Grove Gear -8 with a thump!

I am putting this out to the Grove airfoil geared RV-8 community. I finished my -8 in 2010 and have put about 160 hrs since then. In that time on four occasions I have experienced what I would call a very low frequency thump or vibration in flight. This thump has always occurred in a climb and in smooth air. No real particulars about the climb (twice three or four minutes after take-off and twice from cruise to a new altitude). When it happens, you feel it in the airplane not in the stick or rudder pedals and nothing that I can see from the cockpit seems to be moving also the engine parameters stay rock solid. My buddy (ex -7 driver and a multi thousand hour airline captain) and I have been discussing all sorts of possibilities (one of the incidences happened while he was flying the -8). We checked for loose stuff on the outside of the airplane, pulled the cowl checked all that stuff and couldn't find anything suspicious. So we decided to jack the airplane up and check the gear, wheel pants and tail wheel. After looking all that over without anything being unusual, my buddy was sitting on the creeper next to the left main gear and reached over and gave it a whack on the outside of the fairing. The gear oscillated side to side for about 2 seconds, dampened and then the right main started to oscillate. This went on, back and forth, for 30 to 45 seconds and quit. We both looked at each other dumbstruck! Neither of us had ever seen a landing gear do that. I jumped up in the cockpit and had him do it again and shazam, that was it. We were able to create the situation over and over. This made us feel great that we had found the problem, but now how to fix it? I spoke with the Grove people and they have never had anyone report anything like this. They, like us, feel that it is the prop wash striking the side of the gear in certain climb configurations. Neither of us feel it is a serious problem or that it has long term consequences for the airplane. I am planning on continuing to fly the -8 as is until it happens again. When it does, I am going to change power settings to see if that helps ( I have already tried slipping, loading and unloading the airplane to no avail). One last thing, my -8 is equipped with an IO-360 and a CS Hartzell Blended Airfoil propeller. So in conclusion, I am wondering if any of you like equipped -8 drivers have experienced anything like this and would love any input you may have. Also, this is in no way a criticism of the Grove gear. I love that gear and wouldn't own an -8 without it.
Thanks for your input!

IO-390, same prop, same gear leg, standard tires, wheels, brakes, nearing 500 hours, and no thump.

The "on the jacks" description sounds like a resonant gear leg vibration, which although interesting, does not, on its face, seem much like a "thump". What am I missing?

BTW, all vibration has a frequency. What is the shake frequency of the gear leg? Measurement is nice, but an estimate is fine here...1 hz, 2 hz, 3 hz?

Does the thump have a frequency? And how loud is it?

Neither of us feel it is a serious problem or that it has long term consequences for the airplane.

Click to expand...

Resonance is *not* your friend when it comes to aluminium structures. If you are feeling the thumping, something is resonating. I agree with your plan, find out what the condition is that causes it. Once you know what causes it, avoid that condition.

Thump

Agree with Dan that the resonance may not explain your thump. However your experience of listening in the cockpit suggest that it may. This makes me think of the resonance and oscillation that occurred with the slave struts on the Pitts S-2B. Anything that is airfoil shaped can develop lift if the air flowing over it comes with a slight angle. If this deforms the structure then the lift can go the other way and a resonance can occur. This did happen to the slave struts on the Pitts S-2B. The cure was simply to tape a piece of string along one side of the airfoil shaped slave strut. This eliminated the resonance by dumping lift on that side. A similar thing might work on the Grove gear. Hope this helps and let us know what happens!

You might try applying the brakes the next time it happens (if you haven't already tried that). On occasion a wheel will spin due to airflow, and if slightly out of balance could cause some strange vibrations.

Maybe that is the initiating event??

Easy to try, anyway.........

Just as a data point, it sounds like I have the same setup. RV-8 (2013), IO-360, Hartzell, Grove gear, and in my 130-ish hours in the plane I have not experienced any "thumps" that can't be explained due to turbulence. Although, I have noticed 2 or 3 times now, when leveling off from a climb over a local mountain range (at 10k ft) a "slow vibration" that lasts maybe 20-30 seconds while I transitioning to cruise airspeed. It is mostly noticeable on the dash and seat-of-pants feel. However, I always chalked it up to the mountain range (6-8k-ish ft), since it has never occurred anywhere but that same geographical area.

Wow, sounds like a textbook example of coupled harmonic oscillators! You should take a video. The passing of the energy from the left to right gear leg (and VV) is an indication of the degree of coupling between the lh and rh gear legs. Two nominally identical tuning forks will do this across a room. Resonance is a really bad thing as it can gather and store an inordinate amount of energy which is used to shake apart your plane. From a a physics perspective, your can damp the resonance, shift one of the oscillator's frequency or decouple them. I suspect that you're going to be trying to damp or decouple; the resonance frequency is sqrt(k/m) where k is the spring constant. Seems like the mass and spring constant are kind of hard to change one side to the other (change the mount on one side to stiffen that side's gear maybe?). Because the Q is so high (this goes on for 45 seconds !) I wonder if a bead of Silicone on the gear leg would provide enough damping to kill this?
If you do get a chance to take a video, please let me know, I would love to see it!
Good Luck,
Steve

Could it be that one of the forward belly skins is "Oil canning"? If the skin is slightly convex, or flat and not supported by structure in a large enough area, it may be oil canning, or deflecting due to air pressure in the climb. If this is what is happening, it can be easily fixed by installing some adhesive backed damping material. EAR makes products for this application.