Cessna 172P Gear Leg Shake On Takeoff

J

Jon Wilder

Pre-takeoff checklist
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Jwylde
Just like the title says. Upon leaving the ground, the gear legs shake as the main wheels spin down. Tubular main gear.

Is this a sign of a wheel that's out of balance? Or some other issue?

Thanks in advance.
 
Likely out of balance. Especially if you are able to get it to shake at certain taxi speeds as well.
 
Static imbalance. We can't dynamically balance the mains due to the brake disc occupying the space where weights would go on the inboard side of the wheel, but we can statically balance it, which is good enough for mains. Nosewheels need dynamic balance to stop shimmy.

"Aircraft quality" tires are often much worse than cheap car tires, when it comes to balance and concentricity. I've had to add a lot of weight to some mains to get them balanced.
 
One of the most overlooked items during Inspections is the security

of the Main Landing Gear.Some aircraft are not easy to jack ( Cessna is one)

so it may be “ overlooked”. I met a relatively low time Cessna with

the tube slightly loose in the fuselage fitting. Cessna would not give me

the tolerances so I had to measure and give them the data. It was in limits.

Since this was some time ago I’ll guess there are now others that are not.


The aircraft I most often find an issue with M/L/G is the Aeronca Champ

and Chief. Some are scary!
 
Magman said:
I met a relatively low time Cessna with

the tube slightly loose in the fuselage fitting. Cessna would not give me

the tolerances so I had to measure and give them the data. It was in limits.
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There have been several SDRs filed regarding sheared NAS bolts that retain that tubular gear leg in the casting in the gear box. I saw that and checked the flight school's airplanes, and sure enough, some of those bolts were wearing. New bolts went in. Not cheap.

Braking loads, both the brake drag and the brake torque reaction, try to rotate that leg to the rear, and with enough force enough times, that bolt can fail. Bog bolt, 1/2" or so.

We had the same sort of failure in the Glastar. 5/16" bolt that sheared at very low time. The leg sheared the bolt, and then the leg came up into the cabin, right through where the copilot could have been sitting. Taildragger version. Bashed up the right wing.
 
Too bad Cessna and some others do not have provisions for Jack Pads

on the wings. I believe every Cherokee has this important feature.

Improper jacking can result in Major Damage,
 
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+. My intent is that ALL aircraft should have a provision to jack them.
 
BoButt said:
all Cessnas?
Click to expand...
The tubular-gear Cessnas sometimes had a jackpoint on the gear leg, but that is little help when you want to pull the leg out or replace the retaining bolt.

Heavier retractable singles like the R182 and 210 have a factory-supplied jackpoint that is attached to the main spar with screws, into nutplates in the spar. The Corvalis has small, threaded jackpoints, removed unless needed.

The 172 service manual has stuff like this in it:

1720456030973.png

A piece of 1x4 padded with rubber. Really? Nothing to prevent it from slipping on the wing, and nothing to prevent the 1x4 from splitting under jacking forces. I made thick plywood, rubber-padded jackpoints, with drilled sockets to prevent sliding off the jack, and screws in the top sticking out a little, arranged to fit into the strut cutout in the wing skin to prevent the wing's sliding off the pad. Worked fine.

We also had jackpads that fit in the main gear "armpit" where the gear meets the fuselage. They were 10-gauge steel sheet (about 3/16") shaped to fit around the gear leg a little to prevent fore-aft slippage, and they lifted at the leg's outer bulkhead casting in the gear box and the leg itself. Good for doing work on wheels and brakes. The airplane with the flat leaf spring gear were easy: a welded-up, tapered U-shaped jackpoint that fit around the leg with a minimum of padding, and it would slide up and wedge itself securely on the leg with jacking pressure. It had a welded-on jackpoint, angled so as to be vertical when on the leg. Had several of those to fit the different leg widths. These things need nice big red flags on them, or you will forget it someday and it will go flying and could kill someone on the ground when it finally falls off. They wedge on tightly enough that you have to tap them loose with a hammer.
 
Ref Dan#10

2 nd para says it ALL. A few nut plates can enable inspection and repair.

3rd para says the consequences of not having those nut plates.

Jacking aircraft is not w/o possible consequences..


When was the last time someone was able to shake the MLG on YOUR

aircraft?

Did it happen?
 
Many times it's a flat spot on the tire. Inadvertent hard braking once can quickly bring a tire out of round and out of balance. If the shaking goes away quickly by tapping the brakes after takeoff, it's a good bet that is your problem.
 
David Loftus said:
Many times it's a flat spot on the tire. Inadvertent hard braking once can quickly bring a tire out of round and out of balance. If the shaking goes away quickly by tapping the brakes after takeoff, it's a good bet that is your problem.
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A flat spot will often result in gear shake during taxi. The amount of missing rubber won't often throw the balance off too bad unless there's a lot ground off. And some mechanics will leave the bearings too loose, no preload, and the wheel will tend to stop in the same place and get ground off on landing until enough rubber is missing to make the tire settle somewhere else. Preloading the bearings a bit makes the stop position more random.
 
Some folks do not preload bearings as they believe it will allow easier rotation.

Lack of preload may also allow the Bearing Races to move in the wheel

which utimately means you need a new wheel.

Just because it is PM does not mean Not Important !
 
All tapered roller bearings need some preload when installed properly.
It has always been a mystery what the perfect preload is to many folks.

Same with adjustable rocker rocker arms and hydraulic cams.

I have wondered if I got it right after a cleaning and repack of grease on 3 axles as I roll down the highway with a 49' trailer. 100 miles later I forget all about it.
 
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Gary Ward said:
All tapered roller bearings need some preload when installed properly.
It has always been a mystery what the perfect preload is to many folks.
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Yes. That tapered bearing will try to move outward when load comes on it, and without preload, it's no longer fully seated in the cup and all the weight is then on the bottom two or three rollers. That doesn't bode well for bearing life, of either the rollers or their races. The weight should be borne by nearly half of the rollers.

If you give the wheel a shove and it keeps rotating (no brake drag at all), it's too loose. With aircraft wheels, it should stop within a couple of turns. That amount of drag isn't going to make any measurable difference to takeoff distances.

Cleveland's manual has this:

1720537413175.png

30 to 40 inch-pounds is about three foot-pounds. They say that the "wheel must rotate freely without perceptible play." That "freely" covers a lot of territory. Should it continue rotating for five seconds or something?

I always preloaded them on the flight school airplanes and never had any bearing failures. We did have an inner race split on a 150's main, not from preload, most likely from a manufacturing defect or a corrosion pit. It caused intermittent howling noises as the inner race slowly crept around on the axle.
 
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A word of caution here for Piper People:

The Bearing Nut is Not Symmetrical and the Cotter Pin hole is closer to one

end. I’ve seen folks reverse the Nut and try to line up the Pin Hole.

This results in Very Loose wheels.

fyi- This was discovered at the Annual and it had flown that way many

times!!!
 
I had a wheel go bad last year on my 172.
Several years before that I found the outer race looser than I would have liked. Should heat up the wheel to get it out and in. Best not to take them out.
I flew it liked that for a couple years, 400 some hours and hundreds of landings.
Eventually I could hear it during taxi and when I pushed it into the hangar.
I found a used wheel on Ebay and swapped it out and all is well now.
First I noticed the brake rotor was not true anymore from the race not being solid in the wheel.
My friend lost a wheel on the highway when he was a kid after greasing wheel bearings with his girlfriend aboard.

Made me realize how important it was to have the right preload on your wheel bearings early on.
 
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When I first put on monster retreads there was a lot more rubber/weight on the outside of the tire requiring me to use the brakes after liftoff.
Now I am wearing them down and It is not as noticeable anymore.
 
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