Wetside of Tunnels...

[Jimboat]

Acceleration & Blowover

There is alot going on with respect to the movement of what I call the 'dynamic center of gravity', while a tunnel boat is accelerating out of a low speed turn. The sponsons are likely to be fully wetted, and there's little aerodynamic lift going on. Consequently, the dynamic CG is quite far forward at the exit of the turn.

On accelerating out of the turn, if the 'wing' design of the hull is configured with it's own center of pressure quite far fore (as many are), then there will be some very dramatic movement of the dynamic CG as the boat velocity increases. Typically, in such cases, the dynamic CG moves aft very quickly, and then can actually begin moving fore again. This is very difficult to react to when you're sitting in the boat - let alone at the RC controls 100 yards away!

You can see a typical movement of the dynamic CG on a tunnel boat, as it accelerates out of a low speed turn...

[SIZE=8pt](based on Tunnel Boat Design software, Version 6.5)[/SIZE]

As the velocity increases under these circumstances, the angle of attack of the hull actually needs to be REDUCED, to respond to the rapid change of dynamic CG.

The best alternative is to design the hull and setup for the particular instances of good cornering, and good acceleration out of the corner. Designing for a controlled dynamic CG is alot easier when the balance of these forces is understood for your particular boat and application.

/Jimboat

 

[Mark Anderson]

Can a model accelerating out of the typical corner an an oval be called accelerating from a low speed turn? Granted the corner speed is lower than the straight away speed but not a lot. Lohring in all your testing have you ever looked at corner speed maybe at the apex bouy with a radar gun? I am guessing 3-5 mph down form the saw speed depending on set up. And many boats are accelerating for the first half of the straight.

Is the boat blowing off due to acceleration or sudden loss of drag from the turning action causing a boat with too aft of a cg to pop when steering input is released? Or are the two one in the same?

Mark

 

[TimD]

I have to agree with Mark, The cornering speed is not that slow in comparison to the straightaway speeds. Very different to the full size F1 tunnels.

I find that my tunnel "double stages on the pipe" just at the exit of the turns and that can be unsettling / scary... I put this down to loss of drag causing a spike in RPM's but once it's "on the pipe" it stays on it all the way until I back it off for whatever reason.

The aerodynamic aspect is interesting though - what if we can prevent this sudden loss of drag in the turns? would that make exiting the turn less critical to a blow-over? Any idea's Jim?

Tim.

 

[Jimboat]

A large oval turn will exhibit the same changes in dynamic CG repositioning as in a 1-pin turn, but not as dramatic. Any tunnel in a turn will wet much more sponson area, which increases hydrodynamic drag significantly. Straightening back out will air out the tunnel, increase aero lift. Along with the corresponding reduction of water-lift, comes a big reduction in water-drag, and the shifing of dynamic CG. It will be the same for boats of any size, but not of any design.

 

[Jerry Dunlap]

I'm not sure I hold the record for most model tunnel boat blow overs, but I'd imagine I'm in the Top 10. It's seems to me that a primary cause of blowing off exiting a corner is the result of allowing the motor to release too soon to neutral. A quick release to neutral rudder following a corner usually causes the bow to lift. In my opinion, this quick elevation of the bow coupled with a slight amount of front or quartering wind, and acceleration are prime ingredients for a blow over.

Been there and done that. Kinda like my golf game - I know I need to slow down my back swing. But knowing and doing are two very different things.

Jerry Dunlap

 

[lohring]

It's been a while since I checked the entrance and exit speeds, in a corner, but Mark is right. The other thing I noticed is that blow overs are especially common when turning into the wind. In that case, the OUTSIDE sponson is the one that lifts first. This usually happens before the finish of the turn and before you begin to straighten the engine as Jerry mentioned.

What is the "dynamic center of gravity"? I understand that the center of aerodynamic lift (a very changable point) is ahead of the center of gravity (a very fixed point) which is ahead of the center of hydrodynamic lift (also a changeable point as the hull sets) under static running conditions. What is happening in the turn?

Lohring Miller

PS I always thought my blowovers were caused by running beside Dave Soleway.

 

[Jimboat]

Dynamic stability of a tunnel hull is affected significantly by the dynamic CG of the hull. All of the forces acting on a ‘flying’ tunnel boat are balance at every speed – that’s what keeps the boat from taking off. These forces include the lift and drag from sponsons, lift and drag from the aerofoil, drag from appendages, lift and drag from propeller and lower unit. The lift of the aerofoil acts through its C of P, which changes throughout the boat’s speed range. The location of the lift generated by sponsons also changes as wetted surface changes. The relative significance of the appendage drags and the lift/drag of the propeller/lower unit changes with speed too. And the contributions to the total, of all these lifts and drags vary throughout the velocity range. This is the reason why “balancing” your boat on dry land (or with full-size boats, while it’s on the trailer) will only help stability while the boat is at zero mph!

The moments of all these forces must balance. The resolution of all the moments caused by the action of these forces about the CG must balance out. But the CG changes with speed, since all these forces move around. Hence, the term...dynamic CG.

The 'dynamic stability' of the tunnel hull is a tricky issue, to say the least. A Tunnel hull being part in the water and part in the air makes the 'moving target' of stability a difficult one to examine. It’s outlined in the STBD book. Computers make it a lot easier, though…that’s why I use TBDP software to do the analysis for me.

/Jimboat

 

[Jerry Dunlap]

The most successful program I've found for analyzing the performance of a model tunnel boat is the RTDB(run the **** boat) program. This program works best when the water is fairly calm. Fully charged batteries for the transmitter and receiver, an assortment of props, some stick on lead, and allen head wrenches to adjust the engine height and prop thrust are utalized when operating the RTDB program. I've only had this program "freeze up" on me once. We ran a straightaway record trials in December about a half dozen years ago the cables for rudder and throttle control actually froze in the tubing. I was able to "reboot" the RTDB program by holding my 3.5 tunnel in front of a propane heater.

Jerry Dunlap

 

[sjhauge]

Right now in this area the RTDB software is froze & mother nature will not unlock it until mid April. As a result, all of the hardware is still in the Beta version. ;D ;D

 

[lohring]

I just did some of the RTDB program testing. What a perfect day! It was so sunny I had to wear sun glasses. I wish my boats would run that well in real race conditions. I was only engine testing, though.

I think I will modify one of my hulls to eliminate the stumble blocks. Because the Leecraft runs relatively flat, it might be touchy. Jerry, why don't you try the same thing on one of your hulls? They usually run at a higher angle of attack and some sizes haven't needed stumble blocks.

Lohring

 

[Jerry Dunlap]

Lohring,

I just ran a new WOF30/CMB Green Head 21 and it does not have stumble blocks. It didn't give any indication it would hook in corners. The WOF 29.5 I ran with the CMB Gold Head 21 last season also ran without stumble blocks. I'm working on another Villain kit and I'm going to try running it without the plastic stumble blocks. The Dunlap 27 I ran in 3.5 Stock before this year also didn't have stumble blocks.

Jerry

 

[TimD]

My 27 inch hull has no stumble blocks and does not exibit any signs of hooking either.

RTDB software sure can get expensive - anyone have a free trial version?