C170A Unusual Take Off ( Elevator Oscillation )

[Patrick Phillips]

Looking for some input as to what was going on during a recent take off from a grass strip........First the details a few weeks ago I visited a grass strip near my home airport.....The take off was an uphill runway so speed was slow to build.....What I feel happen was the tail started to fly a few inches off the ground and then banging back into the ground to bounce up again......and the cycle continued a half a dozen times.....All the time this happens the elevator is hard to control...going back and fourth about 10 inches total.......Now one important thing you have to consider about the "A" Model is that the elevators are not balanced there fore they have no lead weights in the tips........This makes them very heavy when there is no air going over them.......The 'B" model does not have this problem.....so every time the tail wheel crashed into the ground the elevator had a equal and opposite reaction....When the plane got finally flying the oscillation stop....but before that it was a handful to control.....has anybody...."A Model Pilots Only".....experienced anything similar to this....I am trying to figure out what happen....I have more than 500 hours on this plane and never had this happen to me before....

[canav8]

Where was your trim positioned on the elevator for that take off?

[Patrick Phillips]

Wow never thought I might have overlooked the trim setting.....it was a few weeks ago and I can't honestly say....although I getting older and starting to make some stupid mistakes.....I normally trim a little nose down ....forward edge of red bar...so the aircraft gets up on the mains for the takeoff roll.....some tail draggers seam to be able to take off from a 3 point attitude ....i am guessing they would be trimmed from the aft edge of the red bar....Thanks for the input

[canav8]

Patrick, I am not sure how to read your response but I offered a solution, because I have seen what you described and as soon as the trim was adjusted the problem went away. I hope your reply was that you honestly overlooked the trim setting and that you are happily flying again rather then wrenching on your plane to find what is broke that caused your flight condition on take off. We have all forgot to reset the trim after landing one time or another. I would appreciate a reply. Thanks, Doug

[Bruce Fenstermacher]

Patrick I'd say what you had happen does not indicate anything wrong with the plane just the circumstance. I'll also say between Doug's answer and your response you have actually given me something different to try with my A model. You see I get this type of isolation, maybe not as bad as you describe, just about every time I leave my home strip.

First things first to set my situation. When I bought into my A model there was no marking on the flap base to set the trim. My partner who has been operating this plane out of short soft grass strips for 35 years, just set it to a nick in the aluminum that was close to what he liked and had been accustom to for the short field take off that he always did. He used low tail attitude, almost a three point take off and max climb technique to clear a 20ft obstacle at the end of the 1100ft runway. We now have an 1800 ft runway but we must clear a 50ft house at the end and winds are always quartering. So his nick in the aluminum base was likely a little far back but that is what he told me to use. I've since painted a red line which is easier to see than the nick. I do have the trim range info but have never checked it against the red line I painted over the nick to see where it fell in the actual trim range. I now suspect it's toward the rear.

Another thing is I always thought some of the oscillation was because of the surface I was traveling over. I try to just lift the tail wheel out of the grass to reduce drag yet keep max angle of attack. if/when the tail wheel hits the uneven ground the tail pops up and of course I counter pulling it down and hitting the ground and it bounces once again. As you mention the '48 and A model elevator are not balanced and it's real easy to get that elevator flapping back there specially if the trim is set off.

I'm going to try a little more forward trim myself. And just maybe I'll remember to take the measure I have for the trim travel out to the airplane and see how my single red line compares to what it should be.

[Patrick Phillips]

Thanks Doug & Bruce for the input.....I suspect Doug is right .....I was improperly trimmed.....I have flown the plane since and I am confident there is nothing wrong with the plane ......only the pilot....This was a quick stop and I don't remember going thru my checklist......Thanks for the lesson learned I will be more careful next time.....Bruce if you like I could measure my trim wheel housing red bar reference and post it here .....I would be interested in knowing the Cessna Spec. is ......Thanks for the help.....Patrick

[Bruce Fenstermacher]

Patrick, I don't have the Cessna specs. What I have is pictures of decals I suspect are original placed by the factory. Here is two I have from a '51 A model.

100_4640-Scaled.JPG

100_4638-Scaled.JPG

Placement of the takeoff range should be pretty easy to place based on the groves in the trim wheel. I've not compared it to a B model and I'd think placement might be different on them as the elevator, trim tab and as I recall incidence of the elevator are different than the 48 and A model.

[lowNslow]

Not sure if the A model is the same as the B but on my B model the takeoff trim range is 1/2" wide and the indicator is dead center when the trim tab is flush with the elevator.

[canav8]

[Poncho73]

Yup, that can happen during rough surface takeoffs on grass runways with my 48, it's somewhat normal and a firm grip on the control wheel is needed. Also, try not to chase the feed back, just hold the column position firmly and with authority until the tail is flying, otherwise you may start a pilot induced oscillation. Also, if you believe this may be a little mechanical, have your mechanic check the primary elevator cable tension - slack cables or out of spec. cable tensions can result in a little cable whip which will feed back through the control column.

[Patrick Phillips]

Thanks for that....I will check cable tension....although I suspect because of my limited experience flying off grass strips there is probably nothing wrong with the plane......this may be the main reason why Cessna start installing lead in the elevators......wish I had balanced elevators

[GAHorn]

The red "take off" trim-setting placard is placed such that, with the elevator in the level position (streamlined with stabilizer), the trim tab is also streamlined in the nuetral position with the elevator.
In other words, during a pre-flight inspection ...(and by the way, everyone should be confirming this on each and every preflight).....the pilot should first visit the cockpit, and while accomplishing the other duties of the cockpit-inspection, the pitch-trim wheel/indicator should be set at the middle of the takeoff-range. Next, while inspecting the tail/empennage during the walk-around, ...in additon to other items regarding the empennage, ...the elevators should be lifted through their entire range of travel,....the elevators should be stressed oppositely against each-other (i.e. lift up on right side elevator while pressing down on left side...then reversing that action)...to detect any looseness/slippage of their mutual connection at the torque-tube flange, ....and the elevator trim tab should be confirmed to be in the streamlined position with it's elevator.
Again, let me stress that this is not an "occasional" or an "annual inspection" event.... This is an "each time you preflight" event.... In order to timely-discover any failure of the elevators and elevator-trim conditions.

(Additional items to inspect at the tail are:
1. elevator balance weights and attachment rivets/screws for security and interference with stabilizer.
2. push/pull tube end and torque-tube flange attachment bolts/nuts.
3. listen for loose and/or rough pulley/cables movement during elevator action. Check elevator hinge-bolts/nuts for security.
4. inspect vertical stab attachment internal-wrenching bolts and angles, especially look for cracked/loose/"smoking" attach-angles. (These latter are especially vulnerable in aircraft which are spun.)
5. Check full travel of rudder versus elevators for contact between the two surfaces, and check the rudder bellcrank stops for damage and condition. Inspect bellcrank attach bolts/forks and links for proper safety-wiring and condition.
6. Check rudder nav-light for security and function.
7. Check all skins for deformity, and counter-balance areas for previous excess stress due to improper external control-locks.

Of course, then there are the multiple tailwheel items to check seperately.