Originally Posted by dbsonic

Originally Posted by alasdair

In R/C Model World, June to September 2009, I wrote a series of articles on "Scale Science", titled Scale Speed, Scale Weight, Scale Power and Scale Spreadsheet. That last one included a spreadsheet which could, for a while, be downloaded from Traplet Publications website.

Scale Science

The science argument goes like this. We use the same lift coefficient as the full size, so the speed and weight are directly related by the common equation which is used to define lift coefficient Cl.
W=L=1/2.rho.V[SUP]2[/SUP].S.Cl
If the speed V of a 1/6 scale model is to be 1/6 that of the full size (Linear Scale) then the model’s weight has to be full size weight divided by 6x6x6x6 or 1296. However models don’t fly straight for long, we are constantly turning and flying manoeuvres. If the size of the turns and loops of a 1/6 model is to be 1/6 that of the full size (Dynamic Scale) then the model’s weight has to be full size weight divided by 6x6x6 or 216. We are talking two vastly different aircraft. The light one flies scale linear speed but it turns and loops impossibly tightly and looks generally jerky. The heavy one flies nice scale manoeuvres but lands like a runaway train. We need to choose a compromise in between these two extremes for realistic scale flight.
I noted in the articles that the Skygate Hawks built down to 20kg for the Jet Masters looked floaty, and would look more convincing at 24 kg

The 20 kg Hawks had a weight index (Wix for short) of about 4. In other words their weight was scaled down by the scale factor to the power 4, which results in Linear Scale flight, but floaty, jerky manoeuvres.

Now look at a 1:2 size Gripen, an aircraft whose weight is, say 18800 lb, thrust (dry) 54000 N

For a light floaty model (Linear Scale) it should weigh 18800 divided by 2^4 or 2x2x2x2 giving 1175 lb or 534 kg

For Dynamic Scale a model should weigh 18800 / 2^3 or 2x2x2 giving 2350 lb or 1068 kg

Those are the lower and upper limits for any semblance of scale realism. At the lower weight of 534 kg my spreadsheet suggests a scale thrust requirement of 4155 N thrust for scale performance.

This model under discussion was a tissue paper model, so far from scale science that it is not surprising that it blew apart. Only the model was damaged, so no harm done.

I am sure it is quite possible to build a vast scale-shaped model that will fly unrealistically but still hold together, but WHY?

Originally Posted by David Gladwin

Surely the maths are very simple indeed, a few multiplications and a division.

However, the simple equation given by Alisdair may not be familiar to some guys. It is a basic equation, one of the very first to be learnt on any aerodynamics course. It defines aerodynamic force for lift and drag, so:
W=L=1/2 rho, v2.S.Cl.

Where :
W is weight.
L is Lift ( lift = weight in level flight)
Rho is air density.
V is velocity.
S is area.
Cl is lift coefficient. ( function of wing section )

So knowing these values lift can calculated.

To calculate drag, substitute Cd (drag coefficient) for Cl to get drag.

I recall that in my RAF time we lost two Gnats with fin failure, it's not an unknown problem, so let's hope the new Scale. Composites Gnat is strongly constructed and flown with consideration of the very tall fin. (Although I am sure John will ! )

Frankly I see absolutely no reason why the fin on the SAAB or any othe model could not be made removable, all it needs is sound design, after all many wings , much bigger than the Gripen,s fin are removable.

David

Originally Posted by JohnMac

But personally I would have grounded the guy because he was flying a barely controlled, high speed model, at one point directly towards the crowd. No imagination! Clearly the safety of the public and other pilots was less important than this guys ego.
John