Wingboat Design

Whoops.....

Will try to post the pic again.....

 

nice one! if it fly's as safe as it looks it may even has a ferry licence?

in the TBP newsbrief some more on the World Water Speed Record

 

That is the way canards operate for low angles of attack. When an aft tail stalls, though, the drag adds to the stabilizing moments - this is why the tail on a kite works even though it's pure drag. A canard is inherently destabilizing - the wing has to be placed well behind the c.g. to provide pitch stabilty - and when a canard stalls, the added drag is destabilizing.

I think we're using the term "skid" to mean different things. By "skid-to-turn" I mean the craft turns by creating side force without banking. A car would fit this category, even when it's not "skidding". A bicycle is of the "bank-to-turn" variety because it keeps the net force aligned with the plane of symmetry and tilts that force to get the lateral component needed for centripetal acceleration. Missiles with cruciform fins are typically skid-to-turn, while manned airplanes are invariably operated in a bank-to-turn manner. Howerver, a pilot may elect to use a skid-to-turn strategy by using the rudder to make fine sideways adjustments to the flight path - such as in formation flight. If you put a glass of water on the dash, a bank-to-turn approach would keep the surface of the water parallel to the dash, while the water surface would be tilted in a skid-to-turn maneuver.

The WIG depicted in Kornev's paper is making a bank-to-turn maneuver, keeping the wing tip at a constant height above the water. If you want to employ skid-to-turn, you definitely need some sort of fin or vertical wing to generate the side force. Hydroplanes have fins for the same reason. They aren't big, but with the density of water and the speeds they're going, they do the trick.

The side-force-generator has to have an angle of attack to produce its lift. How much of an angle is required to produce the required lift determines the character of the maneuver. At one end of the spectrum, you have the sailboat with its keel. The keel produces a large amount of side force for a small change in leeway, so a sailboat turns very easily and the hull only points into the turn a very small amount. At the opposite end of the spectrum is the hovercraft. There is no lift generated at the surface, and the body of the hovercraft doesn't provide a great deal of lift even at large angles of sideslip. So when the hovercraft turns, it has to point practically at the center of the turn and use thrust to provide the centripetal acceleration as is slides sideways. Both the sailboat and the hovercraft would be classified as "skid-to-turn" but the sailboat accomplishes the turn with far less skid.

A V-hulled powerboat would be somewhat in between skid-to-turn and bank-to-turn. When the V-hull starts to turn, the sideslip causes the hull to roll and bank into the turn. So the planing lift on the hull is directed into the turn like a bank-to-turn airplane. But the heel is not 100% of what's required to pass the glass-of-water-on-the-dash test, so it's also skidding, too.

The wingspan of the craft we've been discussing would limit the amount of roll. So it will have to depend on skidding even more than the V-hull. The most effective way to provide the side force would be to mount a vertical wing in the middle, or to have a fin in the water. Or both.

That's also why I think a forward rudder mounted at the step would be a good way to go. It would provide the sideforce directly and the rest of the vehicle just has to be stable so as to follow it.

I don't think there's much compression in the sense of changing the air density. Both phenomena really come from the circulation about the wing. That's why the vortex lattice method works well to predict the lift.

BTW, I once attended a talk at which Alexander Lippisch described how he invented the WIG. His thinking went like this: Imagine the flow about a tear-drop body of revolution. There's a stagnation point at the nose and low pressure on the sides. Now slice the body in half lengthwise and place it on a surface. Ignoring boundary layers, the flow about the half body is the same as the flow about the full body due to the symmetry constraint of the surface. Now hollow out the half-body and open a modest hole at the nose. The flow about the body is very little changed from before - stagnation pressure at the nose and low pressure on the shoulders. But now the inside is at the stagnation pressure. Since there's high pressure inside and low pressure outside, the body will want to lift off the surface. His reverse-delta WIG is just a stretched version of that hollow half-body.

Liebeck has designed a highly cambered high-lift airfoil sections that produces near stagnation pressure over much of the bottom surface, even without the proximity to the ground. With the ground present, it's as though you had two wings in close proximity, belly-to-belly, so the component of the circulation moving forward on bottom surface tends to add together for both of them and slow the air even more. That's the source of the "compression".

Sure - like I said, put the planing surface forward and the aerodynamic lift aft in order to achieve a stable balance.

 

Tom,

Thanks very much for the discussion and information.

Your description of Lippisch and Liebeck seems very similar to the aerodynamic considerations in "ground effects" packages on open wheel racing cars except with the exact opposite desired end result.

Can you point me to a graphic illustration of the Liebeck airfoil?

Doug Carlson

 

Liebeck section

Doug,

It may be will help you ?

http://www.desktopaero.com/appliedaero/airfoils2/lowcmsections.html

http://www.desktopaero.com/appliedaero/airfoils2/airfoildesign.html

http://www.nasg.com/afdb/list-airfoil-e.phtml

Dim.

 

Dim,

Thanks for the excellent references.

Doug

 

You can find the coordinates for just about any section known to man at http://www.aae.uiuc.edu/m-selig/ads.html

Here's a typical Liebeck section: http://www.aae.uiuc.edu/m-selig/ads/afplots/lnv109a.gif The design pressure distribution for these sections has a flat rooftop, followed by a concave Stratford pressure recovery, with the corner between the two shaved off to form a small transition ramp. The Stratford recovery is an adverse pressure distribution where the boundary layer has the same margin from separation all along the its length. It drops very steeply at the start, then flattens out. It is the shortest, steepest way to get from low pressure/high velocity to the higher pressure/low velocity at the trailing edge. The lower surface pressure distribution on a Liebeck section typically has a flat segment at the front, with a linear, favorable gradient to the trailing edge. Compared to other high lift sections, this makes them more front loaded with less pitching moment. It also results in the reflexed trailing edges as opposed to the more hooked trailing edges on other sections (like this Wortmann high lift section http://www.aae.uiuc.edu/m-selig/ads/afplots/fx74cl5140.gif)

Other sections from this family, designed for higher and higher lift coefficients, essentially maintained the same upper surface contour and became thinner with more concave on the under surface. The section I mentioned is totally impractical, but has a Clmax of 3. It has essentially zero thickness except near the leading edge. Imagine a wingmast & sail with the same lee side contour as the LNV109A.

There have been similar sections specifically designed for WIG human powered vehicles, but I can't point you to any references. They looked kind of like the LNV109A, but with flat bottom and LOTS of thickness. They were designed to operate very close to the surface.

 

After reviewing more wig info in the past few days than my current occupation will justify, I'm wondering if, in that this is a type A wig (cannot function out of ground effect), the Jorg dual wing approach wouldn't solve most of the potential pitch problems discussed?

The techniques for achieving flat turns would still be relevant.

 

I think what makes this craft really different is it's not intended to fly 100% at all. With a twin wing, if it pitches up and the stern contacts the water, it's going to flip over backwards just like a racing hydroplane. I think it's possible to design a boat that won't do that.

It would have to be stable in the air and have enough aerodynamic lift that as it pitched, it would fly up faster than the stern went down. Then it could glide back to the surface. The key would be to prevent it from pitching down too much and hitting the water at an excessive angle. This would be a good application for a feedback control system driving trailing edge flaps.

 

Roll on flaps to counteract a pitch up then roll off flaps to ease back into the cushion by moving the center of lift forward relative to the center of gravity?

Pitch up and/or ascent would result primarily from the forward planing surface encountering a lump in the water?

Pitch down and/or descent would result from a sudden absence of water? Are you assuming that a neutral wing would carry you over the troughs or would you need to transfer lift even further forward which would then lift the bow or drop the stern?

Maybe some combination of slats and flaps could give a temporary bump in lift without a corresponding change in attitude.

Once the basic feedback loop had been established, it would seem fairly straightforward to develope an automatically optimizing ride control system based on amplitude and number of oscilllations over time.

 

The pitch up could come from a variety of sources, most likely running into a wave. It's the thing hydroplanes experience all the time - a wave pitches the boat up enough that aerodynamic lift is greater than the weight, and the boat pivots about the stern and flips over.

Tails and especially trailing edge flaps on delta wings have the characteristic that the initial vertical response is the opposite to the net response you ultimately get. For example, if you're flying your delta-winged Mirage jet fighter trimmed up nice and level, then pull sharply back on the stick, the trailing edge flaps deflect upwards, momentarily reducing the lift and accelerating the aircraft downward, but starting to rotate the plane nose up. Once the rotation has increased the angle of attack enough, the increase in lift overcomes the loss due to control deflection and the aircraft accelerates upward. This is normally an adverse characteristic, but for this applicaiton we can put it to use.

Let's start with a simple rate damper, feeding back pitch rate to the trailing edge flaps. As long as the craft is maintaining a constant pitch attitude, whatever it is, there's no flap activity. But as it starts to pitch up, for whatever reason, the feedback will start deflecting the flaps down. This will create a nose-down pitching moment, but it will also tend to lift the craft off the water. Both of which we want.

If the craft zooms up, as it starts to pitch down the pitch rate feedback will move the flaps trailing edge up, creating a nose up pitching moment but also reducing the lift.

The wing is not neutral - the stern of the craft is being supported by the wing well clear of the water while the stepped bow is planing on the water.

Slats and leading edge flaps have very little effect on the lift when they are deflected. They make it possible for the wing to go to higher angles of attack without stalling, but they don't have much authority by themselves. And this craft is not intended to go to high angles of attack, so slats won't be of any use.

If you want direct lift, that would require a surface forward to work in conjunction with the flaps at the tail to produce vertical force with no net change in moment. But there's no way you're going to create enough control power to overcome the heave stiffness of the planing surface. You'd have to be completely flying to get the kind of ride control you're suggesting.

Nothing about ride control on this craft is going to be straightforward! Waves have a very broad spectrum and their statistics are not stationary. You might be able to do some filtering, but the ride is going to be totally dominated by the fact that it's a surface craft. The place to be, however, will be as far aft as possible. There will be two longitudinal modes to the motion. One will be the "pitch" mode because, although it will involve both pitch and heave, it will be mostly pitch as the craft rotates about the step. The other is the "heave" mode, and it will consist of rotation about a point behind the craft. The closer you are to the heave center, the smoother your ride will be. So put the cockpit in the stern, but take care not to block the view too much with the fins.

Of course, you don't have to use just a rate damper for a control system. You could add in attitude feedback as well, to make the craft want to maintain a constant atittude. This will also help to stop the pitch-up and to make it glide back down if it gets bounced off the surface. You wouldn't ever want to use pure attitude feedback, however, because this would create an undamped system that would oscillate. You need the rate feedback along with the attitude feedback to have adequate damping.

The flap characteristic I mentioned at the start, what controls engineers call a "non-minimum phase system", means that at frequencies higher than it can respond in pitch, the flaps will tend to move the craft up and down in response to the bow moving up and down (opposite of what you'd expect from their pitch effect), but without much change in attitude. It's a bit like the suspension system on an Austin Mini, in which the shock absorbers on the front wheels are connected to the shock absorbers in the rear, so when one hits a bump, the front shocks pump fluid to the aft shocks and the rear end is already starting up before the rear tires hit the bump. But if the system has to wait for the attitude to change in order for the feedbacks to move the flaps, this won't happen.

So for high frequencies, you want to mount an accelerometer over the step and feed that back to the flaps as well. When the bow goes up on a sharp wave, the accelerometer will move the trailing edge flap down and start to raise the stern before the pitch attitude changes, much like the suspension system on the Austin Mini.

At low frequencies, the attitude control will keep the craft from pitching as it rides up and down with the swells. At higher frequencies, the accelerometer and rate gyro keep it skipping over the waves. If it ges airborne, the attitude feedback will help it flare back at its normal attitude as it drops back to the water. How fast it drops depends on how much weight was being carried by the planing hull - at constant attitude the glidepath will steepen until the angle of attack increases enough that the lift makes up for the planing load.

So that's how I'd implement the control system for the pitch axis.

 

Its crude but just to see if we are on the same page.

 

Another angle.

 

I like this better.

 

Again from a different view. It seems the vertical stabilzer might need to extend above the wing to get a rudders with enough power.