Sponson down force

[David Murany]

Looking to get some insight on sponson down force. I have 2 sets of sponsons that seem to have too much lift and I'm wondering about changes to help that. The first set is from a Jag 1 and the second is a similar design but has an angle cut from the top down to increase the down force some. If you cut the back off say an inch in from the end and cap it does this increase down force on the top or no? Im sure it reduces some of the lift. Also these are both 40 boats. Thanks for any input shared.

Dave

 

[Elias Vazquez]

Flat top, and around 3 to 4 degrees of a.o.a. should work.

Good boating!

 

[gil sonsino]

To counterbalance sponson lift you also have to work with negative strut angle and lifting props mainly.I agree that 3-4 degrees should work but there are more variables here involved like the ones I pointed out.Gill

 

[David Murany]

Did some youtube searches and found wind tunnel videos that explained what I was looking for. The sloped top of the sponsons cause low pressure and turbulence just behind the sponson and at high speeds I lose the down force. Gonna make some changes to the tops and see what happens. Thanks for the suggestions.

Dave

 

[Terry Keeley]

Changes to the bottom have much more effect than changes to the top since the bottom packs air between it and the water's surface.

Google "ground effect"...

If you can't deepen the strut some angle as Gil mentioned will help.

See ya next weekend!

 

[Steven Bryant]

David

my Jag 1 which i am still running has the front AOA set to 3 and 3.25 degrees and it seems to be very well balanced

Steve

 

[dwilfong]

There is a reason a Eagle sponson looks the way it dose.

Build some foam test pieces and stick them on the back.

When you change the bottom AOA it also changes the top AOA.

You really need to make new sponsons to do a AOA change.

 

[Jeff Schindler]

I've been trying to design a JAE type sponson with more aero downforce, but its pretty hard to do and still make it easy to build. The tail of the JAE sponsons angle up on the bottom creating some ground effect downforce. The design is pretty good, and you will not see much gains from changing it. Instead, have decided to add ram wings to the front for added downforce.

If you are getting lighter in the turns, you may look at spoiling the air coming over the upwind sponson. I have seen this work alot on sport & scale hulls.

See attached schematic:

 

[gil sonsino]

In the early 80's the Prather Piranha's rigger was built with spoilers in the rear of sponsons...I had one that was a pain in the heck with left turns :angry: I wouldn't compare a rigger with a sport or scale hydro but you can make your own tests to figure out what Works in your model.Gill

 

[David Murany]

David

my Jag 1 which i am still running has the front AOA set to 3 and 3.25 degrees and it seems to be very well balanced

Steve

The one I am running has 2.6 AOA and when the boat starts getting around 70 it blows off out of nowhere. The boat runs very stable right up until it goes over. I also have 1.7 degrees in the strut. Bums me out though cause the boat runs great then kite time.

 

[Mike Betke]

What is the angle on the back of the sponson? More angle on the back will create lift. I have tested this and found 5 to 10 degrees difference will cause a boat to blow over. From what I remember your sponsons are like what I am running, not the JAE style.

Jeff is right on the money with his sketch based on everything I have tested thru the years.

 

[David Murany]

After looking at Jeff's drawing I checked my setup on the table and I can see an issue that needs to be addressed. Thanks for the help guys.

Mike are you gonna run anything in Flint this weekend?

 

[Jeff Schindler]

After looking at Jeff's drawing I checked my b setup on the table and I can see an issue that needs to be addressed. Thanks for the help guys.

Mike are you gonna run anything in Flint this weekend?

Looks like maybe the .20 boat is a Jag III? If so I would be reluctant to make changes to it. The .20 and twin jag III's I have seen have handled very well. Steve Ball's is fast as anything else in smooth water, and it really handled the rough water well too.

 

[shoboat]

Jeff,

It is an old Stealth 21 model that I have had for many years.

I will talk with David this weekend on the remedy to his problem.

He may already of noticed it now. Need to cut just a touch off of

the front sponson trailing edges and cap them with a little piece of wood.

We found thru a lot of testing years ago you need to break the air of the back

of the sponson. I will bring my boat and show you the fix. It is a very stable design

Once it is fixed. This is the design of the first 21 hydro that I went 81 MPH with at

a Flint, MI time trial many years ago. A "Lutz powered"Valvola-21 and and a H-7

Lynx-Cut propeller at 3.46" of cup. I will never forget it. Very short pipe under 7".

Looks Nice David,

Mark Sholund

 

[David Murany]

Jeff,

It is an old Stealth 21 model that I have had for many years.

I will talk with David this weekend on the remedy to his problem.

He may already of noticed it now. Need to cut just a touch off of

the front sponson trailing edges and cap them with a little piece of wood.

We found thru a lot of testing years ago you need to break the air of the back

of the sponson. I will bring my boat and show you the fix. It is a very stable design

Once it is fixed. This is the design of the first 21 hydro that I went 81 MPH with at

a Flint, MI time trial many years ago. A "Lutz powered"Valvola-21 and and a H-7

Lynx-Cut propeller at 3.46" of cup. I will never forget it. Very short pipe under 7".

Looks Nice David,

Mark Sholund

After seeing Jeff's drawing it makes sense. I'm going to first try cutting it up at an angle and then if I need to cut it perpendicular to the table. The 21 we had Rick now has and it runs really good. See you this weekend.

 

[David Murany]

After looking at Jeff's drawing I checked my b setup on the table and I can see an issue that needs to be addressed. Thanks for the help guys.

Mike are you gonna run anything in Flint this weekend?

Looks like maybe the .20 boat is a Jag III? If so I would be reluctant to make changes to it. The .20 and twin jag III's I have seen have handled very well. Steve Ball's is fast as anything else in smooth water, and it really handled the rough water well too.

After looking at Jeff's drawing I checked my b setup on the table and I can see an issue that needs to be addressed. Thanks for the help guys.

Mike are you gonna run anything in Flint this weekend?

Looks like maybe the .20 boat is a Jag III? If so I would be reluctant to make changes to it. The .20 and twin jag III's I have seen have handled very well. Steve Ball's is fast as anything else in smooth water, and it really handled the rough water well too.

Jeff they are both 40 boats that are blowing over on me. I agree that Steve's boat runs well. He came up to a couple of races last year and that thing works great. Very fast.

 

[Christian Lucas]

Hi,

Wanja show how i reduce airpressure between sponson and watersurface. Let a tunnel be between aerodesign sponson an airfoilpart running plate .At low speed it produce higher lift as it workes like a hydrofoil and at higher speed air can flow over the plate but air has less lift at this small airfoil then water.

Other with a shokabsorbing moving round plate . Spring is on topside of the sponson.

Also very smoth running was the withe Norwegan rigger with the front mount sponsonholderplate the can bend and be this give a smothe ride like suspension .He has also some spoiler on the sponson for downforce.

Läst the tethered line airprop rigger USSR with pin hold plates ,but the touch the water only one time a circle.

 

[Mike Betke]

After looking at Jeff's drawing I checked my setup on the table and I can see an issue that needs to be addressed. Thanks for the help guys.

Mike are you gonna run anything in Flint this weekend?

I'm not going to run anything this weekend. Getting the new house finished and getting ready to move. Rob or I might stop by for a little while.

Glue a piece of balsa on the back of that sponson and get rid of that radius causing all of the lift. You can shape it multiple ways and tune that boat in.

 

[David Bryant]

In the early 80's the Prather Piranha's rigger was built with spoilers in the rear of sponsons...I had one that was a pain in the heck with left turns

Actually I'm building a brand-new Piranha kit as I speak. Gonna use new Picco 45R. I'm begginning to wonder whether to even use the stainless sheet "spoilers" on the sponsons and, instead, just use negative strut angle to neutralize instead? Would think the spoilers are mostly adding drag (and some weight).

 

[gil sonsino]

David there are some important improvements to be done to your Piranha:I had one with a P80 that won some races here .

-change for a new turnfin(bend)that will add benefits for left turns as the straight fin acts like a hidrofoil .As I said this boat doesn't like left turns at high speed with the original turnfin.

- you can keep the spoilers if you want.

- mount your sponsons with a little toe in(one degree is enough)

- shim the right side of the strut to counteract propwalk

- 1455 is a good prop for tests then go with 1655 or 1657

Hope this help

Gill