tomcat

Just did some searching on the web for simple explanations of wing lift and ground effect. I wanted to get a feel for how much lift is actually possible with a tunnel hull "wing". As usual I started at square one.

A Cessna with 160 square feet of wing area generates about 1600 lbs of lift at 55 mph. We know that because that's its minimum flying speed and its weight.

When any wing flies close to the ground, about 75% more lift is generated at the same speed. The proximity of the ground causes higher pressure beneath the wing and less upwash in front of the wing to deal with (upwash causes negative lift). This increase in lift comes without the penalty of increased drag, a rare bargain in aerodynamics.

Lift is mostly the result of angle of attack, not the curved upper surface of the typical wing shown in a textbook.

Most of the lift generated by moving a wing through the air is due to the movement of air far above the wing. To maximize this effect, the upper surface of the wing should be as free of obstructions as possible.

Most of the lift is generated in the first 1/4 of the wing chord, (the distance from the leading edge to the trailing edge).

A 40 foot cat with a tunnel 4+ feet wide would also have 160 square feet of wing area. Operating in ground effect, that would generate 2800 lbs of lift at 55 mph, most of that acting on the front half of the boat, IF the wing behaved exactly like a Cessna wing.

Since the tunnel "wing" has much less leading edge to set in motion the air movement that leads to lift, and the deck is obstructed, I'm sure that it doesn't behave like a Cessna wing, and your guess is as good as mine how much lift is actually created. But I'll bet it's thousands of lbs, not hundreds.

When you see the Supercats dancing on their sponsons as they come towards you, lots of daylight in the tunnel, you know they're close to lift off.

Which big cat team was it that had a blowover a couple of years ago? Is it possible to make a big cat light enough and the tunnel wide enough to lift off? Would such a hull stand the impact of a wave? If the hull did lift off, how would you control it? (design of the drive skeg?) Didn't the original Power Cat actually fly? If porpoising starts when the center of hydrodynamic lift moves behind the center of gravity, would increasing the wing area at the front of the boat compensate by both increasing the amount of aerodynamic lift and moving the center of aerodynamic lift forward?

So many questions, so little time.

Dixie Doug

I don't think the boat is producing lift because of the venturi effect.I think it is trapping air.(Air Packers)

C_Spray

iTrader: (2)

Tomcat - you've definitely started on the right track with your homework. Unfortunately, there won't be a whole lot of relevant information on ground effect as it applies to raceboats, because the most recent application of ground effect in racing (cars) has been to generate downforce rather than lift. Having the ground (or water) in close proximity to the lifting surface creates an enormously powerful combination. In the unrestricted downforce days of the early 80's, we were able to get over 4,000 pounds of downforce at 170 mph in our Indy cars. In a 1800 (wet) car, that lead to cornering forces close to the 5G range. The rulesmakers reduced that amount by mandating larger air gaps under the cars, in effect "breaking the seal" to the ground. (More about the relevance of this later...)

The relationship between the aerodynamic center of pressure and the center of gravity is very important to maintain an appropriate angle of attack, and therefore stability. As you noted, the fastest cats definitely appear to be teetering on the edge of disaster. (It was Patel that went over in Barnegat Bay several years ago in a Skater, but I have not heard a good first-hand account or analysis of what happened.) The problem arises in that once the nose of the boat starts up, it inherently packs more and more air into the tunnel and moves the CP forward. (Not a good thing.) If the aero and mechanical forces can be properly balanced, the boat will be happier. Given the amount of lift available these days, it should be possible to get the boat up and "dancing" on top of the water enough that any increase in lift as as result of the bow coming up will be enough to also lift the stern edge of the sponsons clear of the water and bleed off the excess air pressure before the whole thing becomes unstable. The other approach to prevent blowovers would be to configure the top deck of the boat so that it stalls and looses lift if the boat gets too high at the front. This could be accomplished with a carefully-placed "trips" placed across the upper surface at just the right place(s). Another approach used by some plane designers like Bert Rutan is to place the control surfaces in front of the main lifting surface (a "Canard" design). The surfaces are configured such that as the angle of attack increases, the front wing stalls first, thereby loosing lift at the front first and settling the plane (or boat) back down. This is what is known as an "inherently stable" system. I am not familiar enough with current rule to know if it is legal to build a cat with two sets of lifting surfaces for offshore racing., since this would require an opening between the top deck and the tunnel somewhere.

There has been some work done of true "surface effect" water craft over the years, and apparently the Russians are experimenting with a very large troop transport based on this principle, so ther must be some technical documents out there somewhere that could be helpful.

It ain't rocket science, but it's very close...

tomcat

C_Spray - Great info based on experience, the best kind!
Dixie Doug - You're right, it's not the venturi effect, but packing air in the tunnel (higher pressure below the wing) is just like saying the venturi effect is what causes lift. So what's really going on?

Here are two good links I found:

http://www.avweb.com/articles/liftsuck/

http://www.aa.washington.edu/faculty/eberhardt/lift.htm

"Packing air into the tunnel" is an intuitive description of how tunnel hulls work, but (I asked myself), "How could a little extra pressure (difference) cause enough lift to matter?" To go back to the Cessna example, Bernoulli's venturi effect (high pressure below the wing and low pressure above the wing) can only explain about 40 lbs of lift at 55 mph. The plane weighs 1600 lbs. Obviously something else is going on.

In an airplane there is higher pressure under the wing but most of the pressure difference across the wing comes from the much lower pressure above the wing. Contrary to what we were all taught in school, this pressure difference does not suck the wing up, it sucks the air above the wing backwards and then downwards behind the wing (downwash). Lift has more to do with Newton than Bernoulli. The downwash is the action and the lifting of the wing is the reaction. So wings don't pack air under the wing as much as they pull air down from above. And a lot of air has to be set in motion to generate a reaction that will lift 1600 lbs.

But in ground effect, the pressure under the wing is much higher, and this is part of the reason for the higher lift. It also makes sense that the sides of the tunnel concentrate this pressure even more, since it can't escape past open wings. So "packing air" is a good enough description as long as we don't forget that lower pressure above the wing is the other half of the equation. And lift doesn't amount to much unless a lot of air is directed into down wash.

The sudden release of this high pressure under the wing is an intriguing possibility for lift control that doesn't exist in an airplane. Air spillling out under the sponsons doesn't sound controlled, but releasing air through a hole in the deck sounds like it might work. Not only do you spoil the low pressure above the deck but opening this flap would create a separate front wing section angled to create downforce. This is not "inherently stable" control; it would require very quick reflexes or some kind of active computer control. Actuators to save a blowover would be high pressure air cylinders, hydraulics are too slow. And the window of opportunity would be very small. Think of your hand out the window at 60 mph. At a small angle of attack, you generate lift, as you increase the angle of attack lift peaks at about 15 degrees, then stall occurs soon after that; no lift. But open your hand a little more and it just becomes an obstruction and the wind tries to push it back hard.

What angle of attack does the tunnel wing have when the hull is running at its normal angle of attack? In an airplane less angle of attack is needed as speed increases, but in a boat we can't change angle of attack, so you have to pick a compromise value for one speed. We would like to achieve this angle of attack without using mechanical trim, which costs us some thrust. I would guess that we especially don't want too much lift, (requiring negative trim) since a sudden drop in speed would drop the bow dangerously.

All this talk makes me think that a Skater designed to go over 200 mph would have to have a purpose built tunnel. A few years ago I saw a documentary about Ken Warby's speed record. They showed some previous attempts that ended in disaster. One of them was an obscenely powered big cat. I think it blew over. Another was a boat designed with a front wing that was supposed to be inherently stable, couldn't blowover. That boat lifted off, went straight up and came straight back down on its transom. Didn't blowover, but it wasn't pretty.

What about tunnel tabs? They are a bit like flaps on a wing, but do they have an aerodynamic function or a hydrodynamic function? Air flaps have the effect of increasing both wing surface area and angle of attack, so they increase lift. Is that what happens on tunnel hulls? Or do they increase the ground effect by preventing air from leaving the tunnel? Or do they just act like a big trim tab, lifting the stern by directing water down?

Are any of the big teams or builders buying wind tunnel time?

C_Spray

iTrader: (2)

Tomcat - I'll have to explore those links later, but for now:

My thoughts on the canard principle is not so much to have a bleed opening to release air on a panic basis, but to have two distinct aerodynamic surfaces/wings separated by a considerable gap between them. The forward wing would be configured to stall much sooner than the rear surface, which would continue to generate (even more) lift as the angle of attack increase. This would be a passive inherently stable configuration if it could be done.

Complicating the whole situation are two more powerful effects: hydrodynamic hull forces and hydro-mechanical propulsion forces. All of these forces have to work in concert with each other. When they do - it's pretty obvious: the boat "flies" straight. Anyone who's been to a boat race and watched carefully has seen the seen the difference between the boats that work well and those that jump all over the course due to an imbalance of forces and/or an unstable combination. (Although it's not a cat, Zinetic's rock-steady behavior is awesome to watch - a very "happy" boat.)

Wind tunnel programs will only go so far until you can figure out how to have an uneven surface (waves and chop) pass under the hull model at speed. No one ever got meaningful results on formula-style car wind tunnel programs without both a moving ground plane under the car and tires that rotated with that surface. If financing could be secured for a program like this, it would be a very challenging, but fascinating endeavor.

In the meantime, there is an option: data acquisition. Instrument the top and bottom surfaces with as many pressure sensors as possible, and then add a ton of ultrasonic or laser sensors to measure the distance to the water at the same points and the same time. Tie it all together with gyros to measure the angle of the boat in all three axes as well as both wind speed and water speed sensors and you'll be ready to go. Once you have the data, all you have to do then is find a dozen or so brilliant data engineers to spend about 10 man-hours looking at every minute of data.....

Anything easy has already been done.

ursus

Some more thoughts

Since most of the lift is generated in the first 1/4 of the wing chord the cessna with a 30 foot wing span will generate much more lift than the cat with a 4 foot wing span,.

The aspect ratio of the wing, span versus chord, is an important fator. High aspect ratios generate a lot of lift for the same area, like a glider haveing a small chord and very long wing span. The low aspect ratio like the cat will make much less lift, haveing only a 4 foot wing and with the extremly low aspect ratio 4' x 28' and most of the lift now being generated in the 1st 10% or so of the wing.

The lift always equals the planes weight 1600 lbs when flying level whether at maximum or minimum speed. As pointed out at slower speeds the wing has a higher angle of attack to generate this lift. The Cessna's minimum flying speed is the speed below which the wing stalls because the maximum angle of attack has been exceeded, in other words the air flow over it just becomes turbulent and ceases to generate lift it is not the speed at which the lift equals the planes weight. The angle of attack is what is important.

The question is how much lift the cat hull (wing) will generate given its angle of attack when running given its aspect ratio, ground effect, venturi effect and a dozen other things I am sure I am not aware of but for sure its angle of attack will not be the same as the Cessnas at 55mph and so its not that simple comparision, apples and oranges.

tomcat

ursus - I knew there was something missing, aspect ratio! Something else that bothers me is where does the downwash go? On an airplane wing the distance across the wing is short and the down wash is free to move down. But if the leading edge of the tunnel creates the low pressure area above the wing and the air above rushes back and down to fill in this void, the downwash hits the deck or at least has a long way back to go before it can spill off the stern. Meanwhile it has run into the windscreen, canopies, heads, scoops etc. Who knows what the lift is or where it is centered on the hull.

C_Spray - I can see the moving ground plane and especially the spinning tires being a factor in wind tunnel testing for Indy cars., but it would be a start just to set a hull on the floor in normal running angle and see how much weight is taken off four scales set at the corners. Yes, we know how to set up stock cars in Canada, and we have the scales, just no wind tunnel.

Hey, that gives me an idea! We'll put the scales under the boat on the trailer, and tow the trailer down the road at 100 mph. Who volunteers to sit in the boat and read the scales?

shifter

The boat that patel went over in was the one that got Carmody this year. It was on its 2nd pass from new iat 154 it took off. Doug Lewis was on the throttles and it was the first time with a tunnel flap for him. No one really knows what happened lots of accusations. Cat can do went over too another Patel boat. Those boats were the start of the real cut down 40's. When the top was cut and then widened it created more under lift(packing) bot as the angle of attack increased a different airfoil design came into effect. I only know of it because I flew on the plane that had it, it was a Fokker. The inverted airfoil at 15 deg creates lift. Strange getting into a plane with the wings on upside down. On forests boat we stretched the foil and sharpened it to change the leading edge. The original foil was semetrical and when they cut them down it became inverted. Most of the boats have gone to sharp airfoils so that they stall at lo angles of attack. They run flatter and don't need high compression tunnels because they sit on the bubble. More caspian sea monster style.
CFD is the way to go for theory in air.
There is next to no way to stop a blow over with current boats. The only person i know that has breathed out of the throttles and didn't chop them. That was Rique Ford .
Just thought I would throw that in.
pat W

tomcat

Thanks again for the info, shifter. I understand what you're saying about the inverted airfoil, stalling at a lower angle of attack. It also has less lift at a "normal" angle of attack. So as speeds have come up and weights have gone down, these boats just don't need as much lift, period. Is it also possible that trimming these boats for the corners is different, i.e. don't trim in to slow down; slow down to drop the bow?

Dueclaws

Great thread.