The Razorback construction thread

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by rtfm, Jul 11, 2008.

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  1. Dec 29, 2008 #101

    rtfm

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    rtfm

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    Question: Wing tip fuel tanks?

    Hi,
    Pazmany recommends wing tip fuel tanks - largely for safety reasons. I can sympathise with this, since a good friend of mine landed hard last year, and the plane cought alight. He had a seat tank, and was terribly burned (no other injuries). He survived, but only after four or five months in hospital.

    On the positive side of the ledger, I can see the following:

    • They LOOK cool (to me, anyway)
    • They place the fuel as far from the cockpit as possible
    • Pazmany suggests that they would act as end plates, increasing the wing's effective AR
    • They would be relatively easy to build as an integral part of the wing.
    • They position the fuel directly on the CG

    However, the negatives include:

    • A source of added drag
    • They place a lot of weight at the extremes of the wing which would compromise manouvreability, and may require a beefier spar
    • Not everyone thinks they look good
    I guess it comes down to how much of a drag penalty they incur. Anyone have any ideas about this? Any experience of this?

    Regards,
    Duncan
     
  2. Dec 29, 2008 #102

    Topaz

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    Re: Question: Wing tip fuel tanks?

    Add one more negative - you need to drain them roughly evenly at all times, lest the lateral CG shift to the point where you're carrying aileron to keep the wings level. That's an awfully large moment-arm you're dealing with, across the entire semi-span.

    Move the tanks inboard to the wing root and you get most of the advantages of wing-tip tanks (lacking the end-plate effect), but few of the disadvantages.
     
  3. Dec 29, 2008 #103

    rtfm

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    Aha!
    I had thought to join both tanks with a through-tube, and to have a single fuel pump dumping gas into a header tank behind the engine. This would bypass the need to manage L/R tanks.

    Moving the fuel into wing roots wouldn't work with the retract gear which will swing inwards.

    Mmmm

    Duncan
    PS Any idea about the amount of drag the wing-tip tanks would add?
     
  4. Dec 29, 2008 #104

    addaon

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    I don't really know numerically, although I think Raymer had some useful approximations based on frontal area. But the people who play with the fighters seem to say that well-designed tip tanks/missiles are about the lowest drag you're going to get on a protrusion. It's definitely going to be higher drag than internal/wing tanks, but I suspect it won't be huge.
     
  5. Dec 29, 2008 #105

    rtfm

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    Hi,
    Well, if it isn't that great, and I can overcome the discrepencies between L/R tank remaining volumes - wing-tip tanks might be not so bad after all...


    Duncan
     
    Last edited: Dec 29, 2008
  6. Dec 29, 2008 #106

    Topaz

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    Yeah, but any extended bank will drain one wing tank into the other, dependant upon the flow rate through the connecting tube - which is given a lower bound by your engine's required flow rate plus a safety margin. The only other option is to use check valves that only allow flow inboard and have a non-vented, sealed header tank (or one with a check-valved vent line to keep the fuel from siphoning overboard through the vent). Which introduces multiple failure modes into the fuel system via the possibility of stuck check valves. You're not always flying straight and level, and failure modes in the fuel system are Bad News. Capitals intended.

    The farther inboard the better. They don't have to be in the absolute root. Put a big header in the fuselage if necessary, and try and mount it so the upper edge is higher than the outboard wing tank in a standard bank. Wing-tip tanks are asking for trouble, if it's the primary fuel system. Anything that diminishes the moment arm for your fuel system span-wise is a plus. 6.08 lbs. per gallon, multiplied by the arm out to the mass center of the tank - pretty much your semi-span plus half the diameter of the tank itself. That's a lot of moment.
     
    Last edited: Dec 29, 2008
  7. Dec 29, 2008 #107

    Midniteoyl

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    I dunno... I kinda like wingtip tanks and, if done correctly, should'nt add drag and might even improve the tip losses. Here a pic of an Express with tip tanks that supposedly did just that:
     

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  8. Dec 29, 2008 #108

    rtfm

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    Hi,
    VERY nice looking tanks. Can you point me in the direction of this aircraft so I can dig up a bit of info on it?

    Cheers,
    Duncan
     
  9. Dec 29, 2008 #109

    Mac790

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    Do you have any signal that something is wrong with them? Did you check? do you have enough space for you "backside" like I told you back in August. I told you please check out these templates before you cut them.
    And because that fuse was designed for side by side, next "we" changed it for stagger version, I'm afraid you will have to make separate mold for a canopy, more buble shaped front might be necessary.

    Seb
     
  10. Dec 30, 2008 #110

    rtfm

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    Seb,
    Hi. You worry FAR too much for a young man! :) The templates look perfect. I am extremely pleased with the shape of the aircraft. The canopy is spot on - no need for an extra canopy mold - it looks exactly right.

    I removed all the "squeezings" today, sanded the foam planks down and added the foam glue to the rear section. Later today I'm going to start on the tail. Once that is added, and the foam planks laid over the foam base, I'll take some more photos and you will be able to see the results for yourself. It is going to be a thing of beauty!

    Duncan
     
  11. Dec 30, 2008 #111

    Mac790

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    Duncan
    You know what I think about those wingtip tanks..., anyway, I did for you a very simple rendering, with and without wingtip tanks just for comparison.

    Seb
     

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  12. Dec 30, 2008 #112

    Jman

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    I rather like those wing tanks!! Maybe you could make wing tanks that are Sidewinder missile replicas. Now that would be sexy.
     
  13. Dec 30, 2008 #113

    Midniteoyl

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    Sorry Duncun.. unfortunately these pics were taken as he (Hans Georg Schmid) was heading out to fly to Oshkosh (from Switzerland) and crashed. The web site has since been taken down. The plane was modified by him and was known as Express 2000 ER (extended range)



    Shame too. Good guy and a World Record holder.

    1997: Mittelholzer Commemoration Flight Switzerland - South Africa and return
    1998: AĆ©ropostale Commemoration flight Switzerland to and all around South America and return
    2000: Millennium Flight: Around the World. Twice. Two times 27 days, roughly 400 flight hours, 50,000 nm. 75 Years after the "Round-the-World-Flight Expedition" of the US Air Service with 4 Douglas "World Cruisers" first around the world flight

    16,000 hours total flight experience
    400 hours in gliders, C in gold, 2 diamonds
    Experience in different singles and twins
    Captain for SWISSAIR on DC-9, F-100, MD-11 (> 10 years)
    Head of Air Safety Board of Aeropers for several years
    Member of the Swiss Accident Investigation Board for ten years
    Member of the board of EAS / Experimental Aviation of Switzerland till 2001, editor in chief of "EAS News"

    I do have more pics, however.
     
  14. Dec 30, 2008 #114

    rtfm

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    Heh heh heh...
     
  15. Dec 30, 2008 #115

    Mac790

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    Sidewinder? I'm afraid that Duncan needs something bigger he's going to fly to Australia, so he needs more fuel, Sparrow replica should be better.:gig: It seems that I'm the only one who don't like these wingtip tanks.

    Seb
     
    Last edited: Dec 30, 2008
  16. Dec 30, 2008 #116

    Topaz

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    Nope, you're not the only one. I think they're going to be trouble, either from an engineering standpoint, a piloting standpoint, and more likely both.

    From an engineering standpoint there's all the valving issues I mentioned before, plus some things I didn't mention then: what they might do to wing flutter, aileron effectiveness, lateral stability, spin recovery... All that mass out at the farthest extremes of a very small airframe. :para:

    Simple cross-tubing won't work, Duncan - the moment there's any bank at all (even a degree or two), fuel will start to pour out of the high tank into the low, and once the latter is full, out of the low-tank's vent and overboard. Even a small bank (five degrees or less) will start pumping fuel overboard if the tanks are fairly full: your turn onto the crosswind leg of your departure will be enough to do it. If the full tank sets up a siphon effect, it could continue to pump fuel overboard even after you level off. So, you're going to have to have a selector valve.

    From a piloting standpoint, then, it's going to be a pain to keep the fuel usage equal between tanks. The pilot will then have to frequently monitor the fuel levels and switch back and forth between tanks to keep the airplane balanced, or have to carry aileron just to keep the wings level.

    Assuming the aerodynamic issues I mentioned above can be dealt with (and obviously, they can be, but it's work and may require some redesign of the wing and/or tails), wing tanks are useful as aux tanks, where they hold extra fuel that can refill the main tanks as they empty. That process is usually handled manually, with valving. As the primary fuel tanks in the airplane, they leave a lot to be desired, IMHO.

    I'm not a big fan of them from an aesthetic standpoint either, but my taste isn't the one at issue here. Beauty will be in Duncan's eye, not mine.
     
    Last edited: Dec 30, 2008
  17. Dec 30, 2008 #117

    Mac790

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    That web site should work through Internet Archive
    He made all these records in modified Long Ez. Definitely he was a brave man, two trips around the world with single engine plane.

    Seb
     
  18. Dec 30, 2008 #118

    Mac790

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    So there are two of us, personally I was thinking about a centersection wing like in Kr2S or Lancair Legacy (with plenty place for a fuel). But Duncan is going to build a single piece wing, which of course is simpler to build, but if I remember correctly Glassair had a single piece wing with internal wing tanks, so maybe his choice (single piece wing) isn't as bad as I thought. Of course he won't be able to take that plane to his garage on a trailer easily.

    Seb
     
  19. Dec 30, 2008 #119

    Topaz

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    I don't see any reason the tanks can't be in the wings in a fairly conventional fashion. If the main gear is aft of the spar, put the tanks in front of the spar. If the mains are in front of the spar, put the tanks aft of it. The gear can't be filling the entire wing root. These locations are so close to the CG that the impact upon the forward-aft CG range won't be any more significant than tip tanks, where the fuel could slosh to the extreme forward or aft end of the tank. In fact, because of that, the impact on the CG (which Duncan seems to have as a concern) is probably even less than that of partially-full tip tanks.

    With a one-piece wing, putting the fuel in the wing roots becomes even easier - there are no big spar-join fittings to work around.
     
  20. Dec 30, 2008 #120

    rtfm

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    Topaz,
    Hi. OK, your reasoning regarding wing tip tanks is quite compelling. I certainly don't want to implement anything which is going to give me that much head pain. Based on your analysis of the technical issues involved, wing tip tanks have moved onto the endangered list.

    So now I'm looking for an alternative location for the fuel. I plan to use retract gear. So that means I can't use all the inboard wing area. But I couold use some of it. There is the possibility (of course) of storing the fuel behind the engine - there is quite a lot of space there. I was going to use this area for a header tank anyway and use the rest of the space for storage, so perhaps this is the option I need to consider also. (Seb: This should make you happy :)) Either way, it does seem like wing-tip fuel tanks are not the wisest idea. Thanks Topaz.

    The reason I'm worried about the CG is that I have placed the pilot much further rearward than is usual. I like the idea of a long nose, and the pilot sitting far back. But this does pose some challenges for the CG.

    Since I like the look of the wing-tip tanks, perhaps I can still use them, but for storage... And if I plan it correctly I can always use the wing-tip storage for extra fuel when I need to make those long hops... Fuel management would then be manual, of course, to overcome Topaz' technical issues. Just thinking out loud.:ermm:

    Duncan
     

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