A Model/B Model wing swap

[skylking]

Anyone have any knowledge/experience putting B Model Wings on an A Model? How about the practical aspect and legal side. Can this be done with a field approval or does an STC exists? Thanks for any advice or opinions.

[GAHorn]

No STC that I know of but STC researching is a tricky endeavor and there may be one “Hidden” somewhere. There ARE some A model fuselages which have been found with B wings and the obverse also… but documentation is scanty at best and to be in-accordance with Hoyle you’d need a field approval.

The struts are different, and the flap cable routing is VERY different and I think it’d be easier to buy a 170 B model if that’s what you want.

[cessnut]

I would want to consider what might be different in the tail to compensate for the pitch effect of the large flaps.

[GAHorn]

I would want to consider what might be different in the tail to compensate for the pitch effect of the large flaps.

THAT…is an excellent point!

The B-model and the L-19/O-1 Bird Dog were developed contemporaneously and the elevator/horiz-stab (H-Z) changed during that development, likely as the result of the effect the DOWN-wash of the large flaps of both L-19/O-1/170-B (Cessna model 305 was the actual designation).

Here’s a short description of the relationship:
The large flaps create a DOWN-WASH of air-flow ahead of the horiz-stab/elevator. The A-model was likely found to have insufficient authority as the result. (The elev/horiz-stab behave as an Up-Side-Down airfoil creating DOWN-Ward lift to offset the C.G. which is forward of the C/L of the wing.)
This relates to the warning to “Avoid Slips with Full Flaps” on the B-model… because the DOWN-Wash ahead of the H-Z has the same affect as an INCREASE in AOA for that flight control surface. This means the HZ and elevator are brought closer to a STALL when full flaps are deployed.

IF… a SLIP is introduced at that moment…the fuselage will BLANK the downstream HZ/Elevator…and the remaining half of HZ/Elevator will Lose Control to keep the nose “up”….resulting in a Sudden DOWN-WARD drop of the nose. Further complicating the issue is the Applied Rudder (which introduced the Slip) … Resulting in ROTATION about the ongitudinal axis….(Think of a SPIN).
If this is down on short-approach to landing… Recovery can be IMPOSSIBLE.

As for usbstituting B-model wings/flaps onto an A-model fuselage/tail….Cessnut makes an interesting point: The Tail-Feathers might have insufficient authority. This would be an Excellent Reason for an Astute FAA Inspector to deny approval on a Form 337.

I believe Bruce F. once owned a similar airframe (an A-model fuselage with L-19 wings. I do not recall if it had B-model/L-19 tailfeathers or not. Perhaps he’ll chime-in.

[Bruce Fenstermacher]

I had a '50 A model with L-19 wings, B model flaps, cable run and Johnson bar and B model struts. It also had B model vertical stab and elevators. There was paperwork for 172 wings (which turned out to be L-19) on a 337 and that is it. No paperwork mentioning any of the other changes required. I believe member Bruce Kown had or maybe still has an A model with B model wings but A model elevator. He also had a Franklin in it. As for frankenstein 170s I've seen a B model with A model struts for no dihedral as well.

The swap is not easy as you must change the cable run from A to B for the flags and that requires a rear carry through spar change with the pulleys.

The major difference between the A and B model elevators is the B model is balanced. The B model elevator also has a few more square inches of area. Today you'd probably have to show the A model elevator with B model wings would meet certification but I bet it would.

[GAHorn]

This demonstrates one reason Bruce’s performance as a convention judge is so admirable.

(I suspect a dataplate swap was performed on one or more of the modified airplanes mentioned, a definite legal No-No.)

The horiz-stab is another difference: The A-model horiz-stab assy carries PN 033200-10 while the B-model is 0530001 indicating significant differences.

ESPECIALLY SIGNIFICANT is that the Angle-of-Incidence of the A-model Horiz-Stab is minus 4-degrees while the B-model is minus 2-degrees 48-minutes. This means that major-alteration documentation and approval is necessary and becomes especially important during stall/spin recovery.