ESTABLISHING C.G IN A UNIQUE DESIGN

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Holden

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Jan 18, 2003
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Orion,

I read your WIG Matrix paper . While I was reading it a question came to mind. The question is about tailoring the pitching moment for a fixed planform by use of walls.

If the walls, being of some thickness, were "S" shaped on the inside, looking from a "top" view, the pressure would increase and decrease due to the volume change along the internal area between the wall, wing and center hull (like on your design). The air would enter and diverge internally causing higher pressure, and along the aft portion of the "S" it would contract inwards causing a lower pressure and flow acceleration. Since the expansion is lateral, it will not suck the craft on the water like a non flat airfoil would.

If we took a rectangular planform or a large chord tapered wing and added highly "straight" walls to the sides, the general effect would be an increase in the ground effect lift as a function of wall seal height and wing/wall height.

Twin hull pontoons could be considered "walls" in this case, just like on your "tri hull" design on your web page, for example. The center of lift would still shift back towards the 50% (center of area where the pressure equalizes along the lower surface) the closer the wing was to the ground. Also, assume a substantially flat bottom to avoid venturi effects in the ram flight regime from the water.

Do you think the "S" shape could be enough to counter the cm change provided the volume was large enough? This would counter the pressure equalization that causes the negative pitching moments.


Some ideas:

Your twin (tri?) hull design you posted on your web page could then be considered "walls" and tailored to counter the cm for a WIG, or have "doors" added that could turn on or off the area change for an airplane. A door could swing into the twin hulls on the inside, or a flat panel swing up in the bottom of the wing (flat bottom wing, for example, flush to a flat wall that swings up), allowing a diversion "nozzle" to be exposed forward. If the door swung up, the hull cavity could be made smooth and well shaped to maintain smooth flow. In flight the door would be closed allowing a flat channel section that is low in drag. In the case of a "tri hull," which seems to be what you are working on, the center hull could also be used to creat the convergence/divergence to change dcm/dh to a point that they equal dcm/dpitch.

Comments on such an idea. This idea must have already been tried, I would think.

Second question: What was the main feature of the floats that made the performance go up? They were NOT symmetrical for one. Were they straight on the insides and "half V'd" on the outside allowing the spray to go only outward, thereby lowering interference spray losses?


Holden
 

foamhead

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Mar 9, 2003
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3
Location
Blue Ridge mountains of VA
Would it be possible to make both seats (wheel chair & passenger seat on a platform that moved both together? Controls to go with platform or maybe mutliple locations for controls?

I use Airplane PDQ and it helps in figuring seat location.

Foamhead
 

orion

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Mar 2, 2003
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Western Washington
Good morning;

Judging by our discussion so far, it might almost be better if we started a new WIG based thread. But for now...

First, the design we have is, as you say, technically a tri hull. However, by definition, a tri hull craft is one where the hulls are of approaximately the same size. Our Pelican design is more a conventional hull with tip floats. These outer hulls do provide a small benefit to the wing but due to their small size and the wing's relatively smaller chord at the tip, they cannot really be considered substantial.

The lateral shaping of the "fences" or hulls has been considered by several other designers and has actually been built and tested by several Navy programs, as well as by a private builder/developer in Florida. The theory is sound however to date none of the designs has been able to substantiate the theory. The best guess is that the system is not as efficient as the theory indicates due to the wave action of the water and the disturbed air you're trying to control. That's however a guess but the bottom line is that to date the idea of lateral shaping has not worked.

Part of the problem though may be that the models and test craft that it's been tried on are too small. One of the things we find in WIGs is that the bigger they are, the better the aerodynamics seem to work. This behavior is predictable due to the scaling effects of Reynold's Number variations and to the lesser effect the water contamination has as the vehicle grows in size. The problem though is taht no one really wants to take a chance of building a bigger proof-of-concept craft before everything is verified.

The Australians with their Flightship program are the furthest along.

As you indicate, the "S" shape should be able to counter some of the trim issues the WIGs have. However what needs to also be considered is the change of characteristics as the vehicle rises off the water and all the aerodynamics change. This is one of our most difficult areas as most of these values change dramatically as a function of only very small changes of operating height. Our solution to the Pelican is to evolve off of work developed by Dr. Lippish and later by Dr. Hanno Fischer. It involves a combination of section and planform design. Then we couple this with our in-house developments in aerodynamic stability control, in order to arrive at a combination that will work both, in ground effect and out of it, although the latter condition introduces other very interesting features. Oh well, everything is a compromise.

There have also been several developments usng the rectangular planform with sponsons and as you indicate, there is a benefit to the cushioning effect generated by the trapped air. The German Hoverwing has actually proven this in flight. It uses retractable doors (the doors move up and down off the "body") to seal the area off at low speed, channeling some of the propellrer air down into the closed off cavity (sort of like a hover-craft). As the vehicle accelerates, the front door retracts automatically due to the pressure of the incoming air. The aft door retracts as the vehicle lifts off the water.

This however has to be done very carefully as the doors do cause very significant changes in pressure distribution. Furthermore, such movable surfaces must be kept clear off the water since impingement there would create dramatic drag, somethint these vehiclees can't afford. So far the Hoverwing has worked well in calm conditions but has failed to get off the water in rougher seas.

The biggest operational headache we have is the difficulty of getting off the water, period. This is especially critical since in order to pass the Coast Guard and the IMO classifications of WIG craft, the vehicle has to be powered and optimized for the ground effect flight. What this results in is a series of vehicles that by aircraft standards are relatively underpowered.

Adding more power (the Russian approach) that is turned off in flight is generally not considered viable just due to the weight increase and to what's considerd a non optimal approach. Putting in bigger engines is also not considered beneficial since then you haveto run the engines at lower throttle settings, which for turbines especially, is not very efficient.

I know all this sounds rather contradictory and it is one of the main reasons that there are no commercial WIGs out there, even after all this time. Yes, the Russians have built many, some even very large however, most of those developments were far from optimal and in general, unstable and very dangerous. I've heard it said that the Russians being proud of their WIG programs is the same as Ford being proud of buiding Edsels. Fortunately Ford knows better; the Russians don't seem to.

Regarging the floats: The efficiency allowing the take-off run to decrease was mainly due to the hull of the float being designed for a higher speed than is commonly used in conventional float lofting. This allowed faster achievement of a planing condition and much lower drag once that condition was reached.

Yes, the floats had vertical walls on the inside. Coupled with the hull design on the outside, we managed to dramatically decrease the formation of spray and thus decreased the spray drag component. They were not "V'd", but actually rounded, pretty much like the racing boat sponsons are. We just chaged the loft to incorporate several techniques of controlling the spray without compromising the beneficial aspects of the somewhat rounded hull bottom.

One of the biggest advantages of this was not so much the performance increase (although many thought even the fifteen percent was good) but the improvement of handlilng, especially in adverse conditions. The airplane would be able to tolerate landing somewhat askew since the hull did not have a hard keel line and so was able to slide sideways a bit without catching (like a skipping rock). If the other float caugt on the inside vertical wall, that was OK since all that would happen is that it would either help straighten the airplane out or come out of the water. It however would not cause the airplane to flip or go out of control.

Overall it was a neat idea - too bad the funding fell short.
 

MLEVEN63

Member
Joined
Feb 10, 2003
Messages
16
Location
central/midcoast Maine
Foamhead,
I've thought about a movable pilots position, but I'm pretty certain the C.G. problem can be overcome with a good design solution. I have someone with then engineering skills who is going to help me out.
What do you think of Airplane PDQ? I have it, but it seems to "assume" many parameters from its database which is fine for conventional designs but mine is a little off the wall. I haven't delved real deep into it yet but I don't think you can choose materials, engines, etc that can vary a designs performance. I maybe wrong, I have little patience for new programs when my head is zooming in a comfortable program (AutoCad). Any tips on PDQ would be welcome.

Thanks for your reply,

Mike
 

wsimpso1

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Oct 18, 2003
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Saline Michigan
Designing your own ship is a significant step. The design issues that you just have to work through keep coming up. I know of what I speak...

As was said earlier take Martin Hollmann with some salt. Some of the things he has written are scary dangerous! Structures, aerodynamics, planning. I own a couple of his books, but I know better than to use his text as a reference.

As to ways to make things happen, look at the Velocity line of ships. They place the front seat between the canard and main wing, with a near full hiegth door. Yeah, all of the Long EZ based ships use runway like a jet. A good idea is a high wing.

The head guy in Challenge Air here in Michigan is a paraplegic, and he flies a Piper Archer. His upper body strength is good, so he may have getting in and out easier than you do, but he transfers to the wing, then into the seat pretty easily. He considers a Cessna to be too tall for his purposes.

There was an article awhile back in Sport Aviation about a paraplegic who built and flies an RV6.

Lots of options. ANd you get to choose which way to tackle it.
 

sr-71builder

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Joined
Oct 15, 2003
Messages
15
Location
houston,texas
Just a thought. If beaching the aircraft to allow pilot access, wouldn't the ramp need to meet another ramp already installed on the beach? I don't know much about being handicapped, so please excuse my ignorance, I mean no offense-but isn't it really hard to push/drive a wheelchair through very loose sand? How would you get to the ramp coming out of the airplane?
 
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