The scale speed is derived by multiplying the actual speed by the square root of the scale factor.

Now I did the calcs in my head before but here it is properly

I.E. at 1/8 scale

Square root of 8 = 2.828427124

At an actual speed of 37.5 mph × 2.828427124 = 106.0660171 mph scale speed

I'm not knowledgeable enough to be able to explain the meat behind the formula, all I can say is that I believe it comes from models used by development engineers in wind tunnel and tank tests. My attention came to it when I started thinking about the margins of some speeds, where some of the numbers just don't look right using the simple scale approach.

For example, if a little tenth scale speedboat wanted to do a scale speed of 100 mph then it would only be doing an actual 10 mph, and in my mind watching a boat at a trotting speed is not representative of 100 mph in scale. So that was why I looked into it really, and when you apply the formula the perception of a models speed as a small rendition of reality when watching models it all fits.

So, if you need to know a target model speed to reproduce a scale speed the formula changes to scale speed divided by the square root of scale factor.

I.E. at 1/8 scale,

Square root of 8 = 2.828427124

A target scale speed of 300 mph ÷ 2.82847124 = 106.0660171 mph actual speed

So it turns out that the scale speed of your estimated figure is the actual speed needed to get the target scale speed of 300. That pans out in all examples.

Firstly to Tim,

What a great job of building your K7, top marks.

Jerry,

Thanks for to the speed conversion formula.

My K7 at 1/12 scale does approx 40 m.p.h. but to get a true scale speed it would need to be 86.7 m.p.h. which is mind blowing.

Earlier in Tims thread I said that I get about a 7 sec run from one end of my club lake to the other at 40 m.p.h..

Last year in the summer on a very flat calm day I had my K7 at Loch Lomond and I was able to send it 100 yds to my right and 100 yards to my left.

I had Mrs T with the camera but unfortunately she finds it hard to shut one eye and focus thro' the lens of the camera with the other, hence the pic enclosed doesn't show K7 flat out.

What actualy happened was that as I openned full throttle K7 took off ( literaly ) as it wound up speed with the amount of run that it had, just as it was level with me it took off just like D.C.'s K7, as it's a prop drive the torque of the prop started to turn it over to starboard and I was quick enough to snap the throttle shut and it flopped down on the starboard sponson which saved me a swim in a very cold loch.

I have a book by Neil Sheppard ( DONALD CAMPBELL AND THE FINAL RECORD ATTEMPT ) which does a very technical study by him and a Dr Kieth Mitchell with so many titles and P.H.D's to his name I will leave out and although a medical man his pastimes are the areodynamics and design of large R.C models and Hydroplanes in particular D.C.s Bluebird K7.

After much study of the last attempt he concludes that at 320 m.p.h. K7 was actually flying and shows some pics of K7 riding on it's stabilising fins and the hull is above the water with only the rear wedge touching and with the long held thought that as the engine cut out and there was no thrust to keep it level the stern dropped down , wind underneath and K7 took of with the resulting disaster and at the speed it was travelling at was way out of it's design criteria.

I seam to have let myself get carried away a bit hear but if you look at the video link with my K7 you can see as it comes at you she is clearly riding on the edges of the sponsons and the prop C/Line and well out of the water and is producing quite a Rooster tail.

I don't think that we should be aiming for a scale speed as you rightly say that Tims K7 would need to acheive 106 + m.p.h. to get up to full size speed.and the spray from the prop will create a rooster tale not unlike the Jet powered K7 of D.C. and Ernie Lazenby's Turbine powered K7 , can't say that I approve of a brake to create a spray as it will only take more power to get it up on the plane.

Hope that this is of interest and the book is well worth the cost to any Bluebird K7 fan and what ever you do Tim don't try to turn your K7 at speed, it just will not do it, I have the TEE shirt to proove it.

George.

Pic at Loch Lomond and link to you-tube.

https://www.youtube.com/watch?v=fGPMBKTA1Wo