Streamlining sponson tubes

[Preston_Hall]

Please explain how a round tube produces positive or negative "lift". It's neutral, isn't it?

Also, if an airfoil is symmetrical and set neutral then how can it have positive or negative "lift"?

 

[Climate Models]

I was wondering the same thing as Preston.

Unless you know something that is going to change the world as we know it, a tube produces no lift, and a symmetrical airfoil at a 0-degree angle of attack does the same thing.

Peter R.

www.climatemodels.com

 

[Guest_scott bouchie]

airplanes fly with symmetrical airfoil so they must make lift cause it supports the weight of the airplane.

 

[DaveMarles]

Tom, like the others, I would like to know how a symetrical airfoil section with zero angle of attack is going to produce lift. I think Boeing would be interested in signing you up if you can make it work. B)

Dave

 

[Eric Perez]

Symmetrical airfoil wings only produce lift if the angle of attack is postive.

 

[Propjockey]

Ok...you ready for the answer? It's called 'ground effect'. (although here I guess it could be called 'water effect') Anyone that flys model planes knows about it.

As an airfoil travels forward in close proximity to the surface, the air between the two is compressed. This pocket of compressed air pushes down on the surface and up on the airfoil...VOILA, lift.

Now, the farther from the surface the airfoil is, the less lift is generated. A 1/2" sponson tube six feet from the surface of the water will not produce any measurable lift, but if it's one inch from the surface, you will see it.

BTW...Boeing can't afford me!

 

[Tom v.d Brink]

I agree the ground effect indeed increases lift. even on a neutral foil.

the only thing that is unknown in a simulation program is the effect of an unsteady ground like water..............

a round tube should be neutral again unless it is in ground effect. besides that the boat pictches in some other effects to. but once more i tested the effect on the whole boat not just the foils themselves.

results are very good when you look at just a foil but the effect on a whole boat is smaller and that was what is was interested in.

the last thing i noticed was if you don't put the foils at the same level

(one higher then the other even slightly)

the airflow of the trailing foil will be effected by the air coming of the first foil.

this can increase lift or negative lift depending on the height offset and the

profile of the first foil

 

[DaveMarles]

As an airfoil travels forward in close proximity to the surface, the air between the two is compressed. This pocket of compressed air pushes down on the surface and up on the airfoil...VOILA, lift.

As air passes over(or under) an airfoil, it speeds up reducing pressure which would be roughly equal on the top and bottom of a symetrical airfoil. Bernoulis principle could possibly apply here as it does in racing cars i.e. a reduction in lift because of the proximity of the tubes to the water. If there were an angle of attack on the sponson tubes then possibly there would be a ram effect such as that used in the russian WIG planes but that doesnt apply here. If there is no angle of attack then there should be no lift with symetrical airfoils around the sponson tubes. Tom saw an increase in lift with his tests but that could be just because there is reduction in drag on the sponson tubes allowing the high lift sponsons to have more effect.

If a hydro was being redesigned by a F1 car designer, would they have high drag round sponson tubes. Noooo.

I think Mike Bontoft has the right idea, don't think about why it won't work, try it, test it, do it

Boeing would have sacked me for being too argumentative

Dave

 

[Ian Inverarity]

Tom's CFD analysis shows that even the round tubes create lift, I think this is indeed a ground effect thing. I would be more concerned with how an air-foil section reacts when the boat pitches up a bit (hits a wave). For round tubes they get further away from the water surface so the lift from the booms should decrease. But with an air-foil section the air-foil now runs at a positive angle of attack and would provide lift.

On a boat with short booms the effect would be lesser.

Also the aerodynamic interaction with the airflow could be quite complex.

I would like to see the boat that an F1 designer would create! I am guessing that lift minimisation and reduced pitch sensitivity would be the aims.

Tom, can you simulate the effects of the hull pitching up a small amount?

Ian.

 

[DaveMarles]

Ian, There is no ground effect lift from a round tube. The opposite is the case. A convex surface with no angle of attack (a tube) will actually have negative lift in ground effect. ie 'suck' down.

I would think that when the hull pitches up it has lots of lift from the body anyway. Why don't you put a wing with zero angle off attack at the rear of the boat. Then when the front of the boat lifts, the rear wing gets some angle of attack to balance out the increase in front lift. I've done it on mono's and its very effective.

Doesnt a rigger have something like that now with the rear sponsons on crapshooters and the winglets on the SG's?

Dave

 

[Watercadet]

What about the full size hydros that use (adjustable) canards (air foils)? Maybe a gyro could be used on a model like the ones in helis. But then the added weight would counteract the decrease in drag from the tubes! :unsure: I guess we will just have to test!

And like Preston said about the boats moving in a vertical manor, that would not change the angle of attack.

 

[Guest_bzubee]

If the airfoils were allowed to be self adjusting float on the tubes as was suggested earlier the lift should be negated. Correct?

 

[Preston_Hall]

I just have to quote myself.

If you used a symmetrical foil I wouldn't think you would gain lift if set properly. What you may experience is a reduction in the current drag that would allow the current lifting ability of the hull/sponson design to be overly effective. If that is the case, some of the lifting ability could be removed from the hull/sponson design and still benefit from the reduced drag.
Lift isn't really a problem if you can control it. I noticed that the Eagles and Crapshooters don't seem to get loose enough to kite off the water. My CS seems to fly nice and level when it comes up. Jumping up off the water in a vertical movement and pointing towards the moon when light are two diffrent things.

Like Dave says a little more clearly, if the back of the boat experiences the same pressures as the front, then you will never flip or pull a wheelie.

Daves is right about groung effects. Closer to the ground, the more negative lift you get.

Ok...you ready for the answer? It's called 'ground effect'. (although here I guess it could be called 'water effect') Anyone that flys model planes knows about it.
As an airfoil travels forward in close proximity to the surface, the air between the two is compressed. This pocket of compressed air pushes down on the surface and up on the airfoil...VOILA, lift.

If you fly planes you will soon learn that on your landing approach at a certain point you will get sucked to the ground. I think what most people experience on landing is over-compensation for the negative lift. Therefore the plane suddenly porposes.

If a plane compresses the air then so would a helicopter. Then the helicopter would have to vary rotor velocity. But it doesn't. It controls the rotor pitch with the same rotor velocity. Everything moves due to changes in pressure on the surface of the object.

 

[Climate Models]

Guys, Guys, Guys, You are not thinking clearly here. You are making general predictions based on a comparison with a known factor that has no resemblance to the problem.

An aircraft wing in ground effect is inherently stable and maintains a general correlation to the ground in its angle. If it pitches up or down a few degrees, the resultant vector is translated in to gains or losses of lift due to the angle of attack and the cushion of air the wing is riding on.

We are talking about a wing with a significant chord compared to a tube that is less than 1/2" in cross section.

You can not compare a full sized F1 car, a full sized boat spoiler, or even a model airplane wing or helicopter blade to a sponson boom tube.

Here is a great little program to see this for yourself. You can experiment with different foils and see that in the sizes we are thinking, there is virtually no measurable difference in lift, drag, and ground effect has almost no effect on the foil.

Have fun with this.

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

Peter R.

www.climatemodels.com

 

[Preston_Hall]

there is virtually no measurable difference in lift, drag,

Then what might Mike be doing wrong to get the measurable diffrences on his test stand?

 

[Climate Models]

Then what might Mike be doing wrong to get the measurable differences on his test stand?

He is not doing anything wrong.

Here is a good webpage that describes the theory.

http://www.princeton.edu/~asmits/Bicycle_web/blunt.html

Mike is simply reducing drag on the surface of a tube, which is less than 1% of the entire surface area of the boat.

The drag that he is reducing is miniscule in relation to other "fixable" points that create significantly more.

What I am saying is if you are trying to fix an area that contributes to less than 1% of the total airflow drag on the boat, while ignoring areas that are 30-40% greater in producing significant, and visible amounts of drag.

Start playing with tub and sponson shapes. This is where your biggest gains will be made.

If you aim for 90% efficiency, and end up with 60%, you have made a significant gain. If you aim for 2% and end up with 1%, what gain have you really made?

Once you have eliminated all of the major points that create drag, you can focus on the smaller ones.

Peter R.

www.climatemodels.com

 

[Preston_Hall]

Backing up a little............

The control area he used seems to be about the length of 1 of the 4 exposed boom tube sections on a boat. The reduction in drag was 120g X 4 tubes = 480g = 1.06lb
A while back we came up with a number of 17lb to accelerate a boat from 30-100mph. So we are talking about 6.2% of the force required being removed.

1lb of measurable drag is removed. Now if it isn't that significant when the entire boat is taken into account then obviuosy there improvement to be made elsewhere. But to remove 1lb of drag force caused by the boom tubes has to be significant.

 

[Climate Models]

120g X 4 tubes = 480g = 1.06lb

First of all, this calculation is flawed. It is a static measurement, not a dynamic one.

Was lateral loading of the bearing surface calculated? What was the bearing surface? Was the surface of the raw tube the same as the streamlined one?

Even if all of this was considered the rear tubes will not see the same air pressure as the front ones, because they are in turbulent air. Also this calculation would only be correct in a straight line. In a turn the resultant vectors would give a significantly different result. Again, miniscule in the grand scheme of things but since we are getting picky.

17lb to accelerate a boat from 30-100mph

I would be interested in seeing the equation that was used to determine this. There are so many factors missing that this statement can not even begin to be taken as fact.

Just in case any of you are wanting to jump on me at this point, I am not trying to discredit anyone’s theory, or say that anything we do to improve our boats is not beneficial, but the amount of miss-information flying around is scary.

FYI I fly full sized aircraft. I have built my own aircraft (from scratch), and I love trying to get the most I can from an airplane. Aerodynamics and hydrodynamics are a lifetime study for me.

Our electric rigger kits are know for being the fastest production riggers in the world. I put all of my knowledge in to their design.

I am not saying I know more about it than anyone else, but I humbly have to say that I have a prety good understanding of it at this point.

Keep up the topic. This stuff is great!

Peter R.

www.climatemodels.com

 

[Ron Olson]

I have to agree with Preston, especially those of us that run smaller boats.

I used to race 1/10 scale on-road cars years ago and by changing bodies alone, I could see a visible difference in the handling of my cars. I didn't think that this was possible with something this small (and at that time) slow as we were racing on small tracks where we really didn't attain any big speeds. One pound of drag can make a difference when we have people that are trying to find a way to lose ounces or grams to go faster.

 

[user 115]

Ron,

Peter is saying that the calculation of a pound of drag is potentially flawed.. No doubt that if a pound of drag was able to be reduced, then this change would be huge. However, it seems to me that Peter doesn't believe that the formula used to calculate the pound of drag is correct.

Sorry if I mis-spoke for you Peter..

Charley