Optimal wing for a "Quad Otter" or "half-size Dash-7" ?


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15 December 2007
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I've been working on a design for a while - carefully checking different possibilities (the design currently has over 70 iterations).

It is closest in performance to a half-sized Dash-7 (i.e. de Havilland Canada DHC-7). It is half the weight, has the same weight/payload ratio, is slightly faster and has the same stall speed. The most unusual feature is that it is designed around a pure-piston layout using TRE750A (developed as a competitor to the PT-6 turboprop). This gives it an exceptional range for its class.

The design uses a high cord wing and propwash from the four props to allow very short take-off and landing runs (<300 metres) while keeping wingspan short enough for small strips (15 metres), efficient cruise, and a wing loading high enough for all-weather flying.

The challenge is the wing

Wing optimisation is never easy - especially without an aerospace engineering degree.

Current wing specs:

- NACA2412 airfoil.

- Incidence 1.2 degrees (decreases to 1.0 degrees for the outer wing), dihedral is minimised.

- 15 metre wingsapn, 2.25 metre cord (For the outer outer 2.3 metres of the wing the cord reduces to 0.7 metres). This yields and Aspect ratio of ~6.4, with a 23.2 sq.m. area, and 425.6 kg / sq. m. wing loading.

- Double slotted flaps with 1/4 ratio to wing cord, and limited to 40 degrees of deflection.

I could do with advice on:

- Whether to change the airfoil shape

- Whether to aim for optimising the cruising speed when powered with two engines (1000 hp combined) or four engines (2000 hp combined). Optimising the cruising speed for two engines allows extreme endurance (>20) configurations to be developed out of the airframe. However, it may undermine performance in the normal full powered configuration.

- Whether to abandon the 15 metre wingspan limit (thus preventing emergency landings on 1A class airstrips). It is tempting to keep a short wingspan, because this allows deployment closer to the area of operations. However, it means a lower aspect-ratio wing and at least some reduction in cruise efficiency.

- Whether to try and improve landing speeds (currently 130 km/h, higher without engine power as it uses propwash to increase the lift of the wing)! Without flaps the landing speed is 190 km/h, which is fine for most airports, but not ideal for a bush-plane.

Does anyone have advice - or know a good place where aeronautical engineers like to hang out online?
Oh, and some more info:

Basic airframe dimensions:
- Length 19.2 metres (63 feet), 15 metres (49.5 foot) wingspan.
- Empty weight 5500kg (12000 lb), max take-off is 10500 kg (23000 lb)
- Cargo cabin 27 cubic metres (6 x 2.5 x 1.8) (yielding 75 kg/cubic metre for a 2000 kg payload or 150 kg/cubic metre for a 4000 kg payload).

Current estimated performance:
- Range: 4200 km with 2000 kg of cargo (12 hours endurance, close to 4800 km / 20 hours with two engines shut down; >1600 km with 4000 kg of cargo)
- Take off roll at full weight = 280 metres, landing roll 285 metres on tarmac (600 meters without flaps)
- Landing speed = 130 km/h (190 km/h full weight / no flaps)
- Endurance Cruise on 2xEngines / 1000 hp = 325 km/h at sea level
- Cruise 4xTRE750A / 2000 hp combined = 550 km/h at sea level

I thought I'd add a few pictures of the simulated airframe.

The wing depicted in these renders is somewhat out of date (it continues to be tweaked - especially the stabiliser configuration).

Of course, the best performance comes from a variable incidence wing - but I'm still trying to optimise a fully conventional design before exploring that in-depth. Any suggestions are welcome.


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Obviously I'm just eyeballing the design at the moment, but I think you're going to need quite a bit more tail. If you're going to add variable incidence, I'd think you're going to end up with a hunchback like the C-141 or Il-76 if you're at all concerned about internal volume.
Re: Optimal wing for a "Quad Otter" or "half-size Dash-7" ?)

Thanks - yes a hunchback would have been required. Variable incidence is pretty tempting. The real issue was the nacelle mounted landing-gear - This means that only the outer-wings would be able to rotate. Variable incidence simply adds too much weight and complexity to be feasible.

I'd already somewhat enlarged the cruciform tail by the time of posting - see the attached pictures. Tests showed that an inverted-y-tail was needed to bring the horizontal stabilisers fully into the propwash when flaps were deflected (required to generate sufficient pitch moment for takeoff rotation at maximum weight).

Anyway, I have a bit of a preference for all-flying tails, and with fly-by-wire and modern actuators it is possible again at the scale of modern aircraft - although no one but me would want one in a transport :) It is kindof like Messerschmitt's designs always having v-tails in the first draft.

P.S. Admin - maybe this should be moved to the User Artwork section? Or the Bar?


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The current state of the design. Some of the changes:

- Inner wing incidence is now ~3.2 degrees, the airfoil has been changed and the cruise attitude.

- 4.5 degree incidence on the wing outside of the engines (this improves low speed response).

- The structural weight estimate was also increased. Overload takeoff weight raised to 11,500 kg. At this weight 450 meters are required for takeoff.

- Switch to an inverted y-tail to generate sufficient pitch moment for takeoff rotation at low speed and maximum weight.

Suggestions are always welcome.


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