Elevator trim adjustment problems?

[mdpesky]

Looking for some advice on elevator trim adjustment.

First some back ground info; I've always wheel landed my 1954 C170B without ever running out of forward trim. In summery, as I approached the threshold and rolled in forward trim to give me slight nose heavy control feeling so that I could just relax my arms when the main wheels touched, raising the tail, dumping the lift and firmly planting the mains on the ground giving me good control, "never" did I run out of forward trim following this technique as long as the aircraft was within C of G regardless of flap settings.

Three winters ago I installed a set of Federal AWB 2500 Wheel skis and Aero tail ski and nothing changed in regards to wheel landings and trim.

This last year we removed the tail to replace some bottom skins on the horizontal stab due to corrosion caused from a mouse nest. The repair was very well done and by the book, no issues.

On reassembly the elevator and elevator trim tab adjustments were all set exactly by the book, no anomalies there. These settings remain accurate, elevator at least 26 degrees up, at least 20 degrees down and the elevator trim is set at 13 degrees up and 25 degrees down.

Now my issue is with the aircraft well within the C of G I'm finding on wheel landings I'm running out of forward trim??? In other words, unlike before, as I approach the threshold on approach, in varying amounts depending on C of G and flap settings, when I go to roll in forward trim to give me a nose heavy feel on the control stick in preparation for a wheel landing I run out of trim!

We have double and triple checked all the assembly, trim and elevator adjustments, C of G calculations etc but cannot find anything out of adjustment??

Before taking the tail apart we never checked what the trim settings were, so we can't compare to the settings now done by the book. It could be possible the original trim tab "up" setting was beyond the 13 degrees we have now, but there is no way of telling?

Does anyone else experience lack of forward trim on wheel landings and does anyone adjust the up trim travel above the 13 degrees listed in the operators manual?

What am I missing here?

All advice appreciated.

Mark

[daedaluscan]

Sounds to me like the trim was set up differently before your repair, though I find it very easy to set in enough nose down trim for wheel landings.

Using forward trim on wheel landings was a revelation for me, as I had always had a hard time pushing forward as the mains touched, somehow relaxing from holding back is just so much easier and sticks the plane down much more smoothly. I was lucky enough to do about a 15 mile straight line in the desert a couple of weeks ago at about 1" AGL and I did maybe 100 wheel landings one after another. More fun in one flight than I have ever had.

Maybe a fresh set of eyes?

[c170b53]

Something is off ..there's no need to change the rigging when removing and reinstalling the tail.

[cessna170bdriver]

Sounds like the horizontal stabilizer was somehow installed with more leading edge down incidence than it had before. It wouldn't take much to cause your problem.

[DaveF]

How about in cruise? Does the trim want to be set where it was before?

[bagarre]

Sounds like the horizontal stabilizer was somehow installed with more leading edge down incidence than it had before. It wouldn't take much to cause your problem.

I've heard about this being done intentionally to gain a little extra speed in cruise vs carrying more down trim.
Most 170s carry down trim in cruise. If you look out the window, you'll notice your elevator is deflected down.
Changing the horizontal stabilizer's angle of incidence is a way to remedy this.

[mdpesky]

Thanks for all the comments, being there is no adjustment on the horizontal stab and it was placed back on as it was removed, I can only assume the original trim up setting was beyond 13 degrees. The issue then is, since when do C170B's run out of forward trim for wheel landings loaded slightly aft of the C of G but still well into the envelope? I've flown mine for over thirty years and never had an issue before but now I do! I've only flown one other C170B and it too never had an issue but it was the 180hp Lycoming so was nose heavy anyways(not a good comparison).

Has anyone any experience in setting up the trim adjustment on the C170B, the only place we can find reference to its settings is in the Operators Manual were we got the 13 degree up figure. The Cessna 100 Series Maintenance Manual does not cover this for C170B's???

If any members have the time to take a digital level and measure how much their max up trim tab position is for comparison sakes I'd sure be interested in the findings. I suspect that most will find more then 13 degrees up settings if they've never experienced limited trim for wheel landings when loaded slightly aft(like having passengers in the rear seats).

Anyways I'm baffled, but more importantly don't like the limitations the limited trim results in for cross wind landings. I know that opens up a whole new discussion on best way to handle severe cross wind landings in a C170 but for me tail up wheel landings is what works best for me, that's another subject for another time. In mean time if any members have time to measure what their trim tab up position is and let me know that would be greatly appreciated. Perhaps too one of the members has some experience with this and may advise to go with more up trim beyond the 13 degrees?

[canav8]

elevator trim tab for the 170B is 12 degrees up and 25 degrees down according to the TCDS. The 100 series service manual describes how to adjust it.
Two things about your comments. First Trim tab adjustment is based on control loading in different configurations. I notice that I have to load the controls with fwd pressure when using full flaps. When I land with flaps 20 that problem goes away.

[mdpesky]

Yes your experience is matching mine, but even with twenty degrees of flap with C of G between middle and absolute aft(half way between the two) at or near gross weight I lack the forward trim I would like for a tail up wheel landing with it set at 13 degrees.

I'm wondering if other members were to check their actual trim up settings they wouldn't find many to be set higher then that?

I'm suspicious this might be the case as no one seems to complain of being short of forward trim(nose down) for wheel landings, at least not from what i've read searching the forum? Certainly until now I never had an issue making me think before we took mine apart it may have been well over 13 degrees up?

If this is just a characteristic of the C170B so be it, but for over 30 years I've been pegging it on the ground in heavy cross winds with the tail high, now, not so easy....

[canav8]

Yes your experience is matching mine, but even with twenty degrees of flap with C of G between middle and absolute aft(half way between the two) at or near gross weight I lack the forward trim I would like for a tail up wheel landing with it set at 13 degrees.

I'm wondering if other members were to check their actual trim up settings they wouldn't find many to be set higher then that?

I'm suspicious this might be the case as no one seems to complain of being short of forward trim(nose down) for wheel landings, at least not from what i've read searching the forum? Certainly until now I never had an issue making me think before we took mine apart it may have been well over 13 degrees up?

If this is just a characteristic of the C170B so be it, but for over 30 years I've been pegging it on the ground in heavy cross winds with the tail high, now, not so easy....

No, your trim is misadjusted at the trim wheel pully. You cannot adjust it higher then that without getting a reduced range on the other side.
When you check it make sure that the deflection is with the elevator to the stops not entrail. Mine was misadjusted. The 100 series manual will help except the threaded pushrod. you will need to play with it to figure it out because it s difficult to discribe how to remedy on a keyboard. I do not run out of trim anymore.

[mdpesky]

So are you saying you should measure the max trim tab deflections(up and down) with the elevator full up on the stop and full down on the stop? So which way do you place the elevator to measure the up deflection of the trim tab(up or down)?

I had a Freind check his C170B to see how many turns his trim wheel would turn from the neutral position(trim tab flush with elevator) to full forward(trim tab up). His trim wheel turned a full 2 and 1/2 turns to full forward from neutral, mine only turns 1 and 1/4 turns. That's a full 1-1/4 turn difference between his aircraft and mine......baffling but it does seem like my trim wheel is out of whack, in the same breath I still get the book value of 13 degrees up at full forward trim. My Freind hasn't checked yet what max up deflection of his is yet, I'm betting it's better then 13 degrees?

[lowNslow]

My trim tab is flush with the elevator when the trim indicator is in the middle of the green band.

[DaveF]

All tab measurements are relative to the tab being faired with the elevator. The 100-Series Service manual says: "Place tab in neutral position by rotating the tab control wheel. Set an inclinometer on the tab. Adjust tab to 0 degrees." In the elevator rigging section, the manual defines "neutral" to mean "streamlined" with the horiz stabilizer, so trim tab neutral means streamlined, or faired, with the elevator.

The rigging procedure in the owners manual says to set the elevator to neutral, then rig the tab.

My control travels are checked every annual, but I don't roll the trim forward for a wheel landing so I don't know if I run out of trim. My airplane is usually forward CG.

[canav8]

Rigging is done with elevator faired. End limits if I remember are measured with full deflection in both directions. the reason why you rig with the elevator in the faired position is to get the threads correct for full deflection limits. If you take it apart and play with it, it is easier to understand what we are trying to type. D

[GAHorn]

The elevator trim tab is set with the elevator in the NEUTRAL position. Here are the instructions for rigging:

8. ELEVATOR TAB

(a) C-170 & C170A
1. The elevator tab is actuated by a cable which has a chain incorporated in each end. The chain in front is actuated by the fingertip tab control, and the one at the rear operates a screwjack, which mounted in the right half of the stabilizer. The travel is 10 degrees up and 27 degree down plus or minus 1 degree.
2. Turn the tab control wheel to the full forward position, and the screwjack to the full up position. Then turn the screwjack back ½ turn. Set the chain on spockets at each end, allowing ½” to 1” overlap in the direction of travel. Tighten the cable tension to approximately 30 lbs.
3. To set the tab travel the elevator must be in neutral position.
4. Turn tab control to full forward position, disconnect the push-pull tube from taband adjust it to hold the tab approximately 11 degrees. This can be done by screwing it in or out. Connect the push-pull tube to the tab and turn the tab control to the full rearward position. The tab should be approximately 28 degrees.
5. Set the stops between first and second bulkhead rear of the baggage compartment on the cables for correct travel, which is 10 degrees up and 27 degrees down.


(b) C- 170B
1. The elevator tab is actuated by a cable which has a chain attached to each end. The chain in front is actuated by the fingertip tab control, and the one at the rear operates the crew jack, which is mounted in the right half of the stabilizer. The travel is 12 degrees up and 25 degrees down, plus 1degree or minus 0 degree.
2. Turn tab control wheel to the full forward position and the screwjack to the full up position. Then turn the screwjack back ½ turn. Set the chain on the sprocket at each end, allowing ½” to 1” overlap at each end in direction of travel. Tighten the cable to approximately 30 lbs.
3. To set the tab travel the elevator must be in neutral position.
4. Turn the tab control to full-forward position, disconnect the push-pull tube from the tab and adjust to hold the tab at approximately 13 degrees. Reconnect the push-pull tube to the tab and turn the tab control to the full rearward position. The tab should be approximately 26 degrees.
5. Set stops between the first and second bulkhead rear of the baggage compartment on cables for correct travel, which is 12 degrees up and 25 degrees down.