Inexpensive trim for those using flaperons

[RIVERFOX]

Tired of hold forward stick pressure for those of you that have only the flaperons for pitch and losing that 5-8 mph? Having sailed catamarans I got the idea to use "Harkin brand" sailing pulleys and a "Harkin" mini cam cleat and a 12" length of approx 5/16" bunge material and apiece of high quality 3/16 sailing line. All the items can be gotten at a sailing shop or on line.You'll have to play with the exact length and thickness of the bunge but with a little trial and error you can trim your plane at MAX cruise and still be able to easily overide it with stick pressure alone. However with the mini cam cleat you merely have to pull down to disengage the whole trim. The neat thing about the cam cleat is that once at cruise and the rope engaged to the cleat you can just pull the rope through the cleat until you've cancelled the stick pressure. This will only eliminate the aft stick pressure from cruise speeds, however I find find that is when I need the trim the most. See the attached photos.

[t j]

Kurt, I like that! It's started some wheels turning in my head. The simpler the solution the better.

I use flaps for pitch trim. My horizontal stab is rigged for hands off pitch trim in level flight with nose tank full and just me in it. As the nose tank fuel is burned I need to add a little nose down trim. Also, any other weight, ie fuel in the wing tank, passenger or baggage is added the cg moves aft so more nose down trim is needed.

Here's a question maybe you can answer with your pitch trim system. How does the drag from using flaps to trim nose down compare to drag from using elevator to trim?

[RIVERFOX]

Hi Tom,
If I cancel stick pressure with flaperons and I'm indicating 90mph then take off the flaperons and use my bunge trim, I see 95-96 mph IAS. If youre asking if the adjustable trim system on the elevator is more efficient I can only say that other Kitfox owners that have gone from flaperons to trim system on the elevator have similar increases as my findings. The big difference is $40-$50 and an hour or two of work or $400-$500 and a lot of labor.

[t j]

Okay that makes sense. I have heard that the most efficient horizontal stab rigging is one that requires a slight forward stick pressure for level flight. yours confirms that.

[jtpitkin06]

I think you can answer this one yourself.

If you need to trim nose down, you have too much down force (drag and dynamic load) on the horizontal stab. Using flaps to move the center of lift does not get rid of the stabilizer down force it merely counteracts it.

If you trim the elevator with bungees you can reduce the down force on the tail, reduce dynamic load, fly with a lower angle of attack and fly faster.

You can prove it yourself. Set up in cruise using flaps for trim. Note the speed. Then take the flaps out and hold the stick with your hand. See where the speed stabilizes.

John Pitkin
Greenville, Texas

[carlisle]

This is a pretty interesting basic subject. It addresses a common principle in design that I found interesting when I first read about it in Flying Magazine many years ago and later on in some basic texts.

Most aircraft, in order to be negatively stable in the pitch axis, are designed so that the horizontal tail in it's neutral position exerts a down force (nose up). This creates a situation such that if the plane is disturbed in pitch, it tends to return to it's normal trimmed condition fairly shortly (in 2 oscillations or less if certified by FAA). That's why when they pick up the pieces of an airplane that's flown into a thunderstorm and broken off it's tail, the wings are almost inevitably failed with downward forces. Any changes in CG cause this to vary and thus a method of trim adjustment is required to maintain neutral force.

So to me, as inconvenient as it is, a little down pressure on the stick in cruise kind of reassures me that things are the way they should be. If I trim out the elevator forces by means of a tab on the trailing edge of the elevator or by a bungee on the push tube, the same thing is accomplished. The basic stable condition is still supplied by the remaining area of the horizontal stabilizer ahead of the elevator.

Anyway, I too have been looking for a simple and less expensive means to do this and this bungee thing looks cool. I've never liked the idea of shifting the center of lift to accomplish trim as it seems like a way of defeating the stability that's designed in and is a drag penalty too. Safe as well since it's probably pretty easily overcome with a little backwards stick pressure and doesn't limit the elevator travel significantly as a result.

Chris Carlisle
Model 2, 582, C box
Sioux Falls, SD

[carlisle]

Well here's a little twist.

Have almost got my bungee trim system, plagiarized from Kurt, finished. Went up and tried it out and it worked great. Today was the first decent weather for quite a while here. It was nice and smooth. Got it trimmed out, 3500 msl, 5800 rpm, and 83 mph indicated with the flaperons neutral.

Then decided to try the old way so released the bungee and trimmed it the old fashioned way with the flaperons. Same conditions, 3500 msl, 5800 rpm, and now....81 or 82 mph.

Not much difference. Hmm. Maybe Dan Denney was right and the flaperon trim system was ok after all.

Oh well, it was a fun experiment and I'll bet others with a faster configuration like a model 3 or 4 with a 912 that routinely cruises a little faster would see a larger difference. For me, it's the usual low and slow fun.

I guess that's why they call them experimental

Chris

[cap01]

chris , ive been doing the same thing the last few times i had the kitfox out . ive never been balls to the wall but cruising in the low 90s and on several occasions have never seen more that 3 mph improvement by holding the nose down with the stick . the stablizer is in the center hole and ive never played with moving it to see if i can get closer to a no stick pressure cruise . i still think after i get a gas tank replaced the next project will probably be to install the rac pitch trim .

[carlisle]

If I had it to do over again, I'd go ahead with the electric trim tab too, even if it doesn't make that much difference. I still think the aerodynamic principle is sound and unloading/destressing the airframe and flaperon hanger bearings seems like sound mechanical sense too. I'm leaving my bungee system in for that reason and also because it looks so simple and elegant.

Chris

[RIVERFOX]

Chuck and Chris my 1050-4 must be a very early model as I have no choice of holes to change my angle on my stabilizer. My stick pressure at 90 is 7-8lbs ! I must be using a lot more flaperon to offset the large amount of pitch up. Chuck what is your serial #?