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Why do tip tanks increase MGW?


bradp

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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 by jetdriven
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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.

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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.


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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.

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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-

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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?

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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.

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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!

IMG_3825.JPG

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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 by Piloto
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