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How much is too much?

Sid Lambert

Well Known Member
After putting few hours on the new tail on my -4 I've noticed it requires more down elevator in cruise than before. I don't think it's hurting much other than a mile and hour or 2. I could put a larger shim under the front and/or move the rear down a bit and drill to a larger size bolt

My question is how much is too much? The tip is elevated 1.5" or more above the HS tip at a high speed cruise. (165 kts) Too much or don't worry about it?
 
Is that in your "typical" flight configuration?

This is just my 2 cents. I would shim the elevator if it were my airplane. That causes a lot of force on the elevator trim tab. The conditions would be even worst with someone in your back seat.
 
I guess the next question would be where is it at with a passenger and full baggage? Sounds like that would agrivate the situation even more. Like Axel I think I would consider fixing it.
Ryan
 
Yes, this is full fuel and solo with no bags. I'm still gaining confidence in all the new stuff so no passengers.

I'll try to find a solution over the winter I guess. Thanks guys.
 
Yes, this is full fuel and solo with no bags. I'm still gaining confidence in all the new stuff so no passengers.

I'll try to find a solution over the winter I guess. Thanks guys.

Sid,

Have you done anything that would change your W & B, like a swap from a metal to a wood prop?
 
Shim it

Sid,
That sounds like things are way off, and I'd look at shimming it. Solo cruise with full fuel should be zero trim as you'd expect. With minimum fuel and an average weight backseater I have about 1/2"-3/4" on the counterbalance LE above the neutral position. I can imagine yours would be way out (with backseater)as your describing now.
 
Ok guys it's time to address the HS needing to be shimmed.

Does anyone have a rough idea of shim thickness to elevator trail ratio? If I add 1/4" shim will that produce 1" elevator trail change? I asked Van's but they were not sure.
 
Might be pointing out the obvious, but the first thing to look at is the W&B. I'd make sure that's dead on before chasing an aerodynamic fix.
 
Look deeper

Hey Sid:

Rule of thumb is to move the fwd edge of the HS 1/2 of the elev horn deflection to get the elevs in trail.

So, it would lead to a LE move of about 3/4", and require a shim of about 3/8" to 7/16" or so. That is far too much shim for this plane. There would seem to be something else wrong to make this deflection angle required.

So, I would suggest that you look at a few other things:
Wing incidence (should be 3/4deg positive)
thrust line (should be about 1/2deg negative)
HS incidence should be zero
Firewall should be plumb with the ship leveled

The battery is up front on this ship, right? Is the #4 bulkhead in the right place?

Get back to us with what you find, please.

Carry on!
Mark
 
Horizontal stab shim

Hi Sid,
Have had my 4 since last May with a similar issue as yours when I got it. I shimed the LE of the stab an additional 1/8". I did not have to force the stab up so I felt no additional pressure was being put on the vertical stab spar and the rudder still moved freely. Initially had the LE of the counterweight about 1.25" above the stab in cruise with no pass or bags. Now I have about 3/8" higher in cruise but am leaving it since I will have to adjust the VS if I do more. W&B is OK with the empty wt at 647.6 = 71.53" arm which is between the 68.7 and 75.9 limits.

O-320 160hp, Prince carbon prop, sky tech starter, 23 lb battery in the well between the pedals.

Just my experience anyway....they all seem different.

Good luck!

Al
 
This is just anew tail on an aircraft with 1000 hours. CG is fine and the battery is on the engine mount.

Before the tips were about an inch above now slightly more.

Al, that is perfect info. So I assume you added a 1/8" to the 1/8" called for in the plans? So an extra 1/8" gave almost an inch of correction? Great place for me to start and that sounds like it won't require a new emp fairing.
 
Tips Up!

Flew my 4 yesterday and the tips are up about 1" in cruise flight. I have about 20 hrs on it. Now I'm wondering if I should shim.....
 
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