Airfoil with rear center of lift?

[nativepaul]

Hi Peeps, This isnt for an airplane aplication but I'm looking for a wing section that has the center of lift and center of pressure at 60% or as far back as possible if 60% cant be reached, instead of the more normal 25-30%.

As its the center section of a catamaran, not a plane wing it will have some advantages and disadvantages:- the wing will allways be in ground efect, the wing will have tip fences(sponsons) right down to the surface, tho if I can get it to ride fractionaly above the surface so much the better, the wing will have a very low aspect ratio, 2'long 1'wide.

A catamaran needs the cg 25-30% from the rear to peform hydrodynamicly, with a normal wing this puts the lift well in front of the CG, and makes them prone to fliping, I'm thinking that if I can get the CoP and CoL back to where the CG is it will lift flat and spill air from underneath, rather than fliping

I'd rather most of the lift came form spillable pressure at the bottom rather than lift onthe top, to regulate ride hight, but maybe the physics to do this are only available a paralel dimension.

I'm not after that much lift 2.5kg at 70 mph.

Hope someone can help me with some of this, even if it telling me which bits arnt possible. Paul

[Sparky Paul]

Try a reversed airfoil in one of the online aerodynamic programs... X-Foil might be able to do this...

[Gary Warner]

Try moving the max thickness back to the 60%. It will allow for a more rearward CP and CG, but the forward portion can reduce the effective loading area, i.e. it will have less effective area of lift then traditional wing of same size.

[Sparky Paul]

I've flown planes with the wing on backwards. Nothing much changes.
An inverted wing might be a better solution.. or a Gurney Flap.

[nativepaul]

Please excuse my ignorance, what is a Gurney flap? I know a few different types of flaps but have never heard of gurney, maybe, being English I know it by a diferent name.

[Sparky Paul]

The Gurney Flap comes from auto racing.. it's a blunt addition vertical to the surface.. used in cars to increase the down force on the aft mounted wings..
Some sprint cars have them on the large center-mounted wings peculiar to these cars..
Here's an aerodynamic investigation..
http://www.lib.ncsu.edu/etd/public/etd-4191713610151931/etd-title.html
On models, they make good trim devices for offset any problems with built-in warps.

[yoyoML]

I think you could just add a horizontal stab. Make twin booms and attach the stab much higher than water/main wing level. Adjust the np with fixed cg as you would with an ordinary plane. Adjust stab incidence to get desired pitch.

[marfish]

I've never seen nor ridden on a catamaran that was only 2' long, so you must be intending to place the relatively short wing near the rear of the platform. OR THIS IS A MODEL CATAMARAN. Which would be very Cool. At those speeds, I expect that your hull is more than 20 feet long or you're sailing a smallish model on a very narrow strip of water that has no wind chop. In the case of the larger boat, just use whatever airfoil gives you the best lift/drag for the total lift you are wanting. Set it toward the rear of the platform, or behind, above, whatever. By 70mph, I assume that you mean airspeed because there ain't no sailboat that can top 60...yet. The windsurfer guys are in hot competition with the fixed mast sailers for the best speed. In the case of a model... I gotta see this thing fly!
Marlan
P.S. I found a foil that gives good rear wing lift at about -2*. "BE6606" The lift is...gobs (.8Cl at 0*) and goes up from there. The bad is that the lift shifts forward as you leave the neg. AoA and the best L/D is at +3* alpha. Look in Profili at the foils and process the pressure coeffecients button.

[nativepaul]

Now were cooking with gas, my cat is a balsa model 34" long and 16" beam the 2 sponsons held apart by a 24" long 12" wide wing, sorry but if you tried to ride it, it would either sink fast or break and sink faster!

Powered by an electric motor so theres no worry about coping with the water you get with 60mph winds, its to be run only on flatish lakes when theres a gentle breeze or less.

Unfortunatly when the nose starts to rise i need the lift to shft backwards not forwards, lift going forward on nose lift will lift the nose more and hence shift the lift fore again and domino into a certain blow over.

That shifting lift is something i hadnt thought about but is very important, is there a foil that moves the lift back with greater angle of attack? this could stabilise itself at a specific AoA if i designed it right, a very potent thought thanks

[shaneyee]

Wouldn't it be more effective if you work on the hulls instead of the wing? For instance having the center of bouyancy shift aft as the hull starts to pitch up.... this could take the form of a fuller stern above the waterline. The problem you seem to be getting is that the middle or forward part of the hull is planing and developing lift as the speed increases and CG is behind the center of lift. IF the hull shape is fixed perhaps you could fit trim tabs on the transom or even strakes on the aft hull just above the waterline. A quik fix might be to just shift weight forward and this couldbe accomplished by moving the battery toward the bow.

Shane

[BMatthews]

Sorry but in the words of that famous TV ad "you can't get there from here". There's no airfoil that has a consistent center of lift that far back or anywhere near there. Altering the shape won't do it either. The center of lift or center of pressure does not follow the maximum thickness point at all.

Your best bet is to move the "wing" back so that the lift at the 25% point coincides with the spot you want it to be and let the hulls stick forward like picklefork tines.

But in the end I doubt it'll matter what the shape is or where it's located. You call for a joiner that is 2 feet in chord by 1 foot in span and that is located well within the ground (water? :D) effect range. Presumably the hulls will be far in excess of the wing size. Given the angles of the wind off the bow that you'll typically encounter the wings will be largely masked by the hulls. So the lift produced will be minimal in any event.

If you really want some lift from the joiner then I would suggest placing it towards the rear to better coincide with the hydrodynamic "lift" center. There may be some benifit to angling the airfoil of this joiner to provide some lift similar to that used by the wing in ground effect skimmers and Akranoplanes (do a search on them, very interesting reading). But be aware that some drag will come from that sort of lift. Is it less than the hydrodynamic drag that you'll avoid by using this lift? I dunno. Good question that.....

[nativepaul]

Thanks BMatthews, you confirn what I hoped wasnt the case, that I cant get the CoL back with a wierd wing section, My cat will have to have more wing at the back and will look funny.

The WiG is exactly what I am after, but with cat config to stop roll in corners, for fast laps rather then the more usual mono for load carying and aerodynamic efficeincy. Mine will be driven by a surfice piercing boat prop for hight stability and acceleration rather than the more usual jet or plane prop (done im guessing for a combination of lightness ease of set up, and that its alot easyer to make a variable pitch plane prop than boat for maximum eflux velocity or jet for fullscale speeds.

Now its either the small rear wing or canard config, I know with a canard I can set the front wing up to stall before the rear to stop me fliping, but that hapens at high angle of attack and the front slaming down can easily lead to submarineing which is very bad, Ive seen a boat lost this way (at least with a flip you get the bits back)
Marfish talked of wings where the CoL moves forward when the AoA rises, are there wings there it moves backward instead? I'm thinking that if as the nose comes up the lift moves back and the nose will drop and maintain level before it gets high enough for a slam down and submarine. or is this just wishfull thinking too!

As for a small wing at the back Im in the dark, im guessing id treat it like a flying wing and put a bit of reflex in it but I know nothing of flying wings the only one Ive seen was unstable but the consencus in the club was the it was too well bashed and the controll surfaces whern't stiff enough anymore.
If I used the middle of a zagi wing, glassed with glassed in place elvons, for my small rear wing. Would that give me the pitch stability I'm after?

[nativepaul]

Hi Shaneyee, Im only askin aboout the wing because Ive made the sponsons allready, I've been into boats for most of my life so I'm better at hydrodynamics than I am at aerodynamics. I am confident that if I can match my hulls to the right center section design I'll have something incredible so i come to a plane forum for help learning more from those who know about aerodynamics.
The hulls are indeed built with more lift at the transom when most of the hulls in the water and it moves back as AoA increses to having the Center of Boyancy about an inch from the stern at 5degrees which is just on the good side of a flip but very unsafe, but thats not what I'm talking about. A cat set up loose for calm wind & flat water will run a couple of degrees nose high and only the last few inches of sponson before the transom will touch the water, so little in the water that, if theres a gust of wind on the upwind leg, the CoL being in front of the CoG overules the fact that the CoB is behind the CoG and you get a flip. :censored: Its definatly an aerodynamic problem.

Theres 2 things im trying to acomplish here:-
1, Get enough pitch stability to run a little nose high, without that damned flipping on gusts into wind.
2, Combine that with enough lift to get the hull right out of the water on the straights for maximum speed. I'm not sure this is the way to go, I think the yaw from rudder wil blank the block the tunnel enough, to drop it back on the water and get some turning done, but I've been wrong before, I may need need a turn fin and that may be more drag than a little weight on the hull. I sure want to test it tho.

It would be great to be able to vary the amount of lift for testing #2 so im now thinking of an elevator. Am I right in thinking that, with the flying wing style small wing at the back, an elevator would directly controll pitch not lift? Just what I'm trying to avoid. and that I would need to mix the canard in with the the elevator/flap to make it a lift controll instead of pitch?

[BMatthews]

The concept about center of lift isthe old way of considering what happens. BUt it has the oddity that as the angle of attack reaches the zero lift point the Center of LIft reaches a point infinitely behind the trailing edge. More recently (not sure when but somewhere around 20 years back?) the method of describing how the airfoil torques was changed to accepting that the lift point occurs at the 25% chord point (which it does for symetrical airfoils of all sorts) and describing the pitching effect as a torque that occurs around that 25% point. This torque alters slightly with changes in the angle of attack and is shown as a graph. It's much easier to deal with this method because nothing runs up to infinity at all and that's a lot tidier. With this new method there's no such thing as the center of lift wandering around on the airfoil.

[rofujiyama]

Hi tackled the same goal in the mid 70's , when i raced boats in namba classes. Sparky Paul's idea of the reversed airfoil is a totally viable solution to blow overs because i have used the concept on both a catamaran and a hydroplane with fantastic results, if they ever broke free, they would just float and then settle back down and keep on running.
The concept revolved around a flat bottom winged trainer airplane , and its ability to fly inverted , but not really able to recover to level flight, right side up, by doing an outside loop.
Also , the same airfoils highest speed ability would be, at a negative angle of attack in level flight, right side up.
Also , since you are dealing with ground effect, while skimming along the water , the underside of the foil shape, trapping air within the tunnel would generate the lift I was hoping to achieve, regardless of the contour of the bottom of the foil shape, hopefully. LOL.
Well, all said and done, the airfoil shape utilized a sharp leading edge upside down, very slight semisymmetrical airfoil with a positive angle of attack with enough thickness for sponson support.
The sharp leading edge, I felt would stall faster and provide less frontal drag.
You can test this idea with a small piece of balsa with a flat bottom airfoil and trying to see how it travels when you toss/push it forward with the airfoil curve on the top , or the curve on the bottom, just a wing section ,now ,no fusealge or tail.
When I see Miss Budweiser blow over, or any tunnel hulls go over , it makes me sad.
I am not sure if it will attain maximum possible speed , but sure makes it less stressfull, when it gets choppy and you want to go FULL THROTTLE! :D
Ro

[JetPlaneFlyer]

Oops... posted in wrong thread :o

[marfish]

Paul:
my thoughts may seem a bit heretical but, here goes: In my understanding and observation, any airfoil that has a movable trailing edge as in flaps has a movable lift point. It would seem that the 25% point of lift is good for non-flapped symmetricals. Other lifting foils exibit pitching moments that indicate more rearward or forward points of lift. It may be true that the lift point doesn't change mathmatically or theoretically on a foil, but try a flying wing and you'll see what I mean. Drop the flaps and you have pitching authority that well surpasses the lifting moment of the "lift point". Without a tail to counteract the pitching moment of the "flaps" now elevons, the wing pitches down(or up depending,,,). What is it that could cause that to happen? :p ;)

So,,,,,,, maybe you could make a 1 foot long wing that has a 6 inch chord set at the rear of your boat. Place it under your 24 inch long deck that has an inverted foil and is pitched with some downward wedge toward the rear(negative AoA relative to the foil OR positive AoA in relation to the water). That way the deck will act somewhat like a jib sail on a boat, aiming the accelerated air onto and around the top side of the rear wing, while subtlely keeping the nose down. Set your wing on pivots that allow it to change AoA or simply put flaps on it to change the lift as it changes the lift center of the wing whenever you want to adjust the lift.

Another way to look at it is that maybe the inverted foil with wedge pitch is all you'd need to satisfy both requirements. While on the water, the leading edge might help lift, but once elevated it might start to pull the nose down. You could experiment with flaps that would "camber" the trailing edge for lift at the rear.

Oh, and by the way, I did a little mock up on FoilSim and it looks like a 6 inch Chord on a 12 inch wide wing with 2% camber set at 2.2* pitch, going 70mph at sea level would produce apprx. 2.5 Kg. of lift.
Have fun,
Marlan

[JetPlaneFlyer]

I think the confusion here is largely on of terminology...

The Centre of Pressure (i.e. centre of lift) of an airfoil moves according to angle of attack and would move dramatically rearward if flats were deployed... So it will be possible to design an airfoil that, at least with a certain angle of attack, will have a centre of pressure at or around the 60% mark

The confusion I believe comes because some people are thinking about ‘Aerodynamic Centre’... The Aerodynamic Centre is something aircraft designers invented to make the their sums easier... It is a point (very close to 25% chord as subsonic flight) where the moment remains constant with changes in angle of attack... This is not the same as 'centre of lift'.


Steve

[marfish]

One more thing I just thought of while drifting off to sleep is that you could use a gyro on the pitch axis coupled to the rear wing, front wing, both....
How far off the water can it float before they disqualify it as a boat? Once the sponsons clear the water.... I think I see a longer prop shaft in your future! Lol.
Marlan

[yoyoML]

The confusion I believe comes because some people are thinking about ‘Aerodynamic Centre’... The Aerodynamic Centre is something aircraft designers invented to make the their sums easier... It is a point (very close to 25% chord as subsonic flight) where the moment remains constant with changes in angle of attack... This is not the same as 'centre of lift'.


Steve

I'd like to think of AC as the point any "additional lift" acts on. By additional lift I mean the difference in lift as you change AoA. Since any lift change acts on this point, there's no torque change around this point (no moment arm), as you change AoA.

It'd be a bit different with drag considered, I guess.

Oh, and this point actually changes with AoA when AoA is large... For example it must move close to 50% chord when AoA is 90 degrees, opposed to 25% chord for small AoA.

[yoyoML]

After a long time, I still think my idea was the best and most simple. Just add a horizontal stab!!! That's what stabs are for in the first place...

You can even tune the stab's AoI to adjust desired main wing lift. How nice is that?

[JetPlaneFlyer]

I'd like to think of AC as the point any "additional lift" acts on. By additional lift I mean the difference in lift as you change AoA. Since any lift change acts on this point, there's no torque change around this point (no moment arm), as you change AoA.


I don’t think this is the case... I believe AC is the point at which the moment is (virtually) constant regardless of AoA... but the moment for any cambered airfoil would not be zero about the AC.

You don’t have to take my word though... here is NASA's words on the subject:

It has been found both experimentally and theoretically that, if the aerodynamic force is applied at a location 1/4 chord back from the leading edge on most low speed airfoils, the magnitude of the aerodynamic moment remains nearly constant with angle of attack. Engineers call the location where the aerodynamic moment remains constant the aerodynamic center (ac) of the airfoil.

Quote from http://www.grc.nasa.gov/WWW/K-12/airplane/ac.html

In fact the point at which the moment is zero is the Centre of Pressure (which pretty much illustrates my point about the confusion between these two terms)

Steve

[yoyoML]

I don’t think this is the case... I believe AC is the point at which the moment is (virtually) constant regardless of AoA... but the moment for any cambered airfoil would not be zero about the AC.

You don’t have to take my word though... here is NASA's words on the subject:

Steve


I don't have to take your word, but your word is my word anyway, if you please reread my post.

I'm not saying there is no torque around the AC, I'm saying there is no change in torque around the AC. This implies that all change in lift with AoA must act on the AC, for the additional lift to exert no additional torque (around the AC).

[JetPlaneFlyer]

Re-reading your post I see that we are generally on the same page...However you did say that the AC moves with AoA... I dont think this is true. Th AC is fixed at very close to 25% regardless of AoA (only moving back at supersonic speeds, which we probably dont need to worry about for models)... its the Centre of Pressure that moves with AoA

[yoyoML]

Re-reading your post I see that we are generally on the same page...However you did say that the AC moves with AoA... I dont think this is true. Th AC is fixed at very close to 25% regardless of AoA (only moving back at supersonic speeds, which we probably dont need to worry about for models)... its the Centre of Pressure that moves with AoA

AC does move with AoA, somewhat. Consider a simple flat rectangular wing which originally flies at 0 AoA, when the AC sits nicely at 25% chord, and the torque around 25% chord is zero. Now increase AoA to 90 degrees and guess where the torque is zero? 50% chord! (I think it's trivial: the wing is for-aft symmetrical.) So in order to maintain the torque around AC zero, the AC has to move from 25% to 50% as AoA goes from 0 to 90.

Now try increasing the AoA to 180 degrees, and guess where the AC is now? 75% chord!

That's why I said the AC moves with AoA, BUT ONLY WHEN AOA IS LARGE. I think anywhere below stall AoA the AC remains very close to 25% chord regardless of AoA.

And supersonic speeds... I think the AC moves to 50% chord when supersonic. Have to check, though.

[rofujiyama]

Here's the boat I was talking about. I cut it up a long time ago,because I forgot what the foil looked like :confused: It used a Taipan .21 w/tuned pipe.
There was a tiny wing aft in the gap by the transom to give the sides a little more support. I think the balance point was at/about the forward stuffing box location

[rofujiyama]

:rolleyes:not uploading sorry

[nativepaul]

Is there any chance of you E-Mailing me the picture?
I'm at paul@nativebeats.co.uk

[rofujiyama]

Hi Paul sent you the pics .hope it goes through. Clicking on the photos ,may make the photos enlarge. Good luck with your projects. :D
Roland

[Sparky Paul]

Personally, I think anything you put back there will so small that any aerodynamic effects would be the same, no matter what the shape.. flat plate or finely tuned profile, ... maybe a gi-normous Gurney flap -might- have some effect.
The RN just isn't there, to have any real control of the airflow.

[rofujiyama]

hope this try works

nope :mad:

[rofujiyama]

whew, I think , IE 7 , and latest java is not together yet , I used the basic upload ,now :( there was a full top cover,too ,but i cant seem to find it :D