Series 7 / SS Trim, Electric vs Trim Tabs

[Dorsal]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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?)