Flat Versus Curved Tunnel Roof...

[Kris Flynn]

I aquired a set of plans from the Graupner Formel 1 tunnel and one of the things that got me was that the tunnel roof of not flat, it is concave...

It also does not have stumple blocks, but has some 'steps' near the front a little like the DPI tunnel does...

It was also designed after a full sized winning tunnel over in Europe, so my question is what would this do to the air flow in the tunnel...maybe it would assist with keep the rear down and stopping 'blow off'??

Kris

 

[GTR]

Hi Kris,

I pressume you mean convex. This is to distribute the lift out towards the sponsons - where the weight is really carried. The step you refer to help hold the nose up when first lauched to air out the tunnel. They will not help spinning out or submarining if they dip in the water. I believe the tunnel it's patterned after is the boat that won F2 OPC in Germany. Quite a nice looking boat if only someone in Oz could get ahold of one.

GT 8)

 

[GTR]

Hi Kris, forgot to say if you want to see something out of the ordinary I can post a pic of the underside of a Maus Champ hull - has a lot of the things you ask about incorporated. cqn't comment on how it handles though as I still haven't got it finished. But I do have power now and expect the production of RTR tunnels to progress rapidly. LOL

GT :

 

[Kris Flynn]

GT, i forgot to mention the concave/vex i was refering to was from the front to the rear

But a picture of the Bandit would be awsome!

Kris

 

[GTR]

Hi Kris,

What I will do over the next few days is post a bunch of pictures of different tunnels and see what people say.

GT ;D

 

[[email protected]]

I've got a 7.5 Maus Champ and it handles like a dream. I ran it recently and it handled very well full speed. My friend ran his mono full speed and I chased him to see how it would handle jumping wakes, turning into and away from waves and it did great. Only downer was if I had to make a hard right over a wae it actually slid like a shovel nose would.

Gary, any ideas how to combat that??

 

[GTR]

Hi Rodney,

That's usually the sign of one or two things -

The inside edges of the sponsons aren't sharp enough, or

Insufficient deadrise angle, eg when it rolls over it slides along the sponsons. Add a strake to the outside edge might help remove this sliding.

GT

 

[Jimboat]

Chris - the "curved" tunnel roof shape you are refering to is really just a different design of aerofoil used in the design for "enhanced" aerodynamic lift.

For example, a high-camber aerofoil shape...We've done alot of research with NACA aerofoil types in application for tunnel hulls (low aspect ratio aerofoils in ground-effect).

Higher cambered aerofoils (with your "concave" shape to lower surface) usually have higher lift characteristics, also higher drag. They also have Center of Pressure further aft than a lower cambered aerofoil.

The Secrets of Tunnel Boat Design book does some explanation of the use of different cambered aerofoils, as well as the effects of thickness, aspect ratio, and height above water surface. The Tunnel Boat Design software allows for use of many different configurations of aerofoil design & configurations.

Call me if you want any more, Chris.

 

[Kris Flynn]

Ahh Jim,

Its good that you came forward with your thoughts

i guess it would be just as easy to increase the thickness of the 'wing' to create more lift...it would also be a fair amount harder to make the concave surface than a flat one! considering you can add lift into the design in different placs...

Kris

 

[Jimboat]

Chris - Yup! You've got it! Adding thickness is pretty easy...so is making the h/c ratio (height above the water surface) smaller - which also increases lift alot.

 

[Kez]

Hi Jim,

Do you mean a boat with a shallow tunnel will have more lift than one with 'tall tunnel', given that everything else being the same?

This is interesting and exactly opposite to my intuition. I have a tunnel (and scale hydro) that are flighty. I was thinking of a surgery to reduce the tunnel depth to reduce the tendency of blow over. So instead I should make the sponsons deeper, right?

Thanks,

Kez

 

[GTR]

Hi Jimboat

Do you mean if you make the tunnel shallower you reduce the ground effect over the water? Is there a rule of thumb for tunnel depth v width & if you extend the centre section beyond the transon ends of the sponsons are you are effectively moving the lift aft given the same airfoil as if it was level.

At this pace I might need to buy your book because there is lots I don't know, clearly.

GT

 

[Slideblues]

OK now I am confused, I got ta agree with Kez, the h/c ratio sounds backward. ???

Used a concept I called shallow draft, basically reduced the size of the tunnel area in width and height and it allowed me to accelerate 10 mph faster that it would stock before blowoff, the airfoil placement and shape was the same, with a flat tunnel roof. How could less volume in the tunnel increase lift?

Hmmmmmmmm..... Dis one got me thinkin 8)

Gene ;D

 

[izitbrokeyet?]

My last 5 tunnels have all had this adjustable feature.......I borrowed it from a Jerry Dunlap Signature Series tunnel I had. I love it so far, since it extends the applications for each tunnel. I can lower the sponsons for rougher water and make them shallow for more lift. Most importantly, I can adjust the center section lift, and I never have to worry about having a true-aligned boat.
It adds a little weight, but so far it's proved to be worth it's weight in aluminum.

 

[izitbrokeyet?]

Jim,

You're giving away all the secrets!!!

at least I was subtle about it

 

[GTR]

Hi Izit

Aren't you also talking about changing the Angle of Attack relative between the sponson riding surface and the tunnel lifting section? :

Shallow tunnel is only useful on very flat water at which point the AofA of the hull and the attitude above the water are at a balance of forces. eg enough lift to get the weight out of the water and a shallow enough attitude to maintain stability in a straight line.

This is making my head hurt.

GT ;D

ps. love those emoticons - keep them coming

 

[Propjockey]

Gene, is it possible that the smaller tunnel volume allows less area for compression? Less area for compression would mean a "stiffer" ride, correct?

 

[izitbrokeyet?]

The advantages of a low center section are increased by a phenomenon called ground effect which has an inverse exponential relationship with height above water. The Bernoulli principal still applies to basic wing lift (remember that from Carb 101?) and is enhanced by Up-wash and down-wash which both impart momentum to the wing (tunnel bottom). The lift is the same but the addition of Upwash and Downwash which is induced by thrust.

The easiest way to think about Upwash and Downwash is diagrammatically:

(air molecules)

___________________ (tunnel bottom)

/ / / / /

/ / / / / (end)

/ / / / /

======================================water surface)

 

[izitbrokeyet?]

The diagram got "shifted"

But I think the principal thought still got across

This phenomenon shouldn't be confused with lift. Up wash and downwash can be mathematically treated more like compressed air, adding an upward force to the whole bottom of the tunnel. Lift can be roughly computed to have a resultant near the low pressure area on a wing.

GTO,

Generally speaking for the same amount of lift, I decrease the angle of attack if I am able to lower the tunnel.

 

[Slideblues]

PJ, I am still letting all this soak in, but it does seem to backup the data in part that I collected running the V2.

GT, Yes the shallow draft requires smooth water to be effective, Optomizing straight line stability while increasing speed was the goal.

Izit, I never really talked about it, but the V2 had removable sponsons also, this allowed me to change ride height and AOA. I tried three different AOA's and setteled on one that had the least adverse effect on handeling but the best speed.

Gene ;D