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Tip: Flap Down Limit Switch

Pmerems

Well Known Member
Advertiser
I installed a down flap down limit switch on my RV-7A and have been flying it for almost 5 years now without any issues with the limit switch setup.

One reason I wanted to install the switch was to limit the flap extension so that I never have the dreaded bent skin condition when someone bumps an extended flap, the under lapped flap skin pops up from under the top skin of the wing and the flap is then retracted. That is a bad day.

So I have posted the link to my installation here so it might help other builders considering putting a limit switch in.

http://www.experimentalaero.com/Flap down limit switch.pdf

It is also on my website under the Builders Tips tab.
 
I like it. Clean approach. You simply put the switch in series with the motor, right?
 
Nice setup but not sure why it's needed, you can adjust the actuator so you have the correct travel without any special limit switches.
 
Hmmmmmm....

Nice setup but not sure why it's needed, you can adjust the actuator so you have the correct travel without any special limit switches.

Hey Paul.....I like the setup and install.....nice job:). But Walt makes a good point so why add the complexity to simplicity? Although in some countries it MAY be a regulation of THE DEPARTMENT OF REDUNDENCY DEPARTMENT :D
 
Why

As I mentioned I wanted to limit the down travel to prevent inadequate flap skin underlap to the top wing skin. There have been many instances of RV's who's flaps were down and were "bumped" by someone getting into the aircraft. When bumped the flap flexed and part of the flap skin "Pops" out from under the top wing skin overlap. Since some of the flap skin is over the top skin and some is not, when the flaps are raised the top skin overhang is bent up and damaged. I personally have seen this damage on a friends RV-7A.

Walt,

I disagree with your statement that you can set up the travel with Van's supplied parts. The geometry of the flap torque weldment (don't recall the part number) provides too much flap travel and can cause the underlap/overlap issue if the flap is bumped as I mentioned, this can not be corrected with pushrod lengths etc. Some builders have shortened the actuator mounting arm to correct for this. I don't know why Van hasn't changed it. Maybe the RV-9 needs more flap travel or Van doesn't see the bumping the flap as a real issue. However for those with top wing skin damage, it is a big deal. However in my case (builder number 71147) the weldment provide too much flap travel.

One thing I failed to mention in my explanation is that I retrofitted this to my RV-7A that had several hours on it. I only had two wires (power/ground-reversing) going to the flap motor and didn't want to run more wires from the switch on the instrument panel to the down limit switch per "standard" limit switch/reversing switch circuit design. So I was able to install the limit switch with diodes that allowed the use of the existing two wires.

I hope you find this helpful.
 
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General comment on the subject.....

I know there has been instances of this problem over the years but I would say it is certainly not common.

Assuming it is related to operating conditions (inexperienced people being around the airplane) then we can assume that Van's demonstrators on display at major fly-ins (Airventure, etc.) would be the worse case for the number of people they are exposed to.
Case in point...
Brand new airplane (RV-14 taildragger) is put on display at OSH for a week and returns home with the paint completely rubbed off of the trailing edge corners of the wing tips by people cutting the corner too close and rubbing against it.

Over many many years of this a displaced flap has not been a problem.

How to prevent it?
1. DO A PRE-FLIGHT INSPECTION
2. Supervise people getting in/out, and instruct them on the importance of not stepping on or pushing against the flap.
 
I agree

I know there has been instances of this problem over the years but I would say it is certainly not common.

Assuming it is related to operating conditions (inexperienced people being around the airplane) then we can assume that Van's demonstrators on display at major fly-ins (Airventure, etc.) would be the worse case for the number of people they are exposed to.
Case in point...
Brand new airplane (RV-14 taildragger) is put on display at OSH for a week and returns home with the paint completely rubbed off of the trailing edge corners of the wing tips by people cutting the corner too close and rubbing against it.

Over many many years of this a displaced flap has not been a problem.

How to prevent it?
1. DO A PRE-FLIGHT INSPECTION
2. Supervise people getting in/out, and instruct them on the importance of not stepping on or pushing against the flap.

I agree that a preflight could catch this condition.

When I initially flew the plane I made sure I had a good underlap of the flap to top skin. However since others had experienced the "bumped" flap and corresponding damage, I felt it is better to have a robust preventive option rather then if for some reason I didn't realize it had occurred and I retracted the flaps. We all have our senior moments......
 
Flaps Up Limit Switch

I set up the stroke on the flap actuator to give 37.7 deg. as Walt has indicated. This seems to provide plenty of engagement at the wing before the flap pops out. Will have to preflight to make sure no one has stepped on the flap and bent it enough to pop out.

Added a up limit switch (ala Vern Little) since the up switch is engaged until you remember to centre it. With cockpit noise I'm sure I would have forgot to do this sooner or later. Diode on the Molex connector is for reversing.

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Alternate Solution

I was planning on adding the limit switch but decided to explore the
internal workings of the actuator. I found the travel can be shortened
by relocating a roll pin in the jack screw. In my case I moved it about
1/2". Not difficult and no additional parts required.
 
I have one similar

Someone bumped my flap while I was working in the hangar one day. I was lucky that I heard the binding of the flap motor when I went to raise them and prevented major damage. I still had to borrow Van's special tool to get the back edge of the wing skin back down a little bit. It could have been disastrous if you look at some of the older postings with wrinkled wing skins on top of the wing.

I use the Showplanes positioning system (works great!) so I built something very similar to Paul's setup to limit the total travel and have had no problems since then.

Andy
 
I opened up the actuator and relocated the split pin as mentioned by Kent N in a previous post. I moved it 5/8" which yielded a reduction in flap travel of approx five degrees. With full down flap I am at 39 degrees and have 7/16" still engaged under the top wing skin.

I couldn't get a drill bit to start drilling on the shaft. Suspecting it was hardened, I took it to a machinist friend of mine who used his milling machine to drill the 3/32" hole. The shaft was case hardened. He secured both ends in a fixture so it wouldn't move which was also part of my problem on the drill press.

It took a little time, but no limit switches or wiring and up and down travel are right where I want it.
 
I have a RV6A that I purchased a few months ago. I'm doing the first condition inspection and one thing I also noticed was the electric flap travel was way too long causing over extensions on both ends. Opening up the jackscrew and relocating the roll pin did the trick. I shortend the travel by 1inch which gave me a 40 degree extension. This is much better than trying to limit extension with switches and brackets. The jackscrew is hardened so I took it to the local machine shop for the new hole.....10 bucks later.... I had a new 3/32nd hole for the roll pin.

Thanks for the info from the two RVers who did it before......it's a great solution to a problem Vans should look into.

Russ
 
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thanks for posting this. I guess I will do this once I have the wings on and find out my flap travel. Or I might add some length to the arm on the yoke, which ever is easier. I have seen several bent flap skins on RVs. It is an issue. I can understand that VANS buys some standard part that probably worked on their prototype, but all of those early airplanes were a bit different. We are homebuilders, we can adapt!
 
I have a RV6A that I purchased a few months ago. I'm doing the first condition inspection and one thing I also noticed was the electric flap travel was way too long causing over extensions on both ends. Opening up the jackscrew and relocating the roll pin did the trick. I shortend the travel by 1inch which gave me a 40 degree extension. This is much better than trying to limit extension with switches and brackets. The jackscrew is hardened so I took it to the local machine shop for the new hole.....10 bucks later.... I had a new 3/32nd hole for the roll pin.

Thanks for the info from the two RVers who did it before......it's a great solution to a problem Vans should look into.

Russ

FYI, the flaps deflect upward by aerodynamic loading so if you measure the travel in the hangar at 40 deg., you will not have that much in flight.
 
I agree.....inflight it looks like about 37 degrees based on my flap markings....which is fine with me. The piece of mind comes on the ground, with the flaps extended, and are accidentally bumped and I don't catch it. Hasn't happened yet. But with the way the previous owner had them set, it would have.
 
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The limit switch, at least in my case, would have electrically stopped the flaps from extending too far due to the flap drive travel being too long. And with me, I leave the flaps down for ease of entry into and out of the airplane. People tend to "bump" the flaps getting in or out which may push the flap out of it's position. The primary problem is the flap drive is too long. IMO, it's better to mechanically limit the flap travel. This may be unique to the RV6.....I'm new to the RV scene.
 
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I have a RV6A that I purchased a few months ago. I'm doing the first condition inspection and one thing I also noticed was the electric flap travel was way too long causing over extensions on both ends. Opening up the jackscrew and relocating the roll pin did the trick. I shortened the travel by 1inch which gave me a 40 degree extension. This is much better than trying to limit extension with switches and brackets. The jackscrew is hardened so I took it to the local machine shop for the new hole.....10 bucks later.... I had a new 3/32nd hole for the roll pin.

If you alter the jackscrew, it should be labeled as to what was done so it doesn't get replaced with a stock part later on.

If a limit switch is used, then the jackscrew can be switched out as a direct replacement.
 
Flap electric actuator disassembly - how ?

I've concluded, similar to the authors of posts 10, 12 and 13 in this thread, that the stock electric flap actuator travel is too great. This is for a flap actuator purchased 10 years ago for an RV-6A. I would like to relocate the pin on the actuator screw that limits the actuator travel. However, I cannot figure out how to remove the outer (aluminum) tube from the actuator gearbox in order to expose the inner (hardened steel) screw. The outer tube appears to be press fit into the actuator gearbox. How did you remove it? Thank you.
 
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