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Design study: RV-7 tapered wing

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gtae07

Well-Known Member
Supporting Member
Joined
Dec 13, 2012
Messages
2,637
Location
Savannah, Georgia
INTRO:
This "project" started as a spinoff of DaveD's thread on posting example calculations: https://www.homebuiltairplanes.com/...-posting-example-design-calculations-hba.html

I'm one of those people that learns by doing things from the bottom up, but unfortunately there are few resources out there for people like me who don't learn well from the traditional deductive approach used in college (starting from arbitrary body B with generalized forces...). I've come to the realization that the only way I'll get a good worked-through example of an entire wing analysis will be to slog through it myself, in full public view.

A bit of a disclaimer: I have a bachelor's in aerospace engineering and currently work in mechanical systems, so there's a lot of background material I'm already familiar with. Some things I'm out of practice on, others I just never really learned properly as an undergrad (a combination of the school's philosophy, poor professor choice, and my own slackness). In particular, my education in structures and stress analysis is very deficient.

NOTE: THIS PROJECT IS FOR EDUCATIONAL PURPOSES ONLY!! I AM NOT RESPONSIBLE FOR ANYTHING THAT MAY OR MAY NOT HAPPEN TO YOU IF YOU TRY TO BUILD THE WING DESCRIBED HERE.


THE PROJECT:

In order to improve my understanding of structural analysis and design (which is my weakest area), I have decided to tackle a project smaller than an entire aircraft. Specifically, I'm going to design a tapered aluminum wing for the RV-7, to see what benefit might be gained in terms of drag reduction. Why that airplane, you ask? Well, I'm already building one. I have ready access to the mating interfaces and substantial structure from which to take measurements. I'm very familiar with RV construction having already built an RV-6 with my dad. And it's something that, if I decide it's feasible and worth the effort, I could build in a shorter time (relative to an entire airplane) and install on the existing airframe for tests.

Assuming I'm able to finish this design and get it to "close", I will then attempt to design a composite wing to the same requirements, though for educational purposes I may drop the requirement to fit the existing attachment points.


GOALS AND REQUIREMENTS:

Every design project has to have goals and requirements. Ohterwise, you end up spending a bunch of time and money and don't have anything to show for it.

In the order I thought of them:

  • The wing must interface with the existing fuselage structure and all systems interfaces (electrical, pitot/AOA, flaps, ailerons, fuel). No modification to the fuselage or tail surfaces is permitted. Flap and aileron pushrods may be changed as required.
  • Stall speed at a given weight with the new wing must not be greater than with the standard wing. Reduction in stall speed is desireable, subject to other requirements.
  • Fuel capacity must not be less than the original standard wing (42 gallons). Additional fuel capacity is desireable, if possible.
  • Loads at structural interfaces to the fuselage must not exceed those calculated for the original wing, unless further analysis shows that the fuselage structure can withstand those loads (I don't plan to get into that unless I just can't make it work otherwise).
  • Wing structural weight must not exceed 110% of the weight of the original wing.
  • G limits of +6/-3 must remain possible with at least one 200lb person and 20 gallons of fuel on board. (note: extrapolating this leads to approximately a half-G reduction in load limit at maximum gross weight)
  • It is desired to maintain roll rate and roll forces equivalent to the stock design, but a reduction and increase (respectively) of 20% is allowable.
  • Each wing must have a threaded connection for a tiedown ring, or other equivalent provisions, suitable for restraining the aircraft.
  • Each wing must have a reinforced section on the upper surface next to the fuselage that can withstand a 300lb person stepping at that location for cockpit access.
  • Construction of the wing must not require complex (5 axis) machining, stretch-forming, or other complex manufacturing methods. All skins must be flat-wrapped. Waterjet or other NC cutting of sheet and forming blocks will be used to best advantage.
  • Use of blind rivets in load-bearing structure must be minimized.
  • Wing deflection at limit load must not exceed that of the standard wing. Further stiffness requirements will be examined as the design matures.
  • Pitch stability of the aircraft must not be detrimentally affected.
  • CG envelope changes must not detrimentally affect the usability of the aircraft.
  • It is desireable to maintain the RV-7's minimal adverse yaw with aileron deflection.
  • The wing must have provisions for a landing light on each side.
  • The wing must have provisions for red/green navigation lights and strobes (note: the white aft-facing nav light is installed on the base of the rudder).
  • Vne of the new wing must not be less than with the stock wing (210kt).
  • * If possible to determine, substantial margin to flutter speed is desired (target 275kt or more). Good practices to avoid flutter will be followed regardless.
  • It is desirable for deck angle at stall to be approximately the same as with the stock wing, to try and minimize the affect on landing characteristics (note: on a stock RV-6/7, the deck angle at stall exceeds the on-ground attitude. A true full-stall landing will have the tailwheel hit noticeably before the mains, and give quite a bounce in my experience).
  • The aileron control installation must accommodate a Dynon autopilot servo or equivalent.
  • Use of composites will initially be limited to non-structural fairings. I may look at composite ailerons after the initial design is complete.
  • Materials used will be limited to 2024 and 6061 sheet and extrusions. Steel may be used where required for bracketry etc.
  • Wherever possible, post-forming heat treating will be avoided.


Comments and discussion will be in this thread to keep the technical content clean and useable: https://www.homebuiltairplanes.com/...esign-study-rv-7-tapered-wing.html#post341381

Up next: baselining the RV-7, part 1
 
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