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.
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.
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