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6A landing technique

G

gerrychuck

Well Known Member
I've read a great deal on here with regard to nose wheel shimmy, bearing preload, breakout force, etc, etc. I now have 12 hrs and 17 takeoffs and landings on my 6A, and have experienced completely shimmy-free rollouts, slight shimmy on braking, and major paint-mixers where the shimmy seemed completely unrelated to speed; it was just there as soon as I touched the brakes, and continued whether I stayed on the brakes or lifted right off. I have come up with a couple of observations, though, and wondered if others have noticed the same things.

First and foremost, I find that the shimmy I get is directly related to how I land the plane; if I hold the plane off with the nose way up in the air for a full stall touchdown, I will usually end up with a lot of shimmy on the rollout, not to mention not being able to see squat in the flare. I am thinking this is because with a full stall touchdown you can't hold the nose wheel off and it comes down fairly firmly. On the other hand, if I flare low and touch down a little faster with the nose a wee bit lower, I am able to touch down more smoothly, hold the nose wheel off longer, get better aerodynamic braking, and usually get no shimmy, even when I do get on the brakes. In both instances, I always retract the flaps before applying any brake.

For me, the landing technique seems to be the main determining factor, more important than tire pressures, etc. I did a rough check of my bearing preload and breakout force this week; I think the preload is pretty close, and the breakout force might be a bit low (I'll check with a calibrated force gauge one of these days) but the fact remains that if I land with the technique noted above, there is no shimmy.

Does this jibe with what others have found? Just curious and interested in others' experiences and observations.

Gerry
 
mysterious thing this shimmy

I thought I had a nose wheel shimmy, and was planning posible fixes. I got a video camara to document the shimmy, and lo and behold, it was actually the right main that was shimmying. in my case it was a minor transient shake that I've decided to just live with. No problem if I watch my taxi speed. On roll out a quick application of brakes stops it.
 
My Experience With Shimmy

Similar to your experiences, early on in flight testing, I found that the occurance of shimmy "seemed" to be related to how well I landed the airplane. I emphasize seemed because that shimmy would seem to come and go depending upon the quality of the landing. Still, exactly if and when shimmy would occur was impossible to predict. Commonly, it would suddenly occur at some point during high speed rollout. The shimmy could be so severe, it would shake the instrument panel. I tried manipulating the weight on the nosewheel via the joystick, adjusting the nose wheel breakout force, harder or softer tire pressures, etc. Those things done on their own seemed to offer little help, nothing I did seemed to completely eliminate shimmy under all circumstances no matter how well I landed or taxiied the plane. Besides, as you probably know by now, it is best practice to always keep as much weight off that nosewheel as long as possible upon landing and taxiing. Shoving the joystick forward just to eliminate a transient shimmy event is nothing more than a desperate bandaid fix that really does nothing to address underlying problems.

Not knowing what else to do, for awhile I considering installing wooden gear leg stiffeners. Before I got around to doing that, however, the time came for the first tire change. At 218 hours TT, I replaced the kit supplied Air Hawk mains with a new pair of Desser's Aero Classic tires. I continued to use the original inner tubes and made no attempt to balance the tires. I can happily report that if ever there was a "silver bullet" fix, that was it. From the day those Aero Classic tires were installed, shimmy has not even slightly occurred regardless of surface conditions or any imperfection on my part when it comes to landing or taxi technique.

Because there are so many variables at work when it comes to shimmy, Van's characterizes the phenomena as "nebulous," I am not saying that in your case, simply changing tires will cure all your shimmy ills, but it is something to consider.

opu3x5.jpg
 
Shimmy or balance?

My experience is similar. My original shimmy was actually a main gear balance issue (with the original tires). I had friends observe a landing and they reported noticeable shaking of the left main gear. Balancing them solved the issue. Then when I replaced them - I got retreads - no balance needed and no shimmy.
 
Another Form of "Shimmy"

On my elderly 6A I occasionally get a shimmy of the nose wheel when I land too fast and with a slight misalignment to the direction of travel. It's my impression that the nose wheel is flapping side to side from stop to stop. If I then lift the nose wheel off the runway the flapping slows and usually stops. (Being able to lift the nose at this time probably means I was going too fast ... shame.) When I then lower the nose again, the shimmy is usually gone. I feel that landing with a slight drift gives the nose wheel a flip to the side and results in the flapping. I have wood dampers on the nose wheel gear leg and the torque on the nut which retains the yoke is set to specs. No one has seen this and these observations are based on my at-the-stick-impressions.
 
Off topic?

Frank Russell said:
....get a shimmy of the nose wheel when I land too fast....If I then lift the nose wheel off the runway the flapping slows and usually stops......
Click to expand...
Frank,

I am only offering the following comments because I have witnessed too many RV drivers shove the nose wheel down long before it really needs to make contact with the surface.

May I respectfully suggest a reevaluation of landing technique? You see, much has been written about the purported fragility of the nose gear. Some of it I happen to agree with, other stuff....not so much. The general consensus I do agree with is this: despite how fast or slow the landing speed is, the nose wheel should be held off the ground for as long as physically possible. The nose gear should only make its inevitable contact with the ground when gravity takes over despite your best efforts holding back pressure on the stick. Even when slowly taxiing the -6A, to avoid unnecessary stress on the nose gear I almost always keep the stick in my gut just as I do when flying a taildragger.

Taken from a video I posted on youTube, the "screen grab" shot shown below depicts my -6A shortly after touchdown and as the plane rolls out, am keeping the nose wheel from making contact with the ground for as long as possible.

My comments assume a runway length long enough so that only a minor application of braking action is actually needed to facilitate a normal exit from the active runway. It should be obvious that among other things, the stresses on the nose wheel assembly will be greatly reduced and as a bonus, if the brakes are judiciously used, those brake pads will last a whole lot longer before needing replacement.

35cojef.png
 
Rick, keeping weight off the nose wheel is fine, however I see some folk holding it so far and for so long that when it comes down under gravity, the arrival on the deck is worse than what they think they are avoiding.

Neither is good airmanship, and not good for the nose gear.

I prefer to see the nose gear gently lowered onto the deck while it still has control. The keep the loads light.

When taxiing do not allow the nose gear to become elevated on rough ground either. Only causes the same heavy nose drop.

There is no hard and fast rule, you must actually think about what is going on, and control it accordingly.
 
RV10inOz said:
Rick, keeping weight off the nose wheel is fine, however I see some folk holding it so far and for so long that when it comes down under gravity, the arrival on the deck is worse than what they think they are avoiding.
Click to expand...

That's pretty much what I think is happening with my landings; when I touch down slow with the nose very high, it tends to come down very shortly after touchdown, and quite hard. When I touch down a bit faster, I am able to bring the nose wheel down a bit more gently, which seems to result in less likelihood of shimmy. Touching down a bit faster also allows me time to retract the flaps while the nose is still up, which again makes it easier to hold it off, makes the contact more gentle when it does come down, and keeps the load on the nose lower during the remainder of the rollout. Makes the brakes a lot more effective too.
 
Practice landing (touch and go) and never let the nose touch the ground. There is no reason for the nose wheel to ever hit the ground hard. You don't even have a heavy c/s prop. Don't expect the nose wheel to stay in the air if you are on the brakes.

I can never tell (feel) when my nosewheel touches the ground. O320 sen. F/P.
 
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Shimmy down here!

Gerry,

I get to fly many different RV's in my consulting business, including lots of A models. Several factors I have discovered help accentuate the shimmy.
1. Air pressure in the tires above 30 psi for some reason increases the shimmy factor on pavement almost every time. This is true in all the RV's except the Rocket and RV-10 which have different gear. I use the Aero Classic 380X150X5 tires on both my former HR2 and my current RVX which handle 40 psi with no shimmy.
2. Hard braking also seems to increase shimmy initially then decrease it but does increase brake wear. Raising the flaps on touchdown and using full aft stick aerobraking as Rick posted earlier eliminates shimmy on most A models and is the best landing technique in any nose-wheel airplane. Aerobraking works well and saves your brakes. We even use it in the F-16 :) Stiffeners and wheel pant balancing may help but I have experienced shimmy with them as well.

My 2 cents is to lower your tire pressure to 25 psi if you have shimmy issues.
Hope this helps.

Smokey

A side note to all this is the age old argument "which RV lands shorter, the nosewheel or taildraggers?" I tested this on my short grass strip in equally equipped RV-6/6A's 7/7A's and the RV10. Every landing at my strip is short as most of the time it is 1500' usable with obstacles on both ends. The 6/7A flown at 60 knots on final with a full stall/full aft stick touchdown will land and stop in an equal or even shorter distance than my X.
 
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Ray; I have also noticed an increased likelihood of shimmy with increasing the pressure in the nosewheel; interesting since so many people say the opposite, but that's what happened when I increased the pressure from 30 psi to 40.

As I had mentioned earlier, I definitely DO use full aft stick after touchdown, retract the flaps immediately after touchdown (easy on my plane; the flap switch is right beside the throttle and I can thumb the switch without taking my hand off the throttle), and use aerodynamic braking. I don't use any brake until aerodynamic braking is exhausted and the nosewheel is on the ground. I don't really have a shimmy "problem"; I just find that the probability of experiencing shimmy seems to vary with touchdown speed and attitude. If I do it right, no shimmy. I guess what initially puzzled me was that the technique that works in the RV is different from what was needed to avoid shimmy in my Grumman Tiger; in that aircraft, holding the plane off for as long as possible and touching down at the lowest possible speed was the recipe for avoiding the shakes, so that was what I assumed would work in the 6A and it took me a few circuits to figure out that wasn't the case.

Thanks to everyone for the input.
 
I think 2 things should be looked at here.

1) what speed are you at touch down.
2) what surface.

I have to admit I often touch down a little hot ( 5kts over ) I know this because I will get a small bounce. As I land on a very long, grass runway, I just hold the nose off until I need to apply brakes, then let the nose settle down gently onto the runway.
When I taxi, I always keep the stick full aft, and at slow speed.
I also use full flaps on all landings, unless I have a big crosswind.

As I dont have a CS up front, I find I can hold the nose up for a long time, so it can settle down at a much lower speed ( just dont let it slap down ) I dont see the need to get the nose down ASAP, when I have a long runway ahead of me.
 
Shimmy

If you have a problem with shimmy I would guess it may have to do with rolling resistance. Lowering the tire air pressure would make sense because you would get more grip to overcome the resistance force.
Try some grease?
Bob
 
Landing technique

I think Rick has it right. Providing the runway is long enough, holding the nose wheel off as long as the elevator has enough authority to do so should not cause the nose wheel to come down hard if you do not use the brakes while it is in the air. The nose wheel should come down slowly, by itself, as the speed bleeds off, even with the stick held all the way back, as it should be. Everyone who flys an "A" model should get some tail wheel time. Bob.
 
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