Corsair/Spitfire hybrid

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Will Aldridge, Oct 16, 2010.

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  1. Oct 16, 2010 #1

    Will Aldridge

    Will Aldridge

    Will Aldridge

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    Designing and building my own plane has been a goal for quite awhile. My first love is the Corsair so I had to have that inverted gull wing(yes I know that's going to complicate things). But as much as I like the Corsair there was at least one major deficiency in the design, which was the way the fuselage tapered before the maximum thickness of the wing which caused a lot of drag. I am big fan Mike Arnold and his AR-5 and studied a lot of the things he has done in his AR-5 and AR-6, which means I don't have the fuselage start tapering until shortly before it reaches the trailing edge and then I added the expanded radius fillets.

    Also i wanted a plane that was uniquely my own. So being a fan of WWII fighters I decided to mix a few features of 2 of the other great fighters from that era namely the Spitfire and Mustang. As you can see from the attached 3-view I am using semi-elliptical wing and horizontal tail plan form. I am going to accomplish this using hotwired foam core in sections. The Mustang DNA is seen in the vertical tail. The Spitfires elliptical vertical tail may be more efficient but I never did like the look so I decided to add some Mustang to the mix.

    So that's the philosophy behind the asthetics of the design.

    Originally I was planning on building it out of aluminum but that extremely complex wing geometry would have been too difficult so I'm going with composites.

    As currently drawn (still tweaking and analysing and learning) it has a 21.5 ft wingspan with 75 sq ft of wing and the fuselage is about 18 ft long. I'm using the NACA 65-415 airfoil for a couple of reasons. First off it allows a fairly deep spar, and second it has a high CL max and benign stall characteristics.

    Horsepower will be in the 85-115 range with gross weight hopefully about 850lbs. If I had all the money I wanted I would use the Rotamax 120hp engine, but economics will probably force the use of a VW or a Corvair conversion.

    The fuselage is 25" wide, 24" inside the cockpit, and designed to accommodate my 6'1" tall frame comfortably. I plan on using vacuum infusion. I can make the fuselage in 3 pieces (I think) by making fairly simple molds.

    The one other feature of the design worth mentioning is the retractable gear. In order to use a simple mechanism to get it to fully retract I am going to mount the pivot on an angle so I don't have to have it rotate in 2 axis'.

    I would like to build the wing in 1 piece that could be removed from the fuselage but given the inverted gull wing it might be easier to have the outer panels bolt to the center section.

    Comments welcome.
     

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  2. Oct 16, 2010 #2

    orion

    orion

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  3. Oct 16, 2010 #3

    Will Aldridge

    Will Aldridge

    Will Aldridge

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    Yeah it is a pretty little plane. Someone suggested that I forget my own design and just add gull wings to the Twister. But I really want something that is all mine not pieced together with another design.

    It's good to have a respected member of this community say it looks good. I was silently bracing myself for some constructive criticism. I know there are things yet to be optimized but if it looks good to an experienced designer I must be on the right track.
     
  4. Oct 16, 2010 #4

    addaon

    addaon

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    Fixed or retract?
     
  5. Oct 16, 2010 #5

    Autodidact

    Autodidact

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    Constructive criticism coming right up! :-]) Well not really. I was just going to say that the swept back, squared off vertical tail looks a little at odds with the elliptical surfaces, but that's not criticism, it's just me voicing an aesthetic preference. Other than that, I like it a lot!
     
  6. Oct 16, 2010 #6

    Joe Fisher

    Joe Fisher

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  7. Oct 16, 2010 #7

    Will Aldridge

    Will Aldridge

    Will Aldridge

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    I mentioned it but it will have retractable gear.

    As far as the squared off corners go they will be rounded when the plane is really built I was just too lazy to do it for the drawing.

    A little more about the landing gear; I'm looking where I can in hopes of scounging a set of used landing gear to use in the plane. Anyone know of any that mount 5x5 tires and would be about 29.5" long in the unloaded condition?

    Given the hp range I'm planning on I'm thinking a 66" dia. prop will be used and since I know the tire dimensions I need to know how much a shock absorber typically deflects so I can design the required 9" prop clearance with a flat tire and compressed strut?
     
  8. Oct 16, 2010 #8

    Jan Carlsson

    Jan Carlsson

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    Hi,

    Nice plane, Maybe a little taller vertical and more rudder area.

    an VW or Corvair will have something like 59"-62" diameter propeller, and isn't 4-6 Inch stroke max what you need?

    Jan
     
  9. Oct 16, 2010 #9

    Topaz

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    I'll second this. Speaking strictly from an aesthetic standpoint, I'd love to see this design with an elliptical vertical tail, rather like the later Spits or the Mosquito. Just seems to complete the visual created by the wings and the horizontal.

    Other than that, I like it a lot. :)
     
  10. Oct 16, 2010 #10

    autoreply

    autoreply

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    As for the comments; how about visibility? The wing seems to block part of the sight, but it's hard to see whether that's significant.

    Did you consider the Jabiru (2200) engine? Cheaper as the Rotax, many built and direct-drive, instead of PSRU.
     
  11. Oct 16, 2010 #11

    Will Aldridge

    Will Aldridge

    Will Aldridge

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    Well I'll admit that the design and proportions have changed several times since I first drew it and calculated the tail volume. It may very well need to be enlarged. Why taller? Is the tail going to get blanketed by the fuselage in the 3 point position?

    Just for all you guys who are saying it should change I'll try drawing in an elliptical tail, but I like the current version. Besides it'll be nice to have one surface that won't require a bunch of extra work to build.
     
  12. Oct 16, 2010 #12

    Autodidact

    Autodidact

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    I wouldn't think it would have to be elliptical, maybe something like the tip of the vertical on this Hawk, or something to round it off a little. Again, these are just aesthetic ideas, it's your design and you aren't required to listen to anyone other than yourself, you know.:grin:
     

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  13. Oct 16, 2010 #13

    Will Aldridge

    Will Aldridge

    Will Aldridge

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    As far as visibility, other than a little forward of strait down I don't think there is much problem there.

    Yes I have considered the Jabirus. They are choice number 2 behind the Rotamax. Really the huge consideration is money. Which will probably eliminate the jabirus and Rotamax's but I really like the narrow profile of the Rotamax and it's light weight.

    Regarding PSRU's I know that it just adds a level of complexity to things but is there a reason other than that to avoid them?
     
  14. Oct 16, 2010 #14

    Autodidact

    Autodidact

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    Maybe something like this:
     

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  15. Oct 16, 2010 #15

    Autodidact

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    Difficult to find one with a proven track record of reliability. Autoflight looks like the best one, but it's in use mainly down under and I don't hear much about its use in the states.

    Geared Drives looks like it may be a winner.

    With the various 5th bearings available, the corvair looks pretty good.
     
  16. Oct 16, 2010 #16

    autoreply

    autoreply

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    A taller (higher aspect ratio) tail is a bit more efficient. A lower aspect ratio (long chord, not so high) is more efficient in sidewind, especially with a low extended part, like many Cessna's have.
    Join the club :gig:
    Complexity isn't the major issue, but it's result (unreliability) is. Of the "known" PSRU's, only the Rotax, Thielert and the Lyc have seen extensive service in our area (50-400 hp engines). All other solutions seem to be amateurish at best and many (if not all) have serious reliability problems, often torsional vibration related.
    Of the "big 3", Thielert had serious problems, keeping the wear down (PSRU overhaul every couple hundred hours), the Lyc doesn't have such a good reputation and the Rotax seems to be well-proven, as long as you don't overload it (they have a 5 minute limitation to max power which - not respected - seriously increases wear).

    Bottomline, no problems with the Rotax. Maintenance might be a tad more expensive, but apart from that, no problems. As for the other PSRU's for car engine conversions, well, just make sure you have a gliders license before you start test-flying ;)
     
  17. Oct 17, 2010 #17

    Will Aldridge

    Will Aldridge

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    tail sizing

    Well I just ran the numbers on my tail surfaces. According to Raymer the Horizontal Tail volume coefficient for most light aircraft is about .5 which would mean 14.1 sq ft of tail. My design as drawn only had 12.4. As mentioned I have studied Mike Arnolds designs and he indicates that with the fillets tail area should be increased about 5% to account for their effects, so that meant lengthening the fuselage by 4.5" and increasing the area to about 14.8 sq ft.

    According to Raymer the Vertical Tail coefficient should be about .04, which by complete and happy accident is almost exactly the size originally drawn. Given the increase in fuselage length I'd say I'm probably covered as far as adding 5% to account for the destabilizing fillets. Btw that doesn't account for the portion of rudder below the Horizontal stab or the strake on the belly so I think I've probably got more than enough tail to keep it stable.
     
  18. Oct 17, 2010 #18

    orion

    orion

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    A TVC of .5 is a reasonable starting point however may have to be adjusted for several variables. In your case, since your airplane is a single seat configuration, your allowable CG range does not need to be all that large so the TVC value should work fine however, if you wish to employ anything other than plain flaps, the added moment generated (single slotted or Fowler) may require a larger TVC value to provide the plane with an adequate control authority and thus a functional allowable range.
     
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  19. Oct 17, 2010 #19

    Will Aldridge

    Will Aldridge

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    I sized my wing for a stall speed of about 50 kts on a Denver hot day. Given the complex geometry I didn't want to have to deal with flaps. So assuming I can keep her under 850 gross I think I should be good without flaps.
     
  20. Oct 17, 2010 #20

    wsimpso1

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    While the elliptical wing is romantic and handsome, even in composites, faithfully and accurately making it could be problematic.

    Next, those big handsome wing root fillets become styling features if you maintain the fuselage shape and size through to the trailing edge of the wing, which you appear to be doing. They may even cost you some drag... Still handsome.

    Once you have bent the wing, I suspect that your gear legs are overlength, which forces weight into them and the wing structures they attach too, and may drive a three-point attitude that is steeper than you really would like or want. Flat strut + flat tire is still supposed to result in prop clearance, but only a couple inches. Then most of us do not want to exceed stall angle of attach when three pointed.

    Most gear legs are designed for a particular airplane weight, landing speed, etc.. When applied to another airplane, they are usually either are wanting or overweight. If you found a good oleo-strut, you might still have to adjust its mountings, its spring rate, and its damping orifices. Some quality time with Pazmany's Landing Gear Design for Light Aircraft might be a good idea.

    Billski
     

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