rough surface (like grass, for instance)? Do you put enough energy into the system to get a large oscillation going?

Knowing this group someone will rig up a camera, go out and try it this weekend :D
 
GPS

RV7Aflyer said:
My airspeed indicator is calibrated in 5 knot increments, not in 1 or 2 mph increments.
Click to expand...

Yes... but my GPS reads in 1 kt increments when taxiing -- taxi speed equals ground speed..... :)

gil in Tucson
 
n468ac said:
Hmmmm... I can plant the stick full aft and be riding on the mains with 50 feet, even at gross weight. We've seen this type of movement in our RV6A, but it was before we added a wood stiffener to the nose gear and changed our take off procedure. I heard from Van's that the nose wheel is only for taxiing not driving down the run way.
Click to expand...
I agree, getting nose wheel off the ground is a priority for me too. When landing I hold the nose wheel off as long as I can, then gently apply the brakes to get the speed down to where there is no shimmy or bouncing. RV gears have always looked like the legs of a cat on a hot tin roof anyway, but hey they work great as long as you baby them.
 
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Grumman Connections....

Hard Knox said:
I might add that my recollection of similar problems with certain Grumman aircraft and several Wichita spam cans.
Click to expand...

I flew my two-place Grumman for a couple thousand hours hours and was pretty well informed in the AYA world for those years. The common problem with the Grumman nosewheels was a side-to-side shimmy, usually occurring when the nut and spring washers holding the yoke to the strut weren't tight enough, allowing too easy of a castoring motion,. Tighten that up, and the problem usually went away.

This video shows an entirely different type of motion (fore and aft) that I never witnessed on a Grumman, and don't ever recall being talked about. Since I've never flown a tricycle RV, I'm not going to begin speculating on what it means...just contributing a little Grumman knowledge to the collective!

Paul
 
robertahegy said:
This test is clearly invalid. First off, you would never taxi at more than 10 mph. SNIP
Roberta
Click to expand...

Roberta, I must respectfully disagree. I am based at a towered airport with 5000' + runways, and you would have a whole stack of airplanes behind you irritated if you taxied to the far end at only 10 mph. Even then, it seems like other planes are breathing down my neck... My normal taxi speed is about 15 to 20 knots, even if no one is behind me. I certainly do not catch up to planes ahead of me at this speed.

When I'm on grass, 10 knots is about the limit for taxi speed.

I do hold full aft stick for all ground operations. The authority of the elevator to hold the nosewheel in the air during landings is very dependent on loading. Low fuel, two px, baggage will allow me to hold it off until around 25 knots (no wind groundspeed). Full fuel, solo, no baggage probably is in the 35 knot range. With takeoff power, perhaps around 15 to 20 knots is when the nosewheel can lift off.

BTW, someone commented that the video was poor quality - I thought it was quite good. Perhaps a different viewer.
 
duplication

these conditions are variables that are most likely gonna be duplicated from time to time. I would be greatly interested to see some more, with more info on the inputs. but dismissing it as bougus is foolish IMHO. fact is these planes do seem to flip over for what ever reason...
 
Technique......

Cytox and others,
I have a nosegeared 6A and was also concerned about flip-overs and have an axe in the airplane - just in case. I asked other experienced RV guys and most of the competent ones said what many on here have. Carefully consider where you fly in to, whether or not the strip has potholes which are a recipe for digging in the nosewheel fork leading edge and always use soft field technique.

Full up elevator unloads the nosewheel even if you're not going fast enough to lift it, especially with prop blast on takeoff. I have come to believe that the design is not flawed, some need wooden stiffeners and some guys tell fibs about why they went over. Bear in mind that quite a few taildraggers have gone over as well; not only RV's but 170's agwagons etc. Without a nosewheel to blame, where does the fault lie?

My .02.
 
That's my video, taken long ago (2002) and extremely compressed to minimize the size of the file. The compression seems to exaggerate the motion. Speed is read in MPH from GPS. Stick held neutral. I was annoyed by the "shimmy" and tried everything to minimize or eliminate it, nothing seemed to help so I wanted to really see what was happening and set up a micro camera to capture the motion. I did send it to Vans and they had no suggestions that were not already tried. Breakout tension, tire inflation, stick position, new tire, tire roundness, close tolerance bolt, balance, etc. Other 6As with the same engine, prop and weight and balance do not exhibit this motion, some just do, some don't. This is Vans second nose gear leg design. It is 1 inch, tapering to 7/8 inch in the middle. The original Vans nose gear leg was straight 1 inch. Empty weight- nose gear 300, left main 403, right main 402. O-360 with CS prop.

A wood stiffener helped so I now have an oak stiffener bonded to the gear leg. No fairing, the stiffener is in the same shape as the original fiberglass fairing. I classify it as an annoyance, not a problem. At 25 miles per hour, the movement begins, so taxi speed is kept below that. It is not a problem on takeoff with correct technique, (stick back slightly) and an occasional annoyance on landing but it is restricted to 35 to 25 mph range.

This video was only done to show the motion occurring and how it changes with increasing and decreasing speed, hoping to figure out a way to eliminate it. During normal operations in 500 hours of flight so far this motion does not happen with proper technique. I would not hesitate to build another -A.

Dave Biddle
http://www.davebiddle.com
 
Good to get the info from the source. What could be real intersting is if some of the engineering types could analyze what type of stiffener material would work best: wood (what type), carbon fiber rod, fiberglass, maybe even some other metal?
 
Like Andy and Kevin, my first guess at exciting frequency was a once per rev tire related issue like balance or out-of-round. However, a few moments of superficial calculator banging says probably not.

A quick count of deflection cycles in a 15 second period yields 47 or so, thus natural frequency would be (47/15), or about 3.1 hz.

Max deflection is stated to be at 42 mph, so that should be the speed with the best match of natural frequency and exciting frequency, ie resonant speed. 42 mph is 61.6 fps.

If a Lamb tire has a diameter of about 11 inches, rolling circumference would be 11 x 3.14, or 34.5 inches or 2.875 ft. At 61.6 FPS, the tire is rotating at about 21 hz (61.6/2.875), which doesn't match the natural frequency of 3.1.

The video may be introducing strobe effects and thus the deflection count may be inaccurate.
 
Interestingly enough, like I said before there was another oscillation at about 6 mph.....

(6*1.467)/2.875 = 3.06

It's also interesting that 21 is a multiple of 3.

As you put more and more energy into a system like this (i.e. roll faster down the runway), the energy tends to come in over a broader and broader frequency spectrum. Everything will more or less start vibrating at it's fundamental and if it's large enough to be noticed then you'll notice it. It's like plucking a guitar string or whacking a tuning fork. You put in a very broad disturbance and out comes a very clean note.

edit: of course I'll add that many things aren't noticable unless there's some strong protagonist (like something else vibrating strongly at the right frequency).
 
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Interesting

Interesting cycles. The frequency appears (by crude count) about 3 Hz throughout all the speed ranges. Try placing a piece of paper across the wing shadow. You can then get an idea of the amplitude. At the higher speeds, I get about a +/- 1.5 to 1.8 inch amplitude (assuming the tire diameter is about 11 inches (from a previous post).


Video at 30 Hz so it is probably capturing all of the cycles.
 
Achieving "proper" technique.

dbiddle said:
It is not a problem on takeoff with correct technique, (stick back slightly) and an occasional annoyance on landing but it is restricted to 35 to 25 mph range.

This video was only done to show the motion occurring and how it changes with increasing and decreasing speed, hoping to figure out a way to eliminate it. During normal operations in 500 hours of flight so far this motion does not happen with proper technique. I would not hesitate to build another -A.
Dave Biddle
http://www.davebiddle.com
Click to expand...

Dave, thanks for the report. Much appreciated. But can you clarify something. On the one hand you describe the phenomenon as "an occasional annoyance on landing". Later you state: "this motion does not happen with proper technique".

From your posting it is obvious that you have gone to extreme lengths to investigate and eliminate this oscillation (without success) so I suppose the question is: Are you saying that the oscillation does not happen with "proper technique" but in the real world you find that you cannot always achieve "proper technique".
 
Hey Dave. Thanks for your insight on your video and sharing first hand what it's all about.

And Alex, you're right. And I do taxi a lot faster than 10 on pavement. More like 20, unless winds are not favorable. I guess it's a throwback from my student pilot days when my instructor yelled at me for taxiing too fast, saying to only taxi at a brisk walk speed. I think Dave's Video would support that speed (20) as prudent as well. Sorry for the rant.

Roberta
 
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Yep its a spring (tapered rod) gear

Yep its a spring (tapered rod) gear. The mains do the same thing on my taildragger to some degree. Is it part of the "problem". Yep its the total design, however if you follow all the rules (fairing/tire clearance, tire pressure, swivel breakout), build it right, fly it with normal procedures (stick back) within its limitations (its not an off-road vehicle or bush-plane), than you are going to be fine. Is it prefect. Well if perfect means a easy to land/taxi, light plane that goes like fast like a scared cat, which is relatively easy and inexpensive to build, than its a success or perfect. If you want a "Heavy Duty" nose gear and 6x6 tires, than how about this.

newgear27yr.jpg


newgear34rs.jpg

The second picture is the HIGH SPEED version with fairings, only 20 mph slower not 35 mph slower than a regular RV. :rolleyes: I think Vans set up is pretty good. If you don't like a nose gear than build a tail dragger.
 
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Achieving "proper" technique

Taxi and takeoff is no problem, (unless I want to taxi faster than 25) but occasionally it does happen on landing deceleration, especially if braking. It is only for a few seconds. Usually I use little or no brakes and there is no problem. I am going to try to find the original tape to repost in uncompressed or lightly compressed format.

Dave
 
Nose gear leg stiffeners

Van's does not recommend putting any stiffeners on the nose gear leg. The old gear legs were of a different design and before the gear leg fairing was supplied, a wood fairing was used. A few gear legs failed at the top and the design was changed. The leg is designed to flex.

Bruce Reynolds
Flying RV-6A from grass for 450 hours
 
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Joel Haynes

I have experienced a similar up and down or fore and aft oscillation with my 7A nose gear as witnessed by an outside observer upon landing. I was also able to witness it myself when landing at sunset and observing the long shadow of my nosegear out to the side. The degree of oscillation was pretty dramatic by eye and by feel. Two things did a lot to minimize it. Balancing the nose wheel and adjusting my landing technique. Rather than holding the nosewheel off for as long as possible and having the nose wheel come down abruptly, I now gently ease the nose wheel down on roll-out while I still have elevator authority. These two modifications have made a major improvement. I never experienced the problem on takeoff.

Joel Haynes,
N557XW 75 hours
 
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