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Elevator trim motor stroke

R

Rick B.

Member
Built the trim tab and mounted it on the elevator. Installed the electric trim servo kit from Vans. Hooked a battery to the servo and stroked the servo to look for interferance between the linkage and the structure. (Three sentances doesn't look like ten hours of work does it)

At full up trim the linkage just touches the skin of the elevator and the tab skin impacts the rear spar of the elevator. I will probably adjust the length of the linkage to prevent this but it's currently set to stroke equally in both directions.
My questions are:

1. If I adjust the linkage toward up trim can the position indicator be "zeroed" to other than mid stroke of the servo?

2. Does the airplane really need that much trim capability? Seems to me that if one needed to deflect the tab to this limit during flight there would already be a whole lot of other issues the operator would be dealing with but I have no experience with this.

The plans said to trim off part of the control horn on the tab for use with electric trim. I would put a slightly taller horn on if it didn't mean throwing the tab away and starting over.
 
The trim motor outputs a signal that, assuming you have the corresponding Ray Allen trim position indicator, will light up the LEDs in the indicator corresponding to the position of the jackscrew. If you really want to set everything up now, you could hook the position indicator to the motor now and set the trim tab to neutral when the display indicates same (thus the jackscrew is at mid-stroke). Or, wait. Until your panel is done, and set it up then.
 
You will need very little down trim (Trim tab in the raised position). I got all I could get for up trim and the balance was down. And then I learned to just pull on the stick for pitch on final (yes, I can trim for neautral pressure). To each his own. (Nose roller, constant speed. I am at the forward CG limit with zero fuel and nobody onboard).
 
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You need lots of pitch up trim (tab deflected down) for single occupant/full fuel, i.e., forward c.g., very little pitch down trim for slow/flaps/landing. Translation for a -7: at the tab trailing edge, 3/4" up, ~2" down. Centering the indicator bar requires equal travel, which isn't possible for the physical deflections required. Since it takes a while to grind full deflection, I usually compromise with a little residual stick force. With aft loadings, very little trim deflection is needed.

John Siebold
 
To answer the second part of your question

The standard trim servo provides much more travel than is necessary to trim for balanced flight throughout the flight envelope in the -6 and -7 (maybe that much trim authority is required in the tandems?? I don't know).

In addition, the trim tab, as driven by the standard servo, is capable of producing flight control loads that you can not override with the stick in the event of a hard-over (runaway) trim servo at higher speeds (I tested this in Phase 1).

This seems particularly true in the nose down direction - with uncommanded nose down, the airspeed would increase to the point that no amount of stick force could override the aero forces on the trimmed elevator (say "outside loop" of death).

At least in nose up, with a reduction of power and airspeed, the elevator load would probably diminish to the point that you could override it, hopefully before you stall (say "decelerating climb").

My contention is, that this is a safety Murphy that most of us fly with. Many have devised ways to mitigate the risk, primarily trim control circuits (like TCW markets) and trim power interruption switchs.

I contend that the only real way to eliminate the risk is to have a servo that is not physically capable of driving the trim tab to an extreme that can not be overridden by the flight controls.

I admittedly have not done this, but it is on my list. Until then I fly with the rationalization that I have a trim power interruption switch (which assumes I will recognize the runaway in time to disable it) and that there has not been a known case.

I mentioned this to the Ray Allen folks, and they declined comment with a shrug. Maybe the simple fix would be to go to one of their servos with less travel on the SBS RV's like the T2-7A instead of the T3-12A that comes with the kit.

Was this more than you wanted to hear??? and yes I know, this is not an issue for manual trim.

Constructive comments welcome
 
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Exactly what I wanted to hear more about...

gereed75 said:
The standard trim servo provides much more travel than is necessary to trim for balanced flight throughout the flight envelope in the -6 and -7 (maybe that much trim authority is required in the tandems?? I don't know).

In addition, the trim tab, as driven by the standard servo, is capable of producing flight control loads that you can not override with the stick in the event of a hard-over (runaway) trim servo at higher speeds (I tested this in Phase 1).

This seems particularly true in the nose down direction - with uncommanded nose down, the airspeed would increase to the point that no amount of stick force could override the aero forces on the trimmed elevator (say "outside loop" of death).

At least in nose up, with a reduction of power and airspeed, the elevator load would probably diminish to the point that you could override it, hopefully before you stall (say "decelerating climb").

My contention is, that this is a safety Murphy that most of us fly with. Many have devised ways to mitigate the risk, primarily trim control circuits (like TCW markets) and trim power interruption switchs.

I contend that the only real way to eliminate the risk is to have a servo that is not physically capable of driving the trim tab to an extreme that can not be overridden by the flight controls.

I admittedly have not done this, but it is on my list. Until then I fly with the rationalization that I have a trim power interruption switch (which assumes I will recognize the runaway in time to disable it) and that there has not been a known case.

I mentioned this to the Ray Allen folks, and they declined comment with a shrug. Maybe the simple fix would be to go to one of their servos with less travel on the SBS RV's like the T2-7A instead of the T3-12A that comes with the kit.

Was this more than you wanted to hear??? and yes I know, this is not an issue for manual trim.

Constructive comments welcome
Click to expand...

So a few comments since I am about at the same point in my RV8 build.

1. I had this discussion with Van's well before making the final decision to order my tail kit. I specifically stated my concern about having no backup method for a runaway electric trim problem, and asked them flat out if I would be capable of overriding a runaway trim problem. They told me that I would be able to manually override under all flight conditions. Interesting to note your phase 1 test results seem to contradict that statement.

2. The Ray Allen trim position indicator is interesting, in that it has 10 LED light positions. So from my point of view, if you want to "center up" the trim tab with the displayed center poistion of the indicator, then you have to set your trim tab to level flight position when the indicator has the two center position lights lit up instead of one. This has proven to be a bit tricky with a 9 volt battery, and I imagine will be even trickier with a 12 volt battery. I have always wondered why this was not configured with an odd number of lights instead of an even 10. Anyway, that is how I have configured mine for now-two green lights is centered.

3. I checked my angle deflection, and I seem to get about 20 degrees one way and 25 degrees the other way (don't remember which is up or down off hand). This is well short of the max trim tab travel specs listed in section 15 of the manual, but is also a well known and well documented fact on VAF. I am well satisified from everyone's posts on this site that state that there is more than enough actual trim tab travel for all phases of the flight using the electric trim system.

4. I don't think that I will ever personally be able to recognize the problem of a runaway trim tab and take the appropriate corrective action necessary in enough time to prevent a condition that may prove difficult to manually overrride. But this depends on many factors. I guess the best defense is to train for it in all phases of flight.

So your phase one test results do concern me a bit. I guess I am balancing the risk with the fact that I have not heard of this happening in RVs frequently, if at all, nor have I ever encountered this problem on any other aircraft I have flown with electric trim systems.
 
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Flyin'Bryan said:
So a few comments since I am about at the same point in my RV8 build.

1. I had this discussion with Van's well before making the final decision to order my tail kit. I specifically stated my concern about having no backup method for a runaway electric trim problem, and asked them flat out if I would be capable of overriding a runaway trim problem. They told me that I would be able to manually override under all flight conditions. Interesting to note your phase 1 test results seem to contradict that statement.

2. The Ray Allen trim position indicator is interesting, in that it has 10 LED light positions. So from my point of view, if you want to "center up" the trim tab with the displayed center poistion of the indicator, then you have to set your trim tab to level flight position when the indicator has the two center position lights lit up instead of one. This has proven to be a bit tricky with a 9 volt battery, and I imagine will be even trickier with a 12 volt battery. I have always wondered why this was not configured with an odd number of lights instead of an even 10. Anyway, that is how I have configured mine for now-two green lights is centered.

3. I checked my angle deflection, and I seem to get about 20 degrees one way and 25 degrees the other way (don't remember which is up or down off hand). This is well short of the max trim tab travel specs listed in section 15 of the manual, but is also a well known and well documented fact on VAF. I am well satisified from everyone's posts on this site that state that there is more than enough actual trim tab travel for all phases of the flight using the electric trim system.

4. I don't think that I will ever personally be able to recognize the problem of a runaway trim tab and take the appropriate corrective action necessary in enough time to prevent a condition that may prove difficult to manually overrride. But this depends on many factors. I guess the best defense is to train for it in all phases of flight.

So your phase one test results do concern me a bit. I guess I am balancing the risk with the fact that I have not heard of this happening in RVs frequently, if at all, nor have I ever encountered this problem on any other aircraft I have flown with electric trim systems.
Click to expand...

For what it's worth, I installed a Matronics "Governor Mark III" trim motor speed controller. Since I haven't flown yet I can't comment on how much, if any, the motor should be slowed (some on this forum seem to think it needs it, others not). It also serves as a relay to allow two switches. In any case, with the motor slowed somewhat it was pretty easy to find center on the position indicator. 10 lights sure don't make much sense though.
 
I can not say about the -8

reason would tell me that you will need more trim authority in the -8 than the SBS RV's because of the tandem loading.

I know in the -6 that you need only about 1/2 the trim available with the T3-12A to trim for any flight regime.

I did not use a scale to actually determine the stick force required to override down trim in the -6. Lets just say that I became uncomfortable with the amount of pull required for fear of damaging the flight controls long before I got to full down trim at 140 KIAS.

There are many threads that mention the very high stick forces that can be generated by trim at the extreme. For instance

http://www.vansairforce.com/community/showthread.php?t=62159&highlight=full+trim

A TCW type trim controller would seem to be a prudent addition to mitigate the risk of a potentially dangerous trim hardover.

Biasing the trim servo so that it gives just enough nose down trim to get the job done (in the belief that you will be able to override extreme nose up as airspeed decreases) also sounds like a prudent course.

It would be interesting to have someone chime in who has tried it in an-8.
 
I have a suggestion to consider for those that are concerned about the forward trim runaway with increasing airspeed condition where the nose down pitch cannot be overridden.

As awkward as it may sound, before the nose gets too low and the airspeed starts to bend the wings back, roll the aircraft inverted and push gently but positively to get the nose up. If you have no inverted fuel system, the engine will cut out and when the airspeed gets to a lower and more controllable point, roll the aircraft the right way up, restart the engine and keep the speed low where the elevator stick forces are acceptable.

This option is probably an improvement on the outside loop death dive.

Cheers,

Dave

(Aerobatic qualified)
 
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