Adjustable Flap Stops

These are very light adjustable stops which can be riveted to the Sonex or Waiex rear spar to set a rigid upper limit to the flaps-up position.Flap stop 1

Mass is only 6 grams which is pretty close to nothing, but they have a big effect.  Stops are supplied with a lock-wire lug and large-head aluminium rivets.

Also available as a printed part incorporating a lock-wire hole. Printed stop 1Printed stop 2

These have been compression  tested and the body is stronger than the same nylon screw that is used in the all-machined stops.  They have not been endurance tested whereas the machined stops on my Sonex are 4 years old and look like they should last for many years.

Why they work

One of the modifications I made very early was to fit adjustable flap stops.  On my first flights I noticed a tendency to roll right.  I tried adjusting the right flap to give more lift, 1/2 turn out on the rod-end, eventually going to 1 1/2 turns out.  The trim was improved but didn’t “feel” right.  I guessed that the problem was torsional deflection of the flap drive shaft.  I know the plans say to fit trim strips which act as flap stops, but how do you get them right in the first place?  Once you have riveted them on, that’s it.  I machined these boss and flange parts from nylon and fitted 8mm nylon screws.

One of the flap stops on my Sonex
One of the flap stops on my Sonex

There are 2 riveted to each rear spar, although I probably could have got by with just one on each  side.  The effect was noticeable immediately.  The aircraft felt more stable.

Now if you don’t believe me on this, here’s a little experiment you can try:

Set the flaps at 10º, 20º or 30º.  Go to the left flap and lift it by the trailing edge enough to take up any slack, then lift until it deflects say 1/4″.  Now try the same thing with the right flap.  You will notice that the right flap is easier to lift.  If you want to be really objective you could use a small spring balance to measure the force to deflect the trailing edge equal amounts.

I tried this on my own Sonex, lifting the trailing edge by 1/2″.  The results were:

Left flap 20 pounds       Right flap 11 pounds.

If you have the Sonex flap stops (light aluminium angles riveted to the fuselage sides) fitted, then your aircraft should be stable, but if you don’t then the trim will vary with any changes in wing load or pressure distribution.

What flap stops will do:

Make the Sonex feel more stable

Help fix minor rigging errors

What the flap stops won’t do:

Fix major rigging errors

Fix a “heavy” left wing

Fitting the adjustable flap stops

Adjustable Flap Stops Mathematical Proof

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