Tunnel Hulls, Wings and Ground Effect
03-20-2002, 11:00 AM
#11
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Excellent thread. There is some great conversation here!
I have to somewhat challenge the two links posted by tomcat, though. The authors evaluations are mostly correct, but not entirely forthcoming. It is true, that a flat plate will certainly create lift when held at an angle of attack relative to an onrushing fluid, but it is the airfoil that creates a great deal more lift.
A wing, fan blade, propellor, etc can be thought of as an Air deflector. Simply deflecting air downward, overcoming the air's inertia will create lift, as in the case of a flat plate.
There is, however, some validity in the bernoili principles when applying them to airfoils. Moreso than the aforementioned authors would care to admit. It explains a great deal, but not entirely. It's silly to believe that ALL lift is generated from air moving "faster on top," but it does also have a great deal of relivance.
Take for example a flat plate. It will create zero lift at a zero alpha. An efficient airfoil, on the other hand, will need to be placed at a negative alpha(negative angle of attack) to create zero lift. Some airfoils as much as 4 or 5 degrees.
No bernoili, huh?
In the case of tunnel hulls and cats, The airfoils are designed, angled and shaped to create desired lift/downforce at certain speeds. But primarily to be efficient---not generate too much drag by promoting laminar flow. (Flat plates aren't good at that).
I think one of the fundamental lifting characteristics of of tunnel hulls can be analogized(is that a word) with nozzles and hydraluics, rather than planes. You are simply trying to cram air into a smaller space than it would originally be at rest in the surrounding environment. ie, large openings towards the bow(more planar area) and then moving it aft and reducing it's volume(less area). This can only be done by increasing the pressure, hence, lift...according to Boyle's law P1V1=Constant=P2V2 provided T and n are constant.
It is quite easy to do this as well. Take for example that 40' cat with a 4 foot wide tunnel. Area=40x4 ft^2=160 ft^2=160x144 in^2.
23040 in^2......So for every 1000# of lift an average pressure difference of only 0.04 psi is required. That's not a whole lot. !
I have to somewhat challenge the two links posted by tomcat, though. The authors evaluations are mostly correct, but not entirely forthcoming. It is true, that a flat plate will certainly create lift when held at an angle of attack relative to an onrushing fluid, but it is the airfoil that creates a great deal more lift.
A wing, fan blade, propellor, etc can be thought of as an Air deflector. Simply deflecting air downward, overcoming the air's inertia will create lift, as in the case of a flat plate.
There is, however, some validity in the bernoili principles when applying them to airfoils. Moreso than the aforementioned authors would care to admit. It explains a great deal, but not entirely. It's silly to believe that ALL lift is generated from air moving "faster on top," but it does also have a great deal of relivance.
Take for example a flat plate. It will create zero lift at a zero alpha. An efficient airfoil, on the other hand, will need to be placed at a negative alpha(negative angle of attack) to create zero lift. Some airfoils as much as 4 or 5 degrees.
No bernoili, huh?
In the case of tunnel hulls and cats, The airfoils are designed, angled and shaped to create desired lift/downforce at certain speeds. But primarily to be efficient---not generate too much drag by promoting laminar flow. (Flat plates aren't good at that).
I think one of the fundamental lifting characteristics of of tunnel hulls can be analogized(is that a word) with nozzles and hydraluics, rather than planes. You are simply trying to cram air into a smaller space than it would originally be at rest in the surrounding environment. ie, large openings towards the bow(more planar area) and then moving it aft and reducing it's volume(less area). This can only be done by increasing the pressure, hence, lift...according to Boyle's law P1V1=Constant=P2V2 provided T and n are constant.
It is quite easy to do this as well. Take for example that 40' cat with a 4 foot wide tunnel. Area=40x4 ft^2=160 ft^2=160x144 in^2.
23040 in^2......So for every 1000# of lift an average pressure difference of only 0.04 psi is required. That's not a whole lot. !
03-20-2002, 04:07 PM
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Great thread.
As i have said here before, i did not take compressible fluid flow, but...
There are more airfoils than there are ants!! (just kidding) Whenever one talks about Bernoulli, remember that all of the so called "Bernoulli lift airfoils" will sustain inverted flight, some much better than others granted, but they will fly inverted. Typical airfoils, a twin engine Piper Aztec to use an example, fly inverted very well even though the airfoil was orininally described as generating lift in accordance with Bernoullis. Obviously something wrong there.
.04 psi is huge! On a 100' by 80' building that is 46,000 lbs. of uplift! Definitely something that needs to be considered when doing the building design. It is about an inch of water which in the scheme of things we are talking about is a lot!
Bernoullis of course is correct...but it most likely doesn't apply to airfoils no matter where they are.
Ted
As i have said here before, i did not take compressible fluid flow, but...
There are more airfoils than there are ants!! (just kidding) Whenever one talks about Bernoulli, remember that all of the so called "Bernoulli lift airfoils" will sustain inverted flight, some much better than others granted, but they will fly inverted. Typical airfoils, a twin engine Piper Aztec to use an example, fly inverted very well even though the airfoil was orininally described as generating lift in accordance with Bernoullis. Obviously something wrong there.
.04 psi is huge! On a 100' by 80' building that is 46,000 lbs. of uplift! Definitely something that needs to be considered when doing the building design. It is about an inch of water which in the scheme of things we are talking about is a lot!
Bernoullis of course is correct...but it most likely doesn't apply to airfoils no matter where they are.
Ted
03-20-2002, 05:55 PM
#13
Thanks for joining Baja Daze - We can agree that some lift comes from the simple pressure difference across the wing, but no where near enough to explain the actual performance. Not all the down wash comes from above the wing, but the majority does.
You raised a new point with the reduced area at the back of the tunnel. Wings create pressure below them to begin with, the sides of the tunnel contain this pressure, and so does the smaller exit at the stern. These things should increase the ground effect, maybe even to the point where what's happening above the wing is not the most important source of lift anymore.
Hello Ted - I agree 0.04 psi, or 1 " w.g. (water gauge) is a lot. To put this into perspective for readers, If you are trying to open a door outward on a building that has negative 1"w.g. due to mechanical exhaust ventilation within the building, over 5 lbs of force per square foot of door area is working against you; that's over 100 lbs of force on an average door. I am reading this out of my Industrial Ventilation Manual.
One of the things I'm going to do this spring when I get the boat in the water, is run some tubing down the tunnel and measure the pressure difference compared to the deck where I will be holding my digital manometer (pressure transducer). I never though the tools of the trade might help me understand my boat better!
You raised a new point with the reduced area at the back of the tunnel. Wings create pressure below them to begin with, the sides of the tunnel contain this pressure, and so does the smaller exit at the stern. These things should increase the ground effect, maybe even to the point where what's happening above the wing is not the most important source of lift anymore.
Hello Ted - I agree 0.04 psi, or 1 " w.g. (water gauge) is a lot. To put this into perspective for readers, If you are trying to open a door outward on a building that has negative 1"w.g. due to mechanical exhaust ventilation within the building, over 5 lbs of force per square foot of door area is working against you; that's over 100 lbs of force on an average door. I am reading this out of my Industrial Ventilation Manual.
One of the things I'm going to do this spring when I get the boat in the water, is run some tubing down the tunnel and measure the pressure difference compared to the deck where I will be holding my digital manometer (pressure transducer). I never though the tools of the trade might help me understand my boat better!
03-20-2002, 06:15 PM
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High tc,
You didn't mention Bernoulli. Just went looking so i could give you a site. Try http://www.geocities.com/galemcraig/
Good simple explanation on why Bernoulli is not involved in airfoil lift calculations. There are many others.
Ted
You didn't mention Bernoulli. Just went looking so i could give you a site. Try http://www.geocities.com/galemcraig/
Good simple explanation on why Bernoulli is not involved in airfoil lift calculations. There are many others.
Ted
03-20-2002, 11:45 PM
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Originally posted by tomcat
... These things should increase the ground effect, maybe even to the point where what's happening above the wing is not the most important source of lift anymore.
And:
One of the things I'm going to do this spring when I get the boat in the water, is run some tubing down the tunnel and measure the pressure difference compared to the deck where I will be holding my digital manometer (pressure transducer). I never though the tools of the trade might help me understand my boat better!
... These things should increase the ground effect, maybe even to the point where what's happening above the wing is not the most important source of lift anymore.
And:
One of the things I'm going to do this spring when I get the boat in the water, is run some tubing down the tunnel and measure the pressure difference compared to the deck where I will be holding my digital manometer (pressure transducer). I never though the tools of the trade might help me understand my boat better!
On point #1: Before the rulesmakers started restricting ground effects in the mid-80's, some Formula One cars were running with no front wings at all and only a token rear wing. In Indy Car racing, we actually ran some test laps at Indy and Michigan with no REAR wing. While the loss of downforce was minor, the loss of a "rudder" at the back had a, shall we say, significant effect on lateral stability. (It's about the only time I ever saw Tom Sneva scared....)
On point #2: Shifter is right in saying that CFD (computational fluid dynamics) is the way to go (long-term), but you still need to confirm your results in the real world. Quality data is the best way to fine-tune your mathematical model. (Note the emphasis on "quality" data - bad data is far, FAR, FAR worse than no data at all.
When all is said and done, you're likely to find that lots of hard work (and occasional blowovers) has gotten the modern cat to a pretty good state of development. My hat's off to all of those who have lead the charge.
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03-21-2002, 02:48 AM
#16
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I remember Sneva doing that, he got the big balls award that day.
Tomcat, I mixed you up a little.
the inverted airfoil only becomes a lifting device a 15 deg plus below that it is is pulling down. It is a violent transition. It seems to have been solved lately with the increased downslope of the deck TR-7 style. It will not allow the bow to raise up. That has allowed the boat not to kite, spill the compression and porpus, or blow over. That has made the boat run flatter and faster but it has induced a bad tendency to bow steer or hook and roll. One of these days they might look at some CFD. When they see it flies better upsidedown and backwards they will make the changes. It helped the hydros.
I have seen cats with big radius 10 inch down to razor sharp leading edges fr the tunnel inlet. big radius toooo much lift. the sharp edge stalls lower angle of attack.
The Caspian sea monster was a very fast troop transport made by the Russians. Picture a C-5 transport with stubby wings and 8 jet engines at the cockpit on a ski. Look it up.
Data is very important. Understanding the data and using it is another battle. You need to know the answer before you get the data that way you can tell people "I told you so".
In 1994-5 the Victory team ran a double air foil boat. It was ok but it had radiused sponsons like Assagi I think that had an ill effect in the boat negating the good. there were other things but that was a major problem. The flat upper deck seems to generate a lot of lift evenly.
pat W
Tomcat, I mixed you up a little.
the inverted airfoil only becomes a lifting device a 15 deg plus below that it is is pulling down. It is a violent transition. It seems to have been solved lately with the increased downslope of the deck TR-7 style. It will not allow the bow to raise up. That has allowed the boat not to kite, spill the compression and porpus, or blow over. That has made the boat run flatter and faster but it has induced a bad tendency to bow steer or hook and roll. One of these days they might look at some CFD. When they see it flies better upsidedown and backwards they will make the changes. It helped the hydros.
I have seen cats with big radius 10 inch down to razor sharp leading edges fr the tunnel inlet. big radius toooo much lift. the sharp edge stalls lower angle of attack.
The Caspian sea monster was a very fast troop transport made by the Russians. Picture a C-5 transport with stubby wings and 8 jet engines at the cockpit on a ski. Look it up.
Data is very important. Understanding the data and using it is another battle. You need to know the answer before you get the data that way you can tell people "I told you so".
In 1994-5 the Victory team ran a double air foil boat. It was ok but it had radiused sponsons like Assagi I think that had an ill effect in the boat negating the good. there were other things but that was a major problem. The flat upper deck seems to generate a lot of lift evenly.
pat W
03-21-2002, 10:24 AM
#17
Thanks again guys for making this thread so interesting.
Ted - Good link. I like the way that guy explains it.
C_Spray and shifter - To sum up, guys with big balls find out what the limits are, then engineers write the equations to match the data. As long as we don't try to extrapolate too far with those equations, the rest of us can stay out of trouble. But every time someone uses technological advances to put more power in a lighter vehicle, we are pushing the envelope again.
Applying everything that has been said here, what do you guys think about the next limit; 200+ mph in a propeller driven cat?
Ted - Good link. I like the way that guy explains it.
C_Spray and shifter - To sum up, guys with big balls find out what the limits are, then engineers write the equations to match the data. As long as we don't try to extrapolate too far with those equations, the rest of us can stay out of trouble. But every time someone uses technological advances to put more power in a lighter vehicle, we are pushing the envelope again.
Applying everything that has been said here, what do you guys think about the next limit; 200+ mph in a propeller driven cat?
03-21-2002, 11:18 AM
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With continued careful attention to aerodynamics, 200 mph should be no problem. The Unlimited exceed that as do the drag boats by a lot for a small amount of time.
Fitting equations to the data is really an important point, not just an aside. This stuff is very empirical as the boat transitions from minimum amount in the water to maximum amount in the air all the while keeping "hooked up."
Complicating it is the "packed tunnel" effect and the aerodynamics of the shape some parts of which are causing turbulence mostly not on purpose. And on a moderately windy day, say 20 mph, adding or subtracting 40 mph to the "numbers" It is a nightmare!!
All of the equations describing all of the phenomena are probably right, how much each applies and when is probably unknowable.
Fitting equations to the data is really an important point, not just an aside. This stuff is very empirical as the boat transitions from minimum amount in the water to maximum amount in the air all the while keeping "hooked up."
Complicating it is the "packed tunnel" effect and the aerodynamics of the shape some parts of which are causing turbulence mostly not on purpose. And on a moderately windy day, say 20 mph, adding or subtracting 40 mph to the "numbers" It is a nightmare!!
All of the equations describing all of the phenomena are probably right, how much each applies and when is probably unknowable.
03-27-2002, 11:20 AM
#20
Here is a good link to a three part article on tunnel boat design.
http://boatdesign.net/articles/tunne...sign/index.htm
Written by the same author as "Secrets of Tunnel Hull Design" that little ad in the back of the performance boat mags for many years. Turns out the author lives about 1 1/2 hours away from me. Small world!
The key point in this article:
The center of lift in an airplane is behind the center of gravity, so if a gust of wind increases the angle of attack and increases lift, it tends to rotate the plane and reduce the angle of attack, therefore self-correcting. In a typical tunnel hull with the engine(s) at the stern, the center of lift is ahead of the center of gravity and if a sudden gust of wind lifts the bow and increases the angle of attack and lift, the situation is inherently unstable. The boat will increase the angle of attack further, creating more lift and maybe a blowover.
Good reading.
http://boatdesign.net/articles/tunne...sign/index.htm
Written by the same author as "Secrets of Tunnel Hull Design" that little ad in the back of the performance boat mags for many years. Turns out the author lives about 1 1/2 hours away from me. Small world!
The key point in this article:
The center of lift in an airplane is behind the center of gravity, so if a gust of wind increases the angle of attack and increases lift, it tends to rotate the plane and reduce the angle of attack, therefore self-correcting. In a typical tunnel hull with the engine(s) at the stern, the center of lift is ahead of the center of gravity and if a sudden gust of wind lifts the bow and increases the angle of attack and lift, the situation is inherently unstable. The boat will increase the angle of attack further, creating more lift and maybe a blowover.
Good reading.
