Strange shaking?

[hevansrv7a]

My -7A, flying without gear fairings, almost always gives a couple of shudders while climbing out a few seconds after takeoff. It usually happens around 85-95 kts and perhaps while raising the angle of the nose. It isn't the wheel brakes and the nose wheel bearing is still pretty snug, so it's unlikely it's spinning after it lifts and that's before takeoff. The shaking is pretty noticeable and can be felt even in the stick although it gives the impression of coming from the front, not the rear. The engine runs smoothly through this and the FP composite prop has not shown any other resonance. In fact, I haven't been able to duplicate this event during slow flight.

Anyone with any thoughts on this, please chime in. Thanks.

 

[mgomez]

Landing vibration?

I've seen this on many factory airplanes. As the wheels slow down after takeoff, they go through a resonant frequency of the gear strut. If you're sure it's not the nosewheel, try tapping the brakes once you're sure you're airborne.

I used to wonder what the heck it was until I was flying right seat in a C-172 with no wheel pants. I could see the wheels wobbling at the end of the strut at the same time that I felt the shudder.

 

[jcoloccia]

I'd have to think it's coming from the wheels. Even if a wheel is only spinning a little bit after takeoff, it can shudder because it's unbalanced. Also, as you pull up into a climb, you're working against the gyroscopic forces of the wheel. Even a tiny amount of wobble could cause a shudder as the wheel fights against the small changes in yaw and roll.

This doesn't help specifically with your plane but I've noticed it with other planes I've flown and it seems to be worse in the summer. Maybe it's just some turbulence off the hot, black runway.

I can't even say this is just my $.02 'cause it's not....it's a total WAG

I'm just curious...does it happen when you do touch and goes.

 

[aadamson]

Ya mean ya never learned to "touch" the brakes *after* takeoff? In all retractables, you learn to do that to stop the wheels from turning before you put the gear in the wells.

I've gotten in a habit of doing it on everything that I fly now-a-days.

So, to figure out if it's gear, as soon as you are airborne, touch the brakes to stop the wheels. If it goes away, you've found the problem, if it doesn't then either a), you have a "sissy wheel" - don't worry, I'm with you on this one ; b) you have some other problem.

If you have a nose wheel and it shudders after you touch the brakes, then you'll have to figure out if it's the nose wheel or some other problem.

Either way, try the brakes first.

 

[hevansrv7a]

It's not the main wheels

I tested the mains/brake theory the obvious way. It's not the main wheels, but that was the first thought. Thanks.

 

[Phyrcooler]

I have noticed something like is being described as well when flying 152/172's. I wondered what that was...! It only occurred occasionally on takeoff - I thought I was flying through some sort of minor layer of air turbulence - sort of like when you do a perfect tight 360 and fly through your own wake.

Learn something new every day here on this board!

 

[aadamson]

I tested the mains/brake theory the obvious way. It's not the main wheels, but that was the first thought. Thanks.

Any chance, you take off with minimal flaps, and it's around or after flap retraction?

 

[aadamson]

One other thot. I was flying a Lancair ES the other day. It didn't have the wing to fuselage seals/tape on it yet. At certain AOA's, it would shake the tail just like a stall buffet. It's related to the turbulence of this wing/fuz area and the lack of a good seal. If you have a poor, or wavery seal in this area, it might be the culprit.

Just a thot.

 

[robertahegy]

My Cherokee sometimes did that during hot, humid, weather conditions on takeoff.

Roberta

 

[Stephen Lindberg]

A contrary view

Ya mean ya never learned to "touch" the brakes *after* takeoff? In all retractables, you learn to do that to stop the wheels from turning before you put the gear in the wells.

Before you adopt this practice check in the POH. Some airplanes (DC-3 for one) warn against it because the brakes can stop the wheels so abruptly that a tire can shift on the wheel and possibly shear off the valve stem. Probably not a problem with our little low mass/inertia wheels, though, but who knows? Rub strips to stop the wheels from rotating are common in wheel wells. Personally, on retracts, I leave the brakes alone. Tapping the brakes is a habit you may have to "unlearn" some day.

 

[jdmunzell]

Echo everything previous poster.

I echo everything the previous poster just said about tapping the brakes after takeoff.

21,000+ hours and I don't believe I ever habitually tapped the brakes prior to gear retraction. Not good for the larger wheels with their mass and inertia. Have done it some on smaller aircraft once in a very great while. As a diagnostic procedure, it can help to isolate a problem like wheel vibration, but not as a regular habit pattern.

I would try tapping one side only first, and then the other side on a subsequent takeoff to try and isolate which wheel is the one that is the problem. If not in the mains, then maybe the nose wheel. I know it doesn't have a shimmy dampener, but perhaps the castering nose wheel has this tendency without a wheel pant on it. Don't know about that. Perhaps Vans would know about this one.

 

[hevansrv7a]

Not flaps

Any chance, you take off with minimal flaps, and it's around or after flap retraction?

No, except for short-field practice, I don't use flaps on takeoff. I'm not even sure it did it when I used 1/2 flaps setting on takeoff.

The theory about wing-fuse gap is at least worth a look, but I have two problems with it. 1] I have the gap seal sheet metal in place and it's not so bad - imperfect, but I don't think it's washing out anything in the tail. 2] I would think the effect could be duplicated in slow flight if that were the case.

Thanks.

 

[rv620mr]

I offer this information primarily as a data point. But, I welcome advice and advisories!

Occasionally, my 6 will shudder during the downwind leg of the landing pattern. In smooth air, I think the shudder would continue indefinitely if I did not reconfigure the aircraft. I've let it go for 10-15 seconds, in the past, in order to investigate. It's a steady, consistent, stable sort of a vibration. Nothing is loose. It's like a mild/moderate pre-stall buffet but it seems to me to be at a higher frequency.

So far, this has only ocurred in the landing pattern. I only remember it ocurring with full flaps--but I'm almost always at full flaps in the pattern. 80 IAS seems more likely to provoke the shudder than does 90 IAS--but I'm almost always at 90. Typically, I run 1700 rpm on downwind.

Although the whole aircraft shakes, I've always had the impression that the cause is closer to the nose than the tail--here, I could easily be wrong. I've tried to isolate the problem by altering engine speed. But, something else always seems to ruin the test--rough air, increased air speed, a pitch change, etc. It doesn't take much to end the shuddering.

The 6 has Van's cowling, fat pants, good fitting intersection fairings from Fairings Etc, and average quality wing root fairings per plans. The underside of the horizontal tail has average to good quality fairings per plans. The empennage fairing fit is average where it meet the vertical tail. It is average where it meets the horizontal tail except at the leading edge--there the fit is pretty poor.

At cruise, the elevator counter-weights stick up above the horizontal tail by perhaps 1/4 inch or less. I've not noticed the problem when practicing stalls. I've done a number of stall series with all combinations of flaps and rpms varying between idle and 1700. The stall really is not preceded by a noticeable buffet. The aircraft has always dropped the left wing 10-20 degrees and pitched down steeply. Recovery has been normal.

The engine/prop combination is as follows: GAMI injected parallel 360, Lightspeed III plus magneto, dynamically balanced Sensenich 72FM (and spinner) pitched at 85 inches, Vetterman exhaust, 9.2: 1 pistons. This has run about 200 hours so far.

I've tentatively accepted this vibration as normal. Should I be afraid?

Monte

 

[George in Langley BC]

In Flight Vibes

Occasionally, my 6 will shudder during the downwind leg of the landing pattern. In smooth air, I think the shudder would continue indefinitely if I did not reconfigure the aircraft. I've let it go for 10-15 seconds, in the past, in order to investigate. It's a steady, consistent, stable sort of a vibration. Nothing is loose. It's like a mild/moderate pre-stall buffet but it seems to me to be at a higher frequency.


I've tentatively accepted this vibration as normal. Should I be afraid?

Monte

Monte you could have another aircraft fly formation (close trail?) at altitude and have a look.

 

[John Clark]

No toucha the brakes

I agree with Steve. Even with 500x5s there is enough energy that touching the brakes could cause the tire to rotate on the wheel. Landing on a flat tire when you are not expecting it can be very exciting.

 

[fodrv7]

Don't doubt it.

In case you doubt the merit of the comments about "keeping off the Brakes when airborne', consider this;
The wheel brakes are designed to arrest the energy of an 1800lb aircraft.
Imagine how quickly they can stop a 10lb wheel.
Pete.

 

[mgomez]

The wheel brakes are designed to arrest the energy of an 1800lb aircraft.
Imagine how quickly they can stop a 10lb wheel.
Pete.

Not too sure about the physics behind this. The mechanical connection between the rim and the tire is also designed to transmit braking loads associated with an 1800 lb aircraft. So why would it be overstressed by stopping a 10 lb tire?

I'll have to do the math to convince myself, unless someone has already done it. Also, let's remember that not everything in aviation scales well...the fact that it's a no-no in a 800,000 lb B-747 doesn't mean it's a no-no in a light single, and vice-versa.