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View Full Version : Series 7 / SS Trim, Electric vs Trim Tabs



Dorsal
01-13-2009, 12:46 PM
I am building a series 7 designed for trim tabs and considering changing to the motor driven trim approach used in the SS (and earlier models). Any experience thoughts or opinions on this, pros cons etc?

Agfoxflyer
01-13-2009, 05:26 PM
I have the electric trim in my Series 5 and fine with it. Some people complain it is too fast. But I have learned to use quick jabs. I understand the rocker switch is subject to stopping, but disassembly and cleaning takes care of the problem.

Dave S
01-13-2009, 08:51 PM
Good Evening,

I have a series 7 which has just completed phase I testing. This aircraft is equipped with the manual trim tab setup , not the electric trimmable horizontal stabilizer.

Do both systems work? I have flown both systems and I believe the answer to that question is yes. Are the two systems different? The answer to that is also yes.

I think a person needs to look at their preferences, bank balance; and, the pros and cons of each system - then make a choice that suits you if you have the opportunity.

1) Theoretically, the trimmable horizontal stabilizer should be cleaner aerodynamically because of having a solid elevator rather than the tabs and associated hardware hanging out in the airstream.

2) People sometimes have a preference for electrical versus mechanical stuff - Personally, I am an old school guy - I like the johnson bar in the old Mooneys for the gear retraction because you can control the rate of operation and have a simpler system with fewer things to break - all you have to worry about is the screws falling out, no concerns for electrons flowing or hydraulics not leaking. I prefer the manual flaps on the old Cessnas for the same reason. Now you know where I am going with pitch trim - I like a manual system over an electrical system - so I am just fine with the manual trim tab system on my series 7. Having said that - we also know full well that a kitfox electrical trim system will work in a perfectly reliable fashion if it is properly maintained:) Remember an airliner took a dip in the pacific ocean a few years back because of failure to maintain the jackscrew in the tail. Stuff has to be properly maintained - period - all of the stuff and all the time.

3) If you look at how the manual trim tab system works - it is pretty clear that you loose part of your elevator effectiveness because the trim tab works opposite of the elevator when the elevator is moved - full up elevator results in a full down trim tab and you have a little rectangular hole in your elevator facing the relative wind ; ditto for full down elevator. I have a trigear and do not see operational issues in this. A tailwheel aircraft might make good use of the little extra elevator up effectiveness to nail the tail to the ground. Here is the deal - My airplane is a trigear with the manual trim; and, I have flown a tailwheel with the horizontal stab electric trim - so I don't actually know how the manual trim tab system would work with a tailwheel. If someone who has a series 7 with the manual trim tab system on a conventional gear - they are in a better position to offer experience with a tailwheel airplane.

4) In any case; I find with the manual trim tab, I run out of trim when using full flaps (22 degrees) and have to hold some back stick down final with full flaps. I have also decided that the airplane works perfectly well with no more than half flaps which allows adequate trim. It seems that flaps don't add much drag no matter how far you hang them out; and, half flaps provides 95 % of the stall speed reduction anyway. Of course, balance changes the equation somewhat but in no case has the manual system been an operational problem throughout the loading envelope on my aircraft. My understanding is that John McBean has a "trim helper" which solves the running out of trim with full flaps on the manual trim tab system.

5) I have seen John Mc Beans new airplane and I believe he has the best of most worlds with the current system with the electric horizontal stabilizer trim, solid elevator and spring assist.

Sincerely,

Dave S
Kitfox 7 Trigear 912ULS

Mark
01-14-2009, 08:21 AM
Dorsal,
I also have the manual trim system on my model 7. I've flown others with the electric trim, too. It is true that some elevator is lost during slow speeds on final approach with the trim tabs. Having the spring assist really helps, too. I only need very slight back-pressure with half flaps on a 65mph approach.

I also find that the manual trim is much more easily tuned than the "hunt-and-peck" electric trim. I like using my manual trim better.

However, (seems there's always a 'however'), I believe I loose some cruise speed and fuel economy without the ability to pitch the horizontal for the load. On long cross-countries with a full load, I can't pitch the nose down where I need it. I am seriously contemplating installing the electric trim with my manual trim to give me the best of both worlds.

Now you have lots of food for thought.

desertfox1
01-14-2009, 08:51 AM
I owned a model 7 with manual trim, flew it for about 150 hours.
I raised the leading edge of the stab aprox 3/4 inch and gained
6 or 7 mph, also lost elevator in landings with flaps. It was very
easy to trim in cruise flight. Now I own the "demo" 78KA. It has
the electric trim and is significantly faster in cruise, has excellent
elevator authority with flaps extended, is a pain to use the peck
peck peck with the switch. I need to install something to slow
down the actuator, that's the next project.

Phil Laker

n85ae
01-14-2009, 08:54 AM
The variable incidence tail has a big advantage over a fixed one, in
theory in that you can trim the plane, and still have full deflection
available from the elevator.

If for example you are trimming for landing with flaperons with a fixed
incidence tail, you are running out of available elevator the more you add
trim. It simple aerodynamics really. Each increment of trim eats a little
more of your available remaining travel. No way around it.

That said, whether it really makes a difference or not? I don't know. I
have a series 5 and the way I fly is most landings I just trim out most
of the heavy stick, but probably not all and just fly it in a little stick
heavy. I can say I have NEVER not had enough elevator authority when
landing.

I don't have gap seals, and I have an IO-240B. My only real consolation
was added ballast in the tail to get the cg a bit more aft.

Jeff

darinh
01-14-2009, 01:32 PM
I don't think Dave said that Mechanical is better...he said he "prefered" the mechanical setup. I don't think anyone would argue that technology these days is amazing and very reliable (I have an all glass panel and love it, not to mention I have the most complicated engine out there it seems). But I think you would find the vast majority of people would feel more comfortable working on a mechanical system as compared to an electric one.

I have the mechanical trim system in my Series 7 and find it perfectly adequate...I have never flown the electric system but am sure it is perfectly fine also.

As far as the mechanical system being much more complicated, well I will have to disagree on that...I think my 6 year old could install it. The electrical on the other hand I don't think he would have the faintest idea of what to do. For us adults, both system are very simple to maintain and install. I don't know about heavier or not...have you weighed both systems? You mention a bunch of maintenance items on the mechanical system but wouldn't it be fair to list the items, or possible modes of failure on the electrical system...like electric motor going out, runaway voltage, switch failure, electrical terminal corrosion, broken mounting lugs, stuck motor, shorted wires, power failure, etc.

In other words, the maintenance of either system is not matter of more or less or simple vs. complicated (by the way, all maintenance on a Kitfox is relatively simple) it is just different.

Use the trim setup you have or feel comfortable with...both are good and will do what you need them to do....Heck, after flying my old Model III, I am just happy to have trim at all!

Dorsal
01-14-2009, 02:42 PM
Thanks for all the great feedback, lots to think about. Anyone know just how much weight the motor adds to the tail? and while I am getting ahead of myself how about trim position encoders to feed an indicator or possibly servo to a given trim position?

n85ae
01-14-2009, 03:52 PM
Well depending on what motor you select you might want weight in the
tail. I have 12 lb.s ballast in addition to the battery with my IO-240
so in my case heavier is better.

Regards,
Jeff

jonbakerok
01-21-2009, 06:34 AM
I'm building an old IV kit and it currently has no trim provision at all. I was wondering if anyone knows how hard it would be to install the variable incidence trim system on a IV?

Otherwise here's my options. I'm not happy with any of them but if these are the only options, I'm leaning toward #3.

1. Swap out my elevator for one with a trim tab cutout. (Very expensive and the truck shipping about doubles the price.)

2. Find a welder to build a cutout in my elevator framework and then order the trim tab kit from John. (What, and ruin the powdercoat? And how do I explain to a welder exactly what I want done without plans?)

3. Order the trim tab kit from Murle Williams. As I understand it, the cutout for his kit is built up from wood with no welding required. (Wooden framework? Is that safe?)

4. Live without trim. (You're kidding! How bad is it?)

wadeg
01-27-2009, 12:46 AM
52 hours on S7 with manual trim. No complaints. Had heard I would run out of trim but have not noticed it. I do pull a little pressure to hold 65 but it isn't significant.

Pull flaps on downwind after passing through 90...then slide trim back to full as I enter the bank for base....all done. The rest is minor stick inputs to get to 65 and hold it.

I had wondered if I would want to make the move to electric trim but 'no thank you'. Happy the way it is.

Skybolt
01-28-2009, 05:55 AM
Hi jonbakerok.

A long time ago, I visited the old Skystar facility. In fact, I saw my Classic IV fuselage being welded (it had my name on the tag/ work order).
When I was there they took me up in the factory demo aircraft. This had an unusual trim system.
I had not ordered a trim system for my kit (trying to keep the cost down. In fact there were a few things that I did not order that I wish I had later)

Anyway, that aircraft had a Rans trim system installed.
I ordered one from Aircraft S. and installed it after the kit was completed. It was a little work but managable.
I am glad I did, moving the flaperons is one thing but the rear mounted trim is much easier to control.
I adjust it for landing and I can gide in at the correct speed without too much stick pressure.

http://www.aircraftspruce.com/catalog/elpages/ranstrimsys.php

Eric

jonbakerok
01-28-2009, 06:18 AM
Well, I'll be darned, another option. It looks like it's just attached to the trailing edge, with no cut-out -- is that correct? If not, how did you make the cut-out in the elevator?

I also discovered another option. Sonex uses a spring trim system instead of a trim tab. I wonder if that would work on Kitfox.

Skybolt
01-28-2009, 08:57 AM
Hi Jon.
A couple of pictures of the Rans trim system.
So I made a couple of adjustments with the stainless steel cover for the interior control.

SkyPirate
12-27-2009, 11:49 PM
I was going to use electric and got to thinking,..dangerous for me ya know :)
but here is what I am probably going to do since I need to add weight,..and I have enough room between the seats for the trim wheel

catz631
12-28-2009, 08:24 AM
I have the elecric trim in my 4 with the toggle on the stick. It was a simple system to install ,speed of the trim is no problem,easy to adjust on climbout/approach and it is right there at your hand. What could be easier.
Dick

HighWing
12-28-2009, 09:13 PM
I have flown for over 900 hours with the electric trim with no issues whatsoever. I didn't use the factory switch which I understand has had some issues, but rather used a miniature toggle (DPDT-Momentary) mounted in the stick grip. Incidentally, I am in the process of building another Model IV and weighed the not yet installed servo and it weighs a tad under 4.6 oz. including the two clevis fittings and allthread connecting rod. no real idea of the wire weight, but it looks like about 24 ga. five wire and is no doubt lighter than the control cable. The three wires not used for servo actuation power a panel mounted trim indicator (two choices avilable).

Also, Matronics, the host of the other list has a speed reducing module as does ACS that makes trim a bit easier- abt $67 vs. $45 and both about an ounce. I used the Matronics unit as the RAC unit was not available when I insalled.


Lowell

jdmcbean
01-01-2010, 01:21 PM
One thing to remember is the trim systems are very different between the Classic IV and the Series 5 through the current S7-Super Sport. The electric trim on the current model is stabilizer trim not elevator trim. The trim tabs that have been used on the Sereis 5 through Series 7 are manual elevator trim tabs and work effectively different than the Classic IV.

SkyPirate
01-01-2010, 01:55 PM
It takes less stabilizer trim to get the desired effect then it does elevator trim,....with exception to ones approach on how well their aerodynamics are of the area where the horizontal stab meets the vertical stab,..it can be a very clean design ,.if someone chooses not make the intersection of the vertical stab and the horizontal stab stream line through all positions of adjustment,...it adds to the total ammount of drag on the plane,..
So for models that this option does not come on ,..your looking at trading the drag a elevator trim tab creates for drag that a dirty intersecion creates,..unless you make it clean thru all positions of travel.

Chase