A Funny Thing Happened on the way to the Forum

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marathonrunner
Posts: 449
Joined: Mon Dec 13, 2010 3:49 am

A Funny Thing Happened on the way to the Forum

Post by marathonrunner »

This will be the first of two posts. I Will try to keep them both limited to the Readers Digest Version as opposed to the Michener version but, there is some technical stuff involved. I almost named this post “The best way to hide an elephant is leave it in plain sight”. This concerns what we learned while getting the slant tail172 series added to our existing STC for 170’s and straight tail 172’s on Aerocet 2200 floats. This should have been discovered shortly after 1925 when Earl Dodge Osborne (Edo), finally took floats out of the plywood age with aluminum.

After much company testing and paperwork we finally were ready for flight test of the Aerocets on the slant tail172G. The paperwork process was continuing at a mind numbingly slow pace. Think running in frozen molasses. While we still had time to kill the engineer involved with us said “Hey why don’t you put spray rails on”.

What appeared to be an easy deal turned into a major project. I mean walk around any float pond and all aircraft on floats have spray rails, herein referenced as to what they really are which is bow strakes.

Many of the same model aircraft with identical floats have strakes that are long, short, wide, thin. Some go back as far as the front spreaders and a few went all the way back to the step. Pondering this I conferred with several engineers and we determined that realistically, you didn’t need them any longer than to the prop face as you are moving forward and the bow wake isn’t going to reverse direction.
On floats you often see ventrals on the fuselage as well as strakes on the sides to help with directional stability. Per CAR 3.118 you have to pass a directional stability test similar to what I call a Sky Brodie. Sort of like going into an icy parking lot with a rear wheel drive car and going like a bat out of somewhere and hitting the brakes while cranking the steering wheel to the stop. In a sky Brodie at various speeds and flap settings, you hold wings level with aileron and input full rudder. You then release the rudder and it has to show a tendency to correct. This means the ball comes off the glass.

We had excellent directional stability in our flight tests with the ventral we added We made our first strake 3 inches aft of prop face and 3 inches below the chine. Taxi tests great. Greatly reduced bow wake in all conditions and wind. Airborne all tests passed except, the sky Brodie. Failed miserably with totally negative directional stability. Back to the drawing board. Incidentally when testing new stuff you use aluminum or Shur tape until you get the desired results before attaching permanently.

Next strake/spray rail at prop face and 2 inches below chine. All tests positive except directional stability which is now neutral. I’m about to throw in the towel and go without strakes but I reduced depth to 1.5 inches below the chine. Taxi tests still same bow wake and we finally passed the sky Brodie test. I tried with 1/2 inch but the bow wake was a bit higher. We stuck with 1.5 inches

Again I pondered all the different strakes on aircraft. I know sometimes at the end of a day your knees did have a workout. I do have the entire IPC’s for Edo’s and several for other manufacturers. To my amazement Edo has 3 that show bow strakes. Their amphibious 185 and 206 float and the tubular gear 172. No others show strakes. What this means is that for nearly 100 years they have been altered at the whim of whoever made the strake decided to do.

The strakes not only affect the aircraft directional stability, they are an alteration. Here is what FAR 43 says regarding alterations

Appendix A to Part 43—Major Alterations, Major Repairs, and Preventive Maintenance

(a) Major alterations—(1) Airframe major alterations. Alterations of the following parts and alterations of the following types, when not listed in the aircraft specifications issued by the FAA, are airframe major alterations:

(i) Wings.

(ii) Tail surfaces.

(iii) Fuselage.

(iv) Engine mounts.

(v) Control system.

(vi) Landing gear.

(vii) Hull or floats.

Hulls or floats are considered a major alteration and for good reason. They do affect flight characteristics.

My next post will involve how floats have serial numbers and should have any STC or alteration properly recorded. Where do we go from here? The great American way of if a little is good more is better usually is not correct. In this case it is really not correct. I do not think the FAA should start busting people because this has been going on for nearly 100 years under their noses. I doubt they would anyway. I had an inspector harass me into getting a one time coordinated field approval for floats on my plane as the engine STC was for landplane only. I have never heard of anyone else ever having that problem.

What I do suggest is to remove your existing strakes including the angles they attach to. Those small angles are larger than wing cover shredders/vortex generators. Start with either a large area one and gradually reduce the size until you still get good bow wake. You could also start small and increase the size until you get a good bow wake. I do not think you need to flight test for directional stability unless you have a qualified test pilot along. I used my iPhone and another pilot to test and video the bow wake. You could also tape on a GoPro

Whatever method you choose once you get good taxi results, then permanently attach your new strakes. They will be effective at reducing prop spray without compromising directional stability.
It's not done till it's overdone
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