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.

[Bruce Fenstermacher]

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.

While I have not actually laid a A model horiz-stab on top of a B model horiz-stab, nor compared each and every internal part, I have looked at both visually and always thought size and shape wise them to be nearly if not identical. The major differences between the two being the A model and B model have a different placement of the hinges and the trim tab on each corresponding elevator is completely different. You can not physically attach an A model elevator to a B model stab and visa versa. The trim tab for each elevator is completely different and it would not surprise me to find a few different parts in the trim tab actuator which is part of the horiz-stab assembly. The '48 170 and the A model have the same horiz-stab and elevator, one reason for the 03 series part number vs the b model 05 series part number.

Probably one of the last old school folks with well over 40 years doing deep repairs and perhaps swaps of components between models of 170s is member Bruce Kown who, last I knew 6 months ago, owns 7 - 170s and major parts such as wings and the wing jigs to do major repairs to them both A and B model.