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PA-18 lift struts on a PA-12

I'm sure that you did. There are a lot of folks here who just do not understand of how one change can alter some other component, with consequences. Those are the ones whose attention I'm trying to raise.
 
As it sometimes happens, this thread has gotten a little off topic. Read the OP again:
" Without sparking a debate as to which is structurally better, does anyone have direct experience with this".
 
But this is a discussion, and it really does not have to stick with what the OP wanted.

It is true I am only looking at the lift struts. All you have to do is look crosswise at an unsupported lift strut and it will buckle. Not so a braced spar - it takes literally tons of force to move a spar in the vertical plane, and what keeps the load in that plane is the ribs and compression struts, not the lift struts. If you have never straightened a metal spar, you really cannot appreciate how strong and rigid they are.
 
Pete, my take on it, in a little more detail, for whatever it's worth. In a statically in determinant system such as this, where the struts affect the jury struts which affect the spars and the same in reverse, one has to take into account the relative stiffness of the contributing members. The stiffer the member, the greater its contribution to resisting deformation of the system. In this case, as Bob is arguing, the spars are much, much stiffer than the struts. Therefore, the spars tend to support the struts (via the jury struts), and not the other way around.

It might be that the -12 design was intended to provide some fore and aft support for the struts, but perhaps it was later learned that the struts are stiff enough on their own in that direction. Just wild guessing on that, but maybe?

In any event I would have to again ask; what are the differences in applied loads between 12 and 18 strut / jury strut / wing system, and again offer that there is none. So by withstanding the test of time, the 18 jury strut configuration has demonstrated its suitability in that system.
 
Bob and Gordon, I'm pleased that I have your attention with your thinking caps on. Picture this: A wing is attached to the fuselage. The lift struts are attached to the wings only, supported with string at the inboard end. You grab the outboard tips of the spars with each hand. Now twist the wing in the wash in/wash out direction. What happens to the strut fittings at the fuselage end of the struts? They move in opposite directions toward and away from the fuselage proving that the wing is not ridged but flexible in torsion, unlike a Cessna wing.

Now pin those struts to the fuselage and twist the wing tip again. It will be very rigid but still flexible. By placing the -18 jury struts this rigidity will not change as the attach point of the jury struts will move up and down in opposite directions. Granted the movement will be small.

Now attach the -12 jury struts to the wing only with the lower ends attached to each other only. Twist the wing again. What happens to the lower ends of the -12 jury struts? Will they move fore and aft in relation to the horizontal tube between the lift struts? I am suggesting that they do. I agree that it will not be a lot, but they will move some. So by next fixing these struts to the center of that horizontal tube between the lift struts, the torsional stiffness of the wing is increased since the greater chord of the lift struts is in the horizontal plane. Thus the struts will oppose this force.

Why am I making such a big deal of this seemingly minor change to the wing structure of a plane? Have any of you been involved in flutter testing? Flutter can and does kill, quickly! Sometimes a very small innocuous change can make the difference in whether a flutter does or does not occur. The change in stiffness of a wing could be one of those causes. I have witnessed the change in flutter critical speed of a plane with the only change being the rigid attachment of an oxygen bottle in the baggage compartment. I have had flutter occur during test flights. Please believe me YOU DO NOT WANT TO EXPERIENCE FLUTTER!

stewart's and now Scooter's -12 which is pictured above no doubt has a "Borer" prop which does not produce high speeds. This -12 also has flaps, big tires or floats or baggage pod or skis or?? all of which change the natural resonance of the airplane. Obviously the small jury strut alteration made no difference in this case. When the jury strut was changed, was the plane flown to Vd and the ailerons pulsed? I doubt it. Do you know what Vd is? Have you ever flown an airplane at Vd? More than likely the answer is NO. In case you are wondering Vne, which you all know as the RED line, is 90% of Vd. ALL airplanes are flown to Vd during flight testing. It CAN be very HAZARDOUS!

You all may think that I am just a kook from Massachusetts and you are entitled to those thoughts. I have experienced flutter while flying at Vd and please believe me it was not a fun experience. Only by the hand from above am I here to pass on this information. I have also turned down an unsolicited job offer by Piper's Chief of flight test. If I can make just one of you aware enough to save your life, my ramblings and your brickbats are worth it.

Another thing, as Gordon pointed out above, the Vne on the -12 is lower than on the -18. Does anyone here know for sure why? Here is a possible scenario. The wider fuselage, altered angle of incidence, more swept back windshield and other minor changes from a -18 may have shown the need during flight test that the Vd should be lower on the -12 than the -18. Perhaps the stiffening of the wing via the jury struts was an attempt to rectify this difference? Just saying. Do you still have your thinking caps on?

Now this is not what the original poster asked. Nor is it directly answering Bill's question. I recognize this. It does have your attention though. Fly safe.
 
During flight ops the struts and jury struts would be in tension, not compression. On the other hand, when the wind's are blowing from the rear the struts are in compression and will fold below the jury struts where the span is greatest. I've not seen jury struts deflect at all until after the wing folds. I've been a human strut a couple of times trying to hold a wing up while somebody strapped a 2x4 to the aft lift strut to protect a Cub wing. The load on the wing is surprisingly heavy when the wind is trying to fold it down. In that role you become very aware of the flex in the lift struts and the relationship between the forward and aft struts as well as the difference in rigidity between them. Wings would fold easier and in less wind without jury struts, no doubt.
 
stewart, I submit that the jury strut loads alternate between tension and compression during turbulent flight and that those loads are not high. The rest of your description is all based on applied loads when the airplane is on the ground. How thoroughly has your new Backcountry Cub been tested? I thought so. When you get it flying take it up to Vd and bang the controls around a bit. See what happens. What number are you going to use for Vd? Try it at different CG loadings. Hopefully nothing, but are you certain? My modified Backcountry Cub has been tested to 160 mph on floats. Piper's certified Vne is 138 mph (Vd 153 mph). Did you ever dive a 185 on amphibs to Vd and have it want to swap ends? Don't get me into telling more true stories.

This is an Airbus 380 Vd test for flutter. https://www.youtube.com/watch?v=s3-g9B6Fgjs

Watch the stabilizer on the Twin Comanche https://www.youtube.com/watch?feature=player_embedded&v=qpJBvQXQC2M You all have seen the pictures of the Tacoma Narrows bridge.

More: Read this and look at some of the attached videos: http://aviation.cours-de-math.eu/ATPL-081-POF/flutter.php
 
Pete, points well taken. I still maintain that the spars are so stiff in comparison to the struts that the struts can not significantly affect the spars' resistance to applied loads, except by acting in pure tension and compression; not in bending. However - - as you suggest, one of my favorite aphorisms applies - - "One good test is worth a thousand expert opinions."
 
stewart's and now Scooter's -12 which is pictured above no doubt has a "Borer" prop which does not produce high speeds. This -12 also has flaps, big tires or floats or baggage pod or skis or?? all of which change the natural resonance of the airplane. Obviously the small jury strut alteration made no difference in this case. When the jury strut was changed, was the plane flown to Vd and the ailerons pulsed? I doubt it. Do you know what Vd is? Have you ever flown an airplane at Vd? More than likely the answer is NO. In case you are wondering Vne, which you all know as the RED line, is 90% of Vd. ALL airplanes are flown to Vd during flight testing. It CAN be very HAZARDOUS!

You all may think that I am just a kook from Massachusetts and you are entitled to those thoughts. Fly safe.

You guys have way too much time on your hands and are overthinking things. VnE in my plane is not a reality let alone Vd or whatever. It couldn't make 130mph let alone VnE aimed at the ground with full power.

Here is my opinion: New FAA guys think just like you are, and that is why there are no easy field approvals. Call in the engineers and slide-rules, computer simulations, wind-tunnels, flight tests. and on and on and on. Just putting -18 jury struts on a -12 give me a break. Older FAA guys would say, no problem, where is your 337 because it made sense, or a few others have been doing it. rant off, have fun arguing about it. The days of sensible modifications and easy approvals are long gone. Maybe you are FAA?
 
Gordon, Another way to think of the situation is to use a tuning fork. If you were to touch the tuning fork at some point along it's shaft you would effect the vibration frequency. The jury struts act in a similar manner to the wing spars. The point of contact would not necessarily have a load applied.
 
Now you are worrying me. I have been using Vne on my Cub the same way we used it on the 737 - a limit for normal operations in smooth air. I never tried a loop entry at less than Vne, although I do not do loops any more - but I use Vne daily to space my aircraft on final approach. We have two turnoffs - one at 1000' and the other at the west end, and one never knows what turnoff the preceding 172 will use, so speeds vary from 39 to 121 to compensate. So far, so good.
 
Flutter analysis requires a very highly qualified aeronautical engineer who is at the top of the engineer food chain. Most engineers always defer to the specialist. I am not an engineer. My knowledge comes only from observation and participation at the very bottom of this food chain. That Twin Comanche video stopped before the final result occurred. I can speak further about that off the air.
 
Agreed. And testing can confound the most exacting analysis. Just for fun, consider that the -12 jury strut arrangement does couple the struts and wing more than the 18 arrangement, increasing effective mass as well as stiffness. Maybe (probably not, but a fun idea to toy with) the uncoupled strut / spar arrangement in the 18 could be less prone to flutter, hence the greater Vne. Again, I'm not suggesting that's true, just playing with ideas.
 
Flutter is nasty mysterious stuff. You may have a valid point Gordon, the slightly increased stiffness could be the reason for the lower Vne. It would be nice to hear from someone who does know. If anyone here knows Gordon Mandell in Anchorage, please ask him. I suspect that he would have the answer on the tip of his tongue.

I know of one incidence where the addition of just one 1/8" rivet on each side of a rudder solved a flutter situation.
 
Used to fly Part 135 freight in a C-310. One day, while getting in, a screw that held down overlapping wing fairing at the door entrance popped out so that the upper fairing stuck up about a 1/2 inch. I'm late, I can get another sheet metal screw at my destination I thought. No biggie, it barely sticks up. I got airborne and it flew in a constant skid. Could not center the ball and the controls were sluggish. I could not figure out what was going on. After all, a minor fairing can't have that much affect on the plane. I even did a low pass at an intermediate stop to see if the boarding ladder had failed to retract. Believe it or not, the problem was gone on the return trip after I replaced the screw. Could not believe it.

Don't get complacent about fairings and such. They actually are part of the airplane!!
 
Excellent article Gordon. It should be REQUIRED READING for ALL pilots. Read and understand! Read it when you have time to think and absorb all of it's content.
 
"VnE in my plane is not a reality let alone Vd or whatever. It couldn't make 130mph let alone VnE aimed at the ground with full power."

^^^^^^ This.

Won't comment on the debate, but I suspect for most -12's where the use of 18 struts might be considered, implications of Vne or Vd flight are theoretical at best.
 
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