bradp Posted April 11, 2018 Report Share Posted April 11, 2018 I was reading somewhere that on a Comanche that tip tanks increase max gross weight ... are there structural changes that allow the max gross weight increase? Empty tip tanks seem like useful load for free. Quote Link to comment Share on other sites More sharing options...
RLCarter Posted April 11, 2018 Report Share Posted April 11, 2018 They produce lift Quote Link to comment Share on other sites More sharing options...
jetdriven Posted April 11, 2018 Report Share Posted April 11, 2018 (edited) They reduce spanwise loading. Since the bending moments are changed, the aircraft can carry that fuel at the tip for free since it doesn’t increase the load on the center section. Its similar in effect to reducing wingspan. Edited April 11, 2018 by jetdriven 3 Quote Link to comment Share on other sites More sharing options...
AH-1 Cobra Pilot Posted April 11, 2018 Report Share Posted April 11, 2018 There might also be structural changes. I heard from B-52 aviators that in the 1970s they had an SOP change from burning the tip tanks first, to burning the tip tanks later. The reason is that the weight in the tip tanks keeps the wings from flexing quite so much. The added weight can result in several feet less of wing flex, contributing far less to wing cracking over time. Quote Link to comment Share on other sites More sharing options...
kevinw Posted April 11, 2018 Report Share Posted April 11, 2018 They increase the gross takeoff weight significantly but not sure why. In a Bonanza the UL increases by over 200 lbs if I”m not mistaken, making it a very popular mod. Sent from my iPhone using Tapatalk Quote Link to comment Share on other sites More sharing options...
Bob - S50 Posted April 11, 2018 Report Share Posted April 11, 2018 When you fly, the wings produce lift upward. That puts tension on the bottom of the wing and compression on the top of the wing. Weight from tip tanks act downward. That puts tension on the top of the wing and compression on the bottom (opposite of lift). So the weight on the tip offsets some of the forces created by lift, thus reduce stress on the wing during flight. If you were to plot the stresses during flight, you should actually see a downward stress (tension on top) starting at the wingtip but decreasing as you move toward the fuselage. At some point inboard of the tip there would be no stress on the wing. Inboard of that, the stress goes back to normal (tension on the bottom) but at a reduced level. 2 Quote Link to comment Share on other sites More sharing options...
carusoam Posted April 11, 2018 Report Share Posted April 11, 2018 We see a few things going on here... Static (not dynamic) load distribution... spreading the fuel weight out over the wing spar.... Aerodynamic end effect blocking.... keeps the high pressure air from escaping to the low pressure side... Added Power needed for T/O and climb... Stall speed may increase... or not? Can we apply this to a Mooney? Fuel weight distribution hasn’t been a challenge since Mr. Monroy did his work.... Could we lift more with more effective wing tip end plates... Can the rest of the fuselage handle the added weight under the usual crash landing test... Keep in mind the end effect is the last foot or two of the wing, about 1/10th the total lifting surface? Eliminating end effect could allow for a 10% addition of lift... If the added weight is balanced by the added lift the actual AOA during normal use doesn’t change. This would keep Stall speeds from increasing. If the added weight is just the outcome of added testing... Then additional AOA is needed to balance the additional weight... Stall speed is going to increase.... Compare to the speed and weight lifting ability of the LB... on the same wing as other Mooneys... If you load another 200 pounds onto the plane... without adding power... T/O runs grow a lot. climb rates decrease a lot. Stall speeds may increase a fair amount. Theoretical numbers for discussion by a PP, not an aero engineer... or CFI. Best regards, -a- Quote Link to comment Share on other sites More sharing options...
201er Posted April 12, 2018 Report Share Posted April 12, 2018 13 hours ago, Bob - S50 said: When you fly, the wings produce lift upward. That puts tension on the bottom of the wing and compression on the top of the wing. Weight from tip tanks act downward. That puts tension on the top of the wing and compression on the bottom (opposite of lift). So the weight on the tip offsets some of the forces created by lift, thus reduce stress on the wing during flight. If you were to plot the stresses during flight, you should actually see a downward stress (tension on top) starting at the wingtip but decreasing as you move toward the fuselage. At some point inboard of the tip there would be no stress on the wing. Inboard of that, the stress goes back to normal (tension on the bottom) but at a reduced level. What about parked on the ramp with tip tanks full? Considering airplanes spend most of their time parked, how does that affect compression? Quote Link to comment Share on other sites More sharing options...
Bob - S50 Posted April 12, 2018 Report Share Posted April 12, 2018 9 hours ago, 201er said: What about parked on the ramp with tip tanks full? Considering airplanes spend most of their time parked, how does that affect compression? The extra weight at the tips will put a larger tension load on the top of the spar and a larger compression load on the bottom of the spar. That would have to be considered when designing the tanks, but assuming the tanks even full of, say 15 gallons of fuel, would probably weigh less than 150 pounds, I doubt it would be a problem. The J is rated at -1.5G so that means each wing is stressed to carry a downward load of a bit over 2000 pounds without any permanent deformation. And it is probably able to carry another 30% over that for a design safety factor. Quote Link to comment Share on other sites More sharing options...
201er Posted April 12, 2018 Report Share Posted April 12, 2018 2 minutes ago, Bob - S50 said: The extra weight at the tips will put a larger tension load on the top of the spar and a larger compression load on the bottom of the spar. That would have to be considered when designing the tanks, but assuming the tanks even full of, say 15 gallons of fuel, would probably weigh less than 150 pounds, I doubt it would be a problem. The J is rated at -1.5G so that means each wing is stressed to carry a downward load of a bit over 2000 pounds without any permanent deformation. And it is probably able to carry another 30% over that for a design safety factor. Well I can personally vouch for the fact that you can have a bunch of Mooniacs on top of that wing plus the grim reaper and it doesn't so much as put a dent! 2 Quote Link to comment Share on other sites More sharing options...
jetdriven Posted April 12, 2018 Report Share Posted April 12, 2018 It’s not even 1G negative loading. Even with Alan’s big ass sitting on there. Quote Link to comment Share on other sites More sharing options...
Guest Posted April 13, 2018 Report Share Posted April 13, 2018 The Comanche gross weight goes up, but it must be fuel in the tips. You can’t install tip tanks and fly at higher gross weights with empty tanks. Clarence Quote Link to comment Share on other sites More sharing options...
Piloto Posted April 13, 2018 Report Share Posted April 13, 2018 (edited) Fuel in the wing tips increases the roll angular momentum and decreases the wing flutter speed. During the Mooney long range tanks STC process I had to do 49 spins with Lucy Young the FAA pilot in different configurations with full aft CG to prove that the plane was recoverable. And Ground Vibration Test GVT to prove that the flutter frequency was not affected by the extra fuel, even though Lucy did high speed (over red line) dives to verify no wing flutter. All tests were done at 10,000ft wearing parachutes and cabin door rigged for quick release. BTW Lucy was the first female Navy combat pilot. Quite a lady. https://en.wikipedia.org/wiki/Lucy_Young José Monroy Edited April 13, 2018 by Piloto 1 Quote Link to comment Share on other sites More sharing options...
Browncbr1 Posted April 15, 2018 Report Share Posted April 15, 2018 There is probably also a zero fuel max weight specified that puts a limit on cabin load. I know the duchess has that. 3500lbs zero fuel and 3900lbs gross Quote Link to comment Share on other sites More sharing options...
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