Pre-war Trim Tab Upgrade Report

[VictorBravo]

Re: Pre-war Trim Tab Upgrade Report

I put photos and measurements in a new thread called "Trim Tab Upgrade PHOTOS pre-war".

Jim and Brie and anyone else who is thinking about this.... wait just a bit longer and let me finish my own testing. I'm not even done with it and there are things I'd change next time. I also have not done any significant testing with numbers. Of course, this is not yet approved at all, and may not ever be if someone is in the wrong mood in the wrong office.

Comments and arguments and opinions are of course welcome, public or private.

[Jim Herpst]

Re: Pre-war Trim Tab Upgrade Report

Bill
Time is about all I have with the clip wing project. I will await your results You asked what I was sending a check for.... It was to another fellow here on the site who was /is selling a very nice elevator with the built in trim tab all set for stripping/painting and modification for my 3 hinge post war empenage on the 1940 BL. I agreed with him to purchase it for a price we agreed upon but was/am having reservations because for the reasons outlined previous I actually LIKE the pre war set up WITH modification (like what you are doing And adding a friction lock) ... I am experimental soo change is ok and good.

Thank you
Jim

[Swick-T N7010S]

Re: Pre-war Trim Tab Upgrade Report

Jim,
I would use the trim tab in the elevator over the pre war set up. The trim tab in the elevator also acts as a servo tab in the swick conversion. My Swick has the trim/servo tab only in the left elevator but some builders put a second servo tab in the right one. Doing acro the elevator is still quite heavy on mine but that was with the O-360.

Mike

[Jim Herpst]

Re: Pre-war Trim Tab Upgrade Report

Mike, I believe everything you say. I am not planning on any real acro which I am sure raises some eyebrows as to the What the heck ? Why then ?type questions but this clip ship will be VERY different as it will look more like a stock BL except for short wings and some other necessary structural mods for strength (in the wings mostly). Yes it will be an open cowl also.(C-85 or 90 hot rodded to over 100hp) Odd I know but thats what I like...... Obviously not after the performance issues here!!

[VictorBravo]

Re: Pre-war Trim Tab Upgrade Report

Test flight (informal) performed yesterday with both new trim tabs installed. Results mixed. On one hand the airplane definitely makes a slight but noticeable improvement in cruise speed... about 2-3 mph perhpas a smidge more. Nothing like 5 or 10 though. Trim tabs were overly effective in pushing the nose down, it was apparent that the new trim tabs would run it right past redline at cruise power at max deflection.

On the other side, the new trim tabs...even with my bendable adjustment strip... were aerodynamically over-balanced. I could find a position where they would stay in one place, but if I moved them just a little one way or another, they would flop over to max deflection. On several occasions a bump of turbulence knocked this balance off and the airplane pitched nose down or nose up. Rather disturbing but I was at least ready for it.

On the negative side again, the new trim tabs failed to reduce the minimum trim speed much below where it had been before. With the original tabs installed it would want to settle down to about 70 mph IAS power off. With the new tabs it was around 65... far higher than the range of approach speeds appropriate for the airplane.

I have two options in my mind; making the tabs bigger and more balanced, or going to the post-war trim system.

I could try to add some more "damping" area behind the pivot point. This would make the tabs more stable, as well as adding a little bit more "downforce" to trim it for a lower speed.

I could also go back to the beginning and make a new set of trim tabs with the pivot point further forward. That would waste the time and money I have spent already on this set of tabs.

On the other hand, I do not have a lot of faith that any reasonable size trim tab will slow this aircraft down to approach speeds of 55 mph. THAT may require a trim tab that is so large that the pivot structure might not be strong enough, as well as so large of a tab that it interferes with the rudder, elevator, brace wires, tailwheel, etc

Now I know damned well that this exercise I am going through right now is why the factory developed the 1941 DeLuxe and later trim system way back 65 years ago. But I don't want to spend the money or the teardown time to put a set of post-war tails on the airplane.

I have only one other possibility, which is that the trim tabs have too sharp of a leading edge, and they are stalling when deflected for slow speed. This would be because the airfoil is not symmetrical.

Anyone have ideas they wish to share on this subject?

[Jim Herpst]

Re: Pre-war Trim Tab Upgrade Report

Bill Hmm In spit of what I write below they did redesign the damn thing did they not? Any way I digress One would think that if you provided enough surface on the tabs PLUS they were counter balanced AND we could work some kind of friction into the control lever we would be set... I know it sounds like more of a pain especially when there is a proven system out there already... just seems that a seperate system is safer . Shawn Tucker just bailed out of his craft last week and control was only by the trim to position him and the plane for this... sure his was part of the existing empenage but it does bring to light.... say the elevator jammed or broke? Separate systems are nice then?

Also I seem to recall that Duan Coles ship still has the orginal trim system installed even as it is sitting in the EAA Museum today. Anyone ??

Point being he last flew the plane with a Lycoming of MUCH higher HP .
Again anyone ?? I have heard the Trim workin on some planes (open AND closed Cowel ) ANYONE ????

[Bob Waldron]

Re: Pre-war Trim Tab Upgrade Report

Bill,

I have been fascinated by your efforts. I have the pre-war trim, and have it working just fine. Unfortunately, my solution involved putting a fixed trim tab on the elevator. I would dearly love to remove that tab, but won't until I can alter the 'stock' trim to work better.

When I saw your 'Zaggi' wing, I figured you might have solved the problem. I did not necessarily like the looks of what you created, but was hoping for good results.

I have decided to take a good look at my trim 'wings' and see if I could alter the airfoil (to something like a blunt-nosed, flat-bottomed Clark-Y) which should greatly improve the lift. I would add a small 'trim tab' to the back of the new trim 'wings' to allow better adjustment. I would try to keep the same wing cord and pivot point and same general shape. I appreciate that there is little room for extending the 'wings' past the horizontal stabilizer wires.

I wonder if your disappointing results might be a result of trying to make too much of a change. Don't give up. Good luck

[VictorBravo]

Re: Pre-war Trim Tab Upgrade Report

Typed out a detailed reply to Bob, but lost it when trying to post. I am making more modifications to the new wood trim tabs in hopes of getting it right. Will post more info later.

[TimHicks]

Re: Pre-war Trim Tab Upgrade Report

Bill,
I'm not familiar with these separate tabs. How much angular deflection are you able to get with the existing control system? Or a better question is: what range of angle of attack do you suppose you are getting?
Tim

[flylo]

Re: Pre-war Trim Tab Upgrade Report

My "little wing" works great! It trims for 60 in the pattern & 90+ at cruise & stays where I put it. I used to have a '40 BL & as I recall it worked fine too. Both ships are under 700# & are O-145 Lyc powered.

[VictorBravo]

Re: Pre-war Trim Tab Upgrade Report

My "little wing" works great! It trims for 60 in the pattern & 90+ at cruise & stays where I put it. I used to have a '40 BL & as I recall it worked fine too. Both ships are under 700# & are O-145 Lyc powered.

I think that's exactly the problem. They worked fine when designed for the 40 horse airplanes, they were marginal with the 65 horse open cowl airplanes up to 90 mph, but they are inadequate with a closed cowl Continental powered 97-100 mph airplane.

Mine will not nearly trim for 60 mph. Perhaps this is because my airplane is heavier and I am heavier than average... perhaps it's the closed cowl... perhaps my fuselage is not straight in some way that I did not see.

If I were re-covering the stabilizer for some reason I'd be awfully tempted to put in some thin wood ribs over the tubes that had a smidge of positive camber

[VictorBravo]

Re: Pre-war Trim Tab Upgrade Report

Bill,
I'm not familiar with these separate tabs. How much angular deflection are you able to get with the existing control system? Or a better question is: what range of angle of attack do you suppose you are getting?
Tim

I have not measured, but I would guess the tab can go about 20 or 25 degrees either way.

The pre-WW2 airplanes had the separate "all flying trim tab" about 8 inches below the stabilizer. Someone at my airport with an early 1941 said that the change to the DeLuxe model with the "normal" trim tab happened in June 1941, but others here will far more about that than I do. That guy has a May 1941 standard airplane with the external trim tabs, and he riveted large extensions on the rear of his to make them work better.

[TimHicks]

Re: Pre-war Trim Tab Upgrade Report

How well did your friend's riveted extensions work?

It does make sense that your surfaces are more effective at higher speeds than lower. Force is a function of the speed squared. You mentioned stalling. I don't want to ask dumb questions, but did you install the Zagi wing "upside down" so you get lift in the down direction? I'm not familiar with the Zagi profile, but I was looking at NACA 0012 yesterday and I remember noticing that it stalled at about 16°. Do you know what it is for Zagi?

[VictorBravo]

Re: Pre-war Trim Tab Upgrade Report

My friend's extensions work for him on the slow speed, but not enough at high speed.

The Zagi seems to be a 23 series derivative, but it is reflexed. It is not symmetrical and I did not want symmetrical. The tial on my airplane needs to lift at cruise (it needs nose down trim) so upside down would not be right for cruise. it would be right for approach.

I am just now epoxy-ing more area onto the trailing edge for a quick fix, and if that works I will make up another "pretty" set of tabs that look right and have the pivot point further forward.

[VictorBravo]

Re: Pre-war Trim Tab Upgrade Report

I've made three flights with my second generation new trim tabs. I added area behind the pivot, rounded the leading edge to a more blunt radius, and tightened up the bolt at the actuator lever for more friction.

The results at this stage are that the airplane now has WAY more nose-down trim than it could ever use, I actually will need to put a stop bolt or some other limit in place. I believe that the new trim tab has given me about 1-2 mph more cruise speed. Not much... but better than a sharp stick in the eye.

The newly enlarged tabs now allow me to slow the airplane down to about 62-63 mph on final approach. But this is not slow enough for an airplane that stalls at 38-40, and that others here say that they approach at 50.

I do not feel comfortable making the tabs bigger than they are now. They look huge as it is (see photos), and I don't want to overload the structure or get into a flutter issue.

If I make a "final" set of improved tabs, I will do the following things differently:

Symmetrical airfoil or even slightly inverted camber.
Planform outline to more closely match the stabilizer/elevator shape.
Wooden block spar and ribs but aluminum outer skin

The existing experimental set of tabs works crudely and allows me to operate the airplane better than before, but they are crude, not very attractive, and not even equal in size. Since the airplane flies better than it did and I solved the initial problem that made me start this whole thing, I intend to work on some other more pressing projects on the airplane, and re-visit the idea of a pretty set of trim tabe a little while later.

Here are a couple of photos of the tabs before paint. The wood block is the "spar" and load bearing portion. The thin metal rectangle is a 12 inch ruler.