Trim Assist Kit Option

[jiott]

Kitfox sell an optional Flap/Elevator Interconnect Trim Assist Kit to "help reduce aft stick forces when making slow landing approaches". John McBean seems to recommend it. It fits 5, 6, 7, and SS.

How many of you guys are using it? Is it so-so or greatest thing since sliced bread? Should I get it for my SS7? Its not too expensive at $85. I will soon be covering the fuse and need to make a decision.

Jim

[Dorsal]

I have it and really like it.

[jiott]

I suppose I better get it. Its ending up with my buying just about every option offered by Kitfox. Its nearly impossible to build one of these things for the base price that gets you hooked at first. I am not complaining mind you, just stating a fact. If a guy is going to go to all the work of building you kind of want it to be the best you can make it, so you just keep adding the options. I'm loving every minute of it though.

Jim

[kitfox812]

Can anybody tell me how it installs and what it looks like? I have an s7 with electric trim. Thanks in advance

[Dave S]

Hi Jim,

I am another satisfied purchaser of the trim assist kit. another vote in favor. :)

I flew off the test period and ran some months later without the trim assist. Bought it - and installed it before morning coffee break. You have to get under the baggage bay & at the mixer & elevator idler just behind the seats - John designed it for an easy retrofit if the plane is already assembled.

While not having it is not a great issue in any case - it is a refinement that one will truly appreciate.

What it does is permit a person to trim off the remaining nose down when flaps are applied at slow speeds like on final approach since full nose up trim doesn't quite quite do that in the landing config. None of the control forces on the elevator are very high in any case on a Kitfox but, it is really nice to be able to trim for neutral elevator stick pressure on final.

BTW - my understanding is that the trim assist is very good for a tailwheel configuration - in any case I have never been able to bury the tailskid in the runway grass with our trigear but I can come darn close.

Our plane has the trim tabs on the elevator rather than the electric stab jackscrew for pitch trim - this configuration benefits from the trim assist greatly.

Cheers,

Dave S
KF7 Trigear
912ULS Warp Drive

[airlina]

Can anybody tell me how it installs and what it looks like? I have an s7 with electric trim. Thanks in advance

Note in the photo the forward attach point of the spring to the elevator idler to provide your up elevator "tug" with flaperon deflection. Aft attach point is flap control horn on the mixer assembly. Bruce N199CL

[kitfox812]

Great informative picture. I ordered the kit from Debra. I have about 900 hrs on my s7 since it was DAR'ed in 2007. I had gotten away from using any flaps on landing because of the excess back pressure required. It will be fun to try this out. Thanks again.

[akarmy]

So the question I have, is what's the tradeoff? I have flown with it both installed and not, both with the series 7 trim tabs and the SuperSport electric trim (full elevator surface).

First, the trim tabs are servo tabs and make the elevator forces VERY light. With the full elevator the elevator is much more solid. I agree that the interlock spring does give a bit more elevator up with flaps out, but I kind of feel like it reduces the elevator feel just a bit more. Feels to me like pulling against the spring makes the elevator even more solid feeling, but It's hard to tell really.

Anyone else have a comparison in elevator feel before vs after? I want to like it, but am still not sure.

The only other thing you get is the stick stays centered with the elevators in trail when on the ground, instead of flopping down with the weight of the elevator.

[Av8r3400]

Would this work on a IV? If not, why not. Same mixer and linkages as a 5-SS, right?

[kitfox812]

I finished the install and tried it out today using all the flap settings. I takes out most of the forward pull when using flaps. I'm going to have to get used to the different feel. I think the installation was worth it.

[jrevens]

It's been a few years since the last post about this subject... how are you guys who have the spring assist on the 7SS (or other models) liking it? Pros and cons?

[airlina]

Hi John, I have a Series 5, with an IO-240 up front.(read heavy) During my build many years ago I did everything possible to keep weight aft and my CG came out fine ,however when at my over the fence speed there was always just a hint of a tug on the stick required. I never liked this trait so when the trim assist kit was introduced I went for it and never looked back. A simple concept that has worked well for me. With the trim assist, I can trim to hands off on final which is what I was after. Later I also added gap seals on the stabilizer which aided with slow speed elevator effectiveness. I have enjoyed following your build and your excellent craftmanship. Sounds like you are getting close . Bruce N199CL

[DesertFox4]

John, they work great and are simple to install. After flying several 7's with trim assist springs installed I ordered one for my 7SS build.

Started typing all the reasons I like them but all those reasons were covered below in previous posts.

BTW, i would think they should work as well on model 4, 5, and 6's. Same mixers.

[jiott]

I have had my trim assist on since day 1, now 520 hours, and am pleased with it although I have no before and after comparison. Everything about it seems positive to me except possibly one thing- if you have to do a go-around at the last minute and don't have time to take out the nose up landing trim, the forward stick pressure is quite heavy, partly due to the trim spring back pressure. This happens so rarely and can be dealt with that I don't really consider it a negative. The nice thing about the spring is that it can very easily be disconnected if you don't like it ($85 wasted). I vote to install it.

[n85ae]

I still think the trim assist kit, can lead to a situation where you don't run the
tail incidence to the proper angle (because the spring is alleviating pressure)
and under the right conditions it can lead to a tailplane stall. Which I
encountered at altitude when screwing around. I did a nose over tail tumble
at 8000 feet, and removed the spring after, and have mechanically blocked
the 22 degree notch ... Never want to do that again.

I don't think I've ever heard of anybody else having it happen, but it happened
to me and it's not gonna happen a second time to me.

I pushed nose over, pulled in 22 degrees of flaperon, and about 2 seconds
later it felt like somebody kicked the tail straight up. I smacked my legs
up into the instrument panel, and everything from the baggage compartment
ended up plastered into the windshield. Scared the **** out of me.

Jeff

[jrevens]

What you describe is similar to an aerobatic "bunt" maneuver, Jeff. There can be a lot of necessary downward force created by the horizontal control surfaces , especially when holding the nose high to perform something like a power on stall, for instance. If that tail gets into some disturbed air, like from the flaps, at a particular angle of attack, it can result in a tail stall, instantly loosing all of that down force. A forward CG can be one of the contributing factors to something like this as well. The result can be spectacular... I experienced it once also, in another aircraft type, and it was a wild ride for a guy who didn't know exactly what was going on. Even though the nose went beyond vertical, and I was hanging from the shoulder harness, the tail remained in disturbed air and stalled until I retracted the flaps, and had lost about 7,000 ft., pulling out less than 500 ft. AGL. My first thoughts were that something had broken in the elevator control system. If I hadn't had mechanical flaps, with the flap lever sticking up to remind me that they were deployed, I would have ridden it into the ground. It was a fast, slick little airplane, on it's second test flight, and even if I had figured out what was happening a few seconds sooner, electric flaps may not have retracted soon enough. Anyway, I think it's important to share your experience like you did. I'm glad you got out of it in time. I don't think it will keep me from trying the spring assist. My belief is that the test flights of a new airplane, performed at altitude, are very important for discovering any adverse traits and the conditions under which they can occur. I'll make sure that I'm looking for that when I'm testing mine. Thank you for letting others know.

[n85ae]

Very similar experience, I grabbed the flaperon handle and shoved it to zero
past vertical, after which the tail started flying again and I got it under
control. In addition to the mods to remove the 22 degree's I also added a
baggage net with tiedowns. I never fly with much more than a map loose
after that anymore.

I have an IO-240B and was solo, so it was a forward CG as well.

One of the very few moments where I was sure I had just killed myself ...
Words can't convey the fear, since it was not even in the realm of things that
I expected at that moment.

[colospace]

I'm not flying yet, but I bought the spring kit (ages ago it seems) and I recall John McBean suggesting that it not be installed until after the initial sorting out flights. FWIW

[southwind32]

I asked Debra about it a few weeks ago, and she suggested the same thing, wait till I do the initial flying. So I will wait.

[jiott]

Jeff brought up a point that got me thinking; when you trim the horizontal stab for hands off straight and level cruise flight with the spring assist what you are really doing is trimming for a constant back pressure equal to the spring force. Seems like this would add some drag to your cruise efficiency. I realize that the spring pressure is small with flaps up (the mixer system adds more spring back pressure as you add flaps, which is when you need it) but with flaps up there is still some spring pressure.

I decided to do a little test today and went up and settled into hands off trimmed cruise flight and carefully noted my cruise airspeed at a stabilized 98 mph. Without touching engine throttle, I then pushed the stick slightly forward with about the same estimated force as the spring backpressure to effectively neutralize the spring assist effect. Of course the nose went down so I retrimmed to return to straight and level flight, but now holding a constant forward stick pressure. After things stabilized in this condition I then noted my cruise airspeed at 99 mph. This effect was so small as to be difficult to measure, but it does seem to indicate that the spring assist does add a small amount of drag in trimmed cruise flight, robbing you of about 1 mph. Personally I believe this very small negative effect does not cancel out the greater positive effects that the spring assist gives during landing approaches. I plan to stick with my spring. What do you guys think? Was this a valid test?

[Dave S]

Jim,

Curious which version of pitch trim you have. Do you have the manual servo tab type trim common to the early S7's or the electric jackscrew on the HS which is currently used?

Wondering if these systems behave differently with the spring assist?

I have the manual servo tab type trim with the spring trim assist and have been happy with it; but, haven't experimented as you have with regard to potential drag effects.

[jiott]

I have the electric jackscrew on the HS.

[jrevens]

Jim,

In my opinion I think it may not be valid test. If you applied the force duplicating the spring force by feel then the possible, and very probable fact is that 1 mph is not enough to prove anything one way or the other. There are too many possible variables.

[jiott]

You are right that accuracy is questionable. A better way would be to measure on the ground with a spring scale the actual force applied by the spring assist. Then knowing this, go fly and use that same spring scale to apply the forward stick pressure. Thinking thru the process of what is occurring with the spring assist, I think it is logical that the spring tends to add drag in a trimmed cruise configuration. The amount is the only question in my mind. Are we in agreement with that?

[jrevens]

This is an interesting discusion... ponder this, Jim - without the spring, the elevator drops down towards the ground because it is not a balanced surface and gravity does it’s thing. In the air (as long as you are upright) there is still that gravity effect, but the air flowing over the surface causes it to come into alignment without the spring. That creates a certain amount of drag, which is eliminated or diminished if the spring is there already aligning or trying to align the surface. So, as long as the spring isn't too strong and “over-aligning” the elevator, it seems that you would usually have less drag with the spring. I believe that the opposing forces and drag created are variable, and at any particular condition there might be more, or less, or even the same amount of drag, with or without the spring. “If you like your spring, you can keep your spring.” ;)

[Esser]

You are right that accuracy is questionable. A better way would be to measure on the ground with a spring scale the actual force applied by the spring assist...

Can you remove the spring for a test flight?

[jiott]

Yes I could easily disconnect the spring, but it would be impossible to compare two separate flights with exactly the same power setting, etc. if you are looking for only 1 or two mph differences.

[jiott]

I think you may be right John, as long as the spring exactly offsets the gravity effect on the elevator it may reduce drag somewhat. In my thinking I was not considering gravity. Apparently Kitfox has done a pretty good job of designing a proper spring force because with flaps up it appears to be quite close to offsetting gravity. A little hard to tell due to friction in the system. Yes, an interesting discussion, and I think we all understand this is a molehill and not a mountain.

[n85ae]

Here's some food for thought.

When you pull in flaperons with the spring, you are deflecting the elevator
to alleviate the back pressure. This changes the camber of the tail to increase
the downforce. Which to the pilot feels good (i.e. no back pressure), but
is this really a good thing? I think it is an artificial good feeling.

Now with that spring increasing the camber, and removing that back pressure
if you then next trim out the pressure the spring is holding, you are changing
the tail incidence back into a less likely to stall angle. So everything is good
(in this condition).

The downside is ... That if you don't trim, and the spring holds the pressure
that camber is increased, and you are much closer to stalling the tail. Which I
believe is what happened to me, when the tail likely flew into a bubble of
turbulated air.

So I am asserting that the spring is a bad thing, because you trick yourself
into thinking the plane is trimmed. Which if you fly along and don't then
trim out what the spring is holding you're on the edge of a tailplane stall.

No matter what the tail can stall at some point, but I think that with
increased camber and a high loading the stall point is at a lower number
than the flatter tail at higher angle of attack.. I might be wrong, but
that's what I believe based on my experience.

In any case my plane flew so horribly with 22 degrees of flaperons
since the control harmony is so bad I've never missed the changes
anyway. I can get down scary fast using a slip anyway so it's not really
needed.

So my humble opinion is that the spring assist is a bad thing.

[jiott]

No matter what the tail can stall at some point, but I think that with
increased camber and a high loading the stall point is at a lower number
than the flatter tail at higher angle of attack.. I might be wrong, but
that's what I believe based on my experience.

Yes, the answer to this question is I believe the crux of your scary incident. If you are right, then the spring may have increased the onset of tailplane stall; however, I don't think the spring effect is nearly as much as you may think. If it really puts us all "at the edge of tailplane stall" then surely there would be many more incidents such as yours. More than likely you experienced a rare combination of flap angle and air turbulence that caused the stall with maybe a very small amount of increase from the spring. I do tend to agree with your reasoning though.

[n85ae]

I think the heavier nose IO-240 is a different airplane. With 22 degrees
flaperon you cannot trim out the back pressure, the spring just fools
you. I only jumped in to share my thoughts, likely with a Rotax there's
no problem ...

Jeff

[jrevens]

You may be right, Jeff. If you are flying with the CG closer to the forward limit than is usual for the lighter engines, then there is more downward force necessary from the tail assembly to trim the airplane. Since the tail is more heavily loaded to maintain that needed force, and at a higher angle of attack to do that, it seems it would be closer and more vulnerable to stalling for whatever reason.

[jiott]

I have enjoyed this discussion, have learned some things.
FWIW, my SS7 with Rotax 912, I cannot trim out all the backpressure even with the spring assist and 22 degrees (full) flaps. Half flaps no problem. As I mentioned on another thread, I screwed in my ballscrew rod eye so my horizontal stab goes all the way to the bottom of the slot in full nose up trim. This gets me close but not all the way to trim out full flap back pressure.

[Floog]

Do wheel landings. Then you won't have to pull so hard:)

[jrevens]

Any tips for stretching the spring to install it? It looks like a pretty hefty spring and a pretty good stretch. Also, what position are most using - the greater, or lesser distance between holes?

[ken nougaret]

I think i used a heavy duty tie-wrap to assist in pulling on the spring.

[bumsteer]

Hi John

Not familiar with the system (yet) so not sure what your dealing with but you could try what we use in gunsmithing. Take a cheap screwdriver and using a cutoff wheel in a Dremel tool cut a V or U notch in the blade and use it on the hook of the spring.

Rick

[jrevens]

One more question, for the Kitfox Aircraft crew... does the new Speedster that Kyle Franklin flew have the spring trim assist?

[ofergd]

Would this work on a IV? If not, why not. Same mixer and linkages as a 5-SS, right?

I think the difference is in the flaps operating handle. In the IV it is held by friction and in the others it is locked in different designated positions.
If the friction is not enough, the flaps will be lowered when pulling the stick.
If you can have a locking mechanism for the flaps, you probably can install it on the IV.

[ofergd]

Jeff brought up a point that got me thinking; when you trim the horizontal stab for hands off straight and level cruise flight with the spring assist what you are really doing is trimming for a constant back pressure equal to the spring force. Seems like this would add some drag to your cruise efficiency. I realize that the spring pressure is small with flaps up (the mixer system adds more spring back pressure as you add flaps, which is when you need it) but with flaps up there is still some spring pressure.

I disagree.
Well, it depends. If you have the full horizontal stab trim, you might be actually adding some drag. The spring pulls the elevator up from its' trimmed position and to compensate for that, you trim the stab down (leading edge up). this change of angle from the trimmed position might add a little drag.
BUT, if you have the small trim tab on the elevator, like I have, the spring reduces the required angle of the trim tab, and this reduces the drag.

[Av8r3400]

I think the difference is in the flaps operating handle. In the IV it is held by friction and in the others it is locked in different designated positions.
If the friction is not enough, the flaps will be lowered when pulling the stick.
If you can have a locking mechanism for the flaps, you probably can install it on the IV.


I put in a gated flap control since I added a trim tab to the elevator...

http://www.avidfoxflyers.com/uploads/monthly_03_2013/post-36-13631308224694.jpg


So, In theory, this should work on the way I have my IV set up.

[jrevens]

I disagree.
Well, it depends. If you have the full horizontal stab trim, you might be actually adding some drag. The spring pulls the elevator up from its' trimmed position and to compensate for that, you trim the stab down (leading edge up). this change of angle from the trimmed position might add a little drag.
BUT, if you have the small trim tab on the elevator, like I have, the spring reduces the required angle of the trim tab, and this reduces the drag.

ofergd,

Are you disregarding the relatively heavy ,unbalanced elevator, on the newer models especially, and the effect of gravity that pulls the elevator down, which must be overcome by aerodynamic forces, causing additional drag? Ideally, I would think that the spring could eliminate or diminish at least some of that additional drag. I think that the spring probably doesn't necessarily "pull the elevator up from it's trimmed position", but locates it in a more trimmed position to begin with. My understanding is that the spring should actually bring the elevator to the streamlined position on the ground, overcoming that "gravity droop", for lack of a better term. I'm certainly no aerodynamicist, and I'd welcome discussion of the "issue". Of course if you fly inverted it might not be so desirable. :confused:

The elevator is really quite "heavy" on the ground... it reminds me a little of a Skybolt I flew once. It took so much force to level the elevator when standing still without the engine running that I thought something was wrong. Of course I was used to flying many hours in my Thorp, which has a wonderfully light and balanced stabilator.

[ofergd]

You are probably right. the weight of the elevator does increase drag.
But that is only the Shape Drag. For cruise speed you can win some speed with the spring system.
In slow flight, the main drag component is the induced drag. The induced drag does not change because it depends on the total drag that the horizontal stab and elevator create.
In my plane the problem is not having enough trim when the flaps are down. It's a short time in the overall flight so the effect will be minimal if not installing the kit.
I have a classic IV.

[jdmcbean]

One more question, for the Kitfox Aircraft crew... does the new Speedster that Kyle Franklin flew have the spring trim assist?

Absolutely!

[jrevens]

Thank you, John. BTW, installing the spring was no big deal... I made a hook out of a piece of piano wire and stretched it right on there. Don’t know why I thought it might be more difficult.

[Flybyjim]

What model Kitfox were you flying when this happened?

[Frontier Fox]

2144421445
Help TKF. I had a little time today to take care of some odds and ends. I adjusted the trim actuator and followed the procedure for setting up the elevator push/pull tubes. Went ahead and bonded the trim position sensor bracket also. Hey so far so good.
I thought I would install the Trim Assist Kit but ran into a snag.
The bracket that was in the Trim Assist Kit looks nothing like what is pictured in the instructions. The half round part on the bracket that I have does not fit the curvature of the bellcrank and I see no way to attach the spring.
Did I get the wrong part in the kit? Am I missing something?

[Esser]

Wrong part. I had that one sent to me too

[Frontier Fox]

Thank you Esser. That’s what I was thinking, glad someone could confirm. Looks like an email to KF is in order. Do you happen to know what that part is for?

[efwd]

I can't recall where you are at in your build but thought you could use the advice that John McBean gave me. Do all the flight control rigging without this connected. After rigging is complete throw the spring on. He says it just complicates the rigging process.

[Frontier Fox]

efwd
That makes good sense to not connect the Assist yet. I was just putting the spring “hangers” on for now.
Any idea what the part is that was supplied with my kit?

[Brandon Petersen]

Joe, that part you have is for an older model....I think 3 or IV....for the under panel choke mount. Do send an email and we'll get you fixed up. We had a couple of those pre-packed kits pulled that were pulled from the bin right next to the one you need.
Brandon