Turn Fins

[Mark Grannis]

Instead of hijacking the sponson offset thread let's talk here.

I visualize the turn fin as just a wing down in the water. Please pardon my lack of correct terms, for I've never flown RC Planes. To increse the amount of lift increase the distance the fluid travels over one side.

To keep this simple just think of a turn fin going dead straight through the water....no shims or tweaked trailing edges.

I've always been told not to sharpen the inside (left) of the fin...(I don't know why). Now if the leading edge of the fin is (sharpened on the outside) a nice tight radius the force to the right is X. Now if you make the length the water travels longer by sharpening the fin farther toward the back of the fin, I'm guessing the new force is greater than X.

Does the blunt edge have a greater drag than the longer transitioned side? Equal or less?

I used to think the sharpened edge was pushing the fin left, but now I admit I don't know. My last test sesssion was interesting. I'm just trying to understand.

Maybe I should check my rudder servo!

MG

 

[Greg Hahn]

Instead of hijacking the sponson offset thread let's talk here.
I visualize the turn fin as just a wing down in the water. Please pardon my lack of correct terms, for I've never flown RC Planes. To increse the amount of lift increase the distance the fluid travels over one side.

To keep this simple just think of a turn fin going dead straight through the water....no shims or tweaked trailing edges.

I've always been told not to sharpen the inside (left) of the fin...(I don't know why). Now if the leading edge of the fin is (sharpened on the outside) a nice tight radius the force to the right is X. Now if you make the length the water travels longer by sharpening the fin farther toward the back of the fin, I'm guessing the new force is greater than X.

Does the blunt edge have a greater drag than the longer transitioned side? Equal or less?

I used to think the sharpened edge was pushing the fin left, but now I admit I don't know. My last test sesssion was interesting. I'm just trying to understand.

Maybe I should check my rudder servo!

MG

I don't think we can compare the lift properties of air and water because of the obvious difference in density.

With a 1/8" turn fin sharpened on the outside only, I look at it like we're displacing (moving) 1/8" of water to the right which exerts a force to the left on the turn fin.

The shape of the fin whether blunt or as you said slowly tapered, determines the amount of drag created while displacing the water. The slower you displace the water the less energy required. So making a gradual taper on the turn fin will be faster while still creating the 1/8" displacement .

I look at the turn fin sharpening outside only, as a counter to prop torque. If the turn fin will make enough left pressure to counter the prop torque so the rudder will require no trim, then you're as clean as possible.

Greg

 

[Tom Foley]

By gradually sharpening the fin over a longer distance I believe you have a flatbottomed airfoil vertical in the water which will give more pull to the right (top of airfoil ) . by sharpening it more bluntly it will push to the left more and correct for prop torque better . So while it may have a little more drag , it has the benefit of better handling which I believe is more valuable most of the time anyway .Mark your boats are so much faster than mine anyway so I might as well try to concentrate a little more on going straighter !!!

 

[Rigged]

while i guess being off-topic, but still on the topic of turn fins... heres something ive wondered while testing different fins on my gas Rigger.

Why can you have two fins, somewhat similar...that one in a turn throws alot of water off of it, and the other you barely see any water coming off the fin?

I see alot of Riggers that throw alot of water off the fin in the turns... my FireFighter with a Crapshooter 90 fin on it throws barely (if any at all) water off of it through the turns.

 

[Mark Grannis]

Tom your stuff is plenty fast who you kidding.

Greg, I used to think the same way as you, but I think I was wrong.

I think the greater the difference the more pull. I'm going to replace what I think is a good servo just to confirm my findings.

The amount of spray is related to the efficiency of the fin. Marty Davis' current .21 has very little if any spray. It gets through the turns so smooth. I can't wait to run it in Atlanta since he can't make it.

MG

 

[John Finch]

Tom your stuff is plenty fast who you kidding.
Greg, I used to think the same way as you, but I think I was wrong.

I think the greater the difference the more pull. I'm going to replace what I think is a good servo just to confirm my findings.

The amount of spray is related to the efficiency of the fin. Marty Davis' current .21 has very little if any spray. It gets through the turns so smooth. I can't wait to run it in Atlanta since he can't make it.

MG

Mark,

I expect what you are saying is that when a lot of water is thrown in the air we are wasting energy in doing that. How then do you re-design the fin so it does not throw all that water?

 

[Mark Grannis]

Tom your stuff is plenty fast who you kidding.
Greg, I used to think the same way as you, but I think I was wrong.

I think the greater the difference the more pull. I'm going to replace what I think is a good servo just to confirm my findings.

The amount of spray is related to the efficiency of the fin. Marty Davis' current .21 has very little if any spray. It gets through the turns so smooth. I can't wait to run it in Atlanta since he can't make it.

MG

Mark,

I expect what you are saying is that when a lot of water is thrown in the air we are wasting energy in doing that. How then do you re-design the fin so it does not throw all that water?

John,

I'm stumbling through different shapes, sizes, locations, material, where the sharpened edges are, because I don't know and wish to learn what combination is best. I'm sure many before me have studied this topic, and I am not claiming to be the expert.

When the turn fin is removed the boat goes much faster, there has got to be some way to tap into that extra speed. The least amount of water displaced should result in a faster boat.

You've got the 8.5 second boat. Did that run have your standard heat racing fin?

Respectfully,

Mark

 

[Chris Wood]

Mark,

I spent alot and i mean ALOT of time on turn fins.. I had one that was so aggressive you could here it cut the water.. but i was to much and would want to continue to turn after bouy 3 and 6. I worked alot on shape hook and materal like you.

I have since changed my shape.. and the boat tracks much better and quiet honestly turns as good or better. i played alot with the hook size and how much.. so i feel your pain on turn fin testing lol.

One thing you may look at for a material if you can figure out how to bend it is spring steel.. maybe a .015 or .030 thick ( depending on the boat ) i always wanted to play with it but never did and had no idea how to roll bend the bottom without distroying it.

good luck and let me know how it goes.

chris

 

[John Finch]

Mark,

The turn fin used on the 8.5 second boat is pretty much the same as I use for heat racing. It does not throw water in the straights, but it throws a wall of water in the corners. I am thinking that for every action there is an equal and opposite action. I know you heard that in school. So, throwing the water upward pushes the sponson downward. That's my thinking, but I'm no engineer. I learn from test and see. What I think I see is a boat that slides away from the buoys in a corner, and as it slides it scoops water and throws it up in the air. The trailing edge of the fin controls the angle of the discharged water. The curve at the bottom scoops the water as the boat slides. I'm sure most people think the curved part of the fin is simply a hydrofoil type device that only acts in holding down the sponson because of the angle of attack as it passes thru the water. I feel it is doing more than that. Controling the slide and the water discharged up in the air is what I have been working on. Preston hall took a picture of the 8.5 second boat at trials one year at the turn and he missed the boat. The only thing in the picture is a wall of water about 25 feet high around all three buoys that looked like the inside of a bowl. It's hard to tell how much is fin and how much is roostertail, but I have a really nice me-time photo of my gas rigger going thru the turn at buoy six where the turn fin wake looks to be bigger than the prop wake. They have some great pictures we can learn from. I love it when they cover our races.

 

[Greg Hahn]

Tom your stuff is plenty fast who you kidding.
Greg, I used to think the same way as you, but I think I was wrong.

I think the greater the difference the more pull. I'm going to replace what I think is a good servo just to confirm my findings.

The amount of spray is related to the efficiency of the fin. Marty Davis' current .21 has very little if any spray. It gets through the turns so smooth. I can't wait to run it in Atlanta since he can't make it.

MG

Mark,

I expect what you are saying is that when a lot of water is thrown in the air we are wasting energy in doing that. How then do you re-design the fin so it does not throw all that water?

John,

I'm stumbling through different shapes, sizes, locations, material, where the sharpened edges are, because I don't know and wish to learn what combination is best. I'm sure many before me have studied this topic, and I am not claiming to be the expert.

When the turn fin is removed the boat goes much faster, there has got to be some way to tap into that extra speed. The least amount of water displaced should result in a faster boat.

You've got the 8.5 second boat. Did that run have your standard heat racing fin?

Respectfully,

Mark

Mark,

You mention trying different shapes, I don't know what all you've tried but I've been trying more leading edge sweep hoping to cut down on drag. I've visually noticed more speed in the turns, haven't been particularly worried about the amount of water being displaced. If you were to lay my latest fin out flat it essentially looks like a triangle, the leading edge is 45 deg. with a very narrow and shallow hook.

I'm trying to learn as are you. I'm all ears and eyes!

thx

Greg

 

[Brad Christy]

Tom,

You're thinking exactly backwards. If you look at an airplane wing, the leading edge is what you call "blunt", but the lift is generated on the "sharpened" side. The lift is actually generated by the tapered trailing edge, which our turnfins DON'T have. The blunt leading edge in water is an trembndous amount of drag. A very slowly tapered leading edge generates a fairly low amount of drag and is therefore faster, but produces less force to the left, so we add some toe-in on the right sponson to counter prop torque.

As John mentioned, the wall of water is caused by a slipping turnfin. There is no change in shape or curl that will eliminate the wall of water. Increasing size, however, WILL decrease the amount of water displaced in the turns. The next race you go to, watch the boats the take the turns really fast. You will most likely notice that the faster they take the turns, the less water they throw up. Then go check out those boats on their stands. I think you will find, on average, that they have considerably more turnfin area that those that seem to stuggel through the turns, and throw less water to boot. I know it seems backwards, but look at it like this: If you take a butter knife and a meat cleaver, which is easier to pull through water at any angle from the direction of travel? The butter knife, of course, slips through sideways much easier. So it can be said that with equal force trying to turn the blade from it's "true" path, the butter knife will displace more water because it will deviate more from it's true path. If you look at the turnfin on the Blazer Whiplash GSH, it is enormous. It also throws much less water than other boats with small turnfins, and yet takes the turns almost like a rigger.

John,

The curl is doing more to hold the turnfin in the water than anything else. Outriggers are so light, that a straight turnfin would just slip up out of the water instead of holding the boat in the turn. The trick with the curl is getting enough horizontal area to do what we want it to do, but not so much as to allow it to lift while running straight. I had this problem when devoloping the turnfin for my BlackJack. The original turnfin worked OK until I started getting some comments from guys running the boat that they were having some trouble holding lane one over about 70MPH. So, at the suggestion of one of the more experienced boats who had a BlackJack, I developed a new, larger turnfin. I had alot of curl and alot of what I call horizontal surface below the curl. There was no actual "horizontal" area, but I had full "length", meaning leading edge to trailing edge, below about 45 degrees of the curl. The boat turned like it was on rails, but it would do a cartwheel coming out of the turns at any speed at all. So I started grinding back the leading edge, starting at the top of the curl, sweeping it back towards the trail "tip". Over the course of about a a month of this and numerous trips to the pond for testing, I now have a turn fin that will still turn like it's on rails, but yet tracks straight and won't lift if I turn left as long as I get out of the throttle a bit if I need to correct some oversteering. I also noticed there was less water thrown in the air in the turns with the new turnfin than with the old one.

Thanks. Brad.

Titan Racing Components

BlackJack Hydros

 

[Mark Grannis]

Thanks for all the overwhelming information.

Time to hit the shop.

MG

 

[Tom Foley]

Tom,
You're thinking exactly backwards. If you look at an airplane wing, the leading edge is what you call "blunt", but the lift is generated on the "sharpened" side. The lift is actually generated by the tapered trailing edge, which our turnfins DON'T have. The blunt leading edge in water is an trembndous amount of drag. A very slowly tapered leading edge generates a fairly low amount of drag and is therefore faster, but produces less force to the left, so we add some toe-in on the right sponson to counter prop torque.

]

Well actually the reason the wing adds or produces lift is because the pressure on the bottom or flat side(shorter ) creates more pressure than the top side(longer, lower pressure due to the increase in speed of the water/ air across its surface ).

 

[Scott Schneider]

An issue just touched off in this thread is the actual shape of the sharpened leading edge.

Many schools of thought on this as well, just going to share what my take is on it ....

I sharpen "CHISEL" having the sharpened edge be flat and not rounded then blended into the side of blade.

Reason for this is too cause a slight swirl eddy at point the displaced water leaves the flat chisel edge. This causes air to be pulled in along side the right not working face of turn fin helping the water stay in a state of cavitation and not wetting the surface.

Have found in most cases the drag of the fin the boat feels and needs to be corrected for is less.

JMO ... <_<

 

[Ron Jefferson]

Maybe Marty could get involved in this thread. Perhaps he could share his profile and design of the ultimate turn-fin. No secrets here eh!

Ron

 

[John Finch]

Maybe Marty could get involved in this thread. Perhaps he could share his profile and design of the ultimate turn-fin. No secrets here eh!
Ron

Marty talked a lot about fins in a thread last summer. Some good info there.

 

[PHM Racing]

EDIT

 

[Marty Davis]

Since I am also not an engineer, I will only indicate what I have found that works for me.

I believe that ANY boat that does not throw water up in the turns is operating more efficiently than one that does. If a boat is throwing up the sheet of water, the boat is NOT pivoting about the fin, but making the fin do something that is difficult. Making the fin uncomfortable rather than happy to just be the pivot point is my way of getting through the turns quickly and without speed loss.

Work on the placement of the fin AS MUCH as the size, shape, thickness, etc.

 

[Boris Mazor]

Hi, Mike! I'm glad to read your article! Resourch instead guessing! Thanks. Boris

 

[Marty Davis]

Hi, Mike! I'm glad to read your article! Resourch instead guessing! Thanks. Boris

I think that one might have been directed at me.

If you worked on turn fins every day for the next year, you still wouldn't have done as much resourch as I have on turn fins.

I just want to be helpful to everyone and pass along those tidbits that have helped me figure out how and why turn fins work.