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Performance amphibian ultralight project

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yellow_submarin

Well-Known Member
Joined
May 10, 2013
Messages
88
Location
Laval, France
Hi everyone

As stated in my fresh introduction thread, I'm into the design of a performance amphibious ultralight.

The goals are as follow : 100 hp, 2 seater, 65 km/h stall speed, 495 kg mtow (450 + 10% as amphibian), variable pitch, retractable landing gear, and other tricks allowed. Single engined of course. I aim at high cruise speed (250 / 300 km/h), excellent manoeuvrability / light aerobatics / fun factor, good range to travel with beloved one. And the ability to land on water, snow, and any slick earth strip ;)

This might seem ambitious. But that's what I am, so, no need to argue. I don't claim to be a know-it-all, I obviously don't, I am always asking to learn. Yet I'm a soon to be graduate in aerospace engineering. Just so you know I'm not that irrealistic as I know partly what I'm dealing with.

About the design, I have spent the previous 4 months reviewing most of the architecture possibilities offered by the constraints, and I must say this forum has been quite helpful. To either open my eyes on other configurations, or discedit what I thought could be a good solution. This is what I have come to.

- Floats are a no-go. Too much drag, too heavy. Buyoyant fuselage is the way to go, though the hull shape and the cusp offer somewhat undesirable airborne drag. So I'll have to get rid of that too. Obviously there has to be a replacement device to stabilize and reduce water suction. I have come to the conclusion that retractable skis, pretty much Sea Dart style, with wheels fitted to the tips, could not get me any closer to what I want. They can be lightweight, offer very low cruise drag, can have the role of landing gear spring & damper, and are easy to build.

- Fuselage features tandem seating, rear engine. Maximum visibility and fun factor, side stick is a must have. Large NACA inlets & outlets for engine cooling. Concerning the engine, it will be a turbocharger Weber MPE 750, which I was fortunate enough to purchase for $1300. Overall, the structure will be carbon fiber built. Bidiagonal cored skin, unidirectional spars and ribs. Same as for the wing. Some wood and eventually aluminum honeycomb would be used for local reinforcement as core material (engine tray for instance). I consider 2 shrouded propeller. I can see the smile (or despair ?) on your faces right now. But this a well thought choice. Let me explain. Since the aircraft will be low on water, a conventional pusher or tractor prop is out of the question. I don't want no nasty engine pod reducing my cruise speed of 50 km/h, and ruining propeller inlet flow (reason why the Piaggio Avanti II is that awfully noisy). I don't want either a Sky Arrow, SeaMax or Freedom look-a-like. Too much drag for me in here. So, the prop has to lay above the engine & fuselage. This might produce high pitching moment, and I don't like how it looks. So the idea is to split the propeller disk into 2 propeller disks, side by side, which have to be shrouded. If not, I suspect the tip losses of 2 propellers versus one single propeller will imply quite a loss of thrust. The shroud would also have an acoustic major role, so as to reduce noise. And most of all, apart from limiting propeller tip losses, it would allow added suction thrust. Talking about iso propeller disk surface, the perimeter of 2 propeller disks is 1.4 that of a single propeller. That means 40% more thrust augmentation compared to a single shroud. One more reason to shroud 2 propellers. I agree, this is a major complication and weight penalty, and weight, most of all, is an ennemy. But if the thrust augmentation (which will CFD computed) is large enough, that's a price I'm willing to pay. Plus it gets a badass A-10 look :) (2 1m diameter propeller would be fitted in lieu of a single 1.7m diameter propeller to keep the same area). Care should be taken to keep a low drag fuselage shape (airfoil like).

At last, let's look as wing planform. I have considered MANY designs : delta, canard, tandem, ... I had a close look at RMT's Bateleur, which close-coupled canard prevents the main wing from stalling at large AOAs. If it does not, that's what I had in mind anyway. Yet I highly doubt Wiki's performance chart : 300 km/h for a 16 square meters wing seems optimistic. Meanwhile, I decided that I wanted a 3 surfaces layout : a canard only for trimming purposes, a low aspect ratio, low surface main wing, and a conventional tail. No fancy T-tail after all, the stabilizer would be blown by the propellers, which I don't want (most of all on its lower side). Plus it's stronger thataway.
The airfoil would be a GAW(1). I like the low cruise drag, high CLmax, high thickness. High AOA drag is after all somewhat desirable for steep approach. The thickness is very interesting for fuel storage and most of all for structural reasons, as it would enable low weight wing. As I aim at 5 square meters wing, for high cruise speeds, very effective flaps and slats have to be used. A 40° fowler flap on the GAW, when well designed, can provide a CLmax of 3.8, compared to 1.7 for the bare airfoil. This doesn't even take into account the slat. Both the full span fowler flaps and extendable slats would provide a 45% wing area increase, so as to get a 7.5 m² area (full flaps). This, including a 3.8 CLmax, is enough to provide the 65 km/h stall speed. It does not even take into account canard lift, nor slats CLmax increase, and numerous other factors such as fuselage lift.


So, that's what I have for now. It is likely to evolve in the future, just like it did during the 4 last months. Moreover if I have your feedback now. Please feel free to comment, in any way you like, I wouldn't be offended, I'm merely looking for an outside, fresh look.

Thanks for reading ;)
 
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