Penant Shape

[Anonymous]

Simple question : what is the best shape of towline penant for minimum drag ?

[Anonymous]

I use 4″high by 10″ long made from rip-stop nylon. It seems to hold up pretty well even in grabby weeds.

You also need an additional 1/2″ to wrap around the line and an overhand knot in the line above and below the flag to keep it in place.

Would be interested in some pictures of better designs also.

Some of the guys have been using ribbon material that has open weave, and they say that has less drag. But it does get caught in the weeds.

The size by the rules is:
c) To facilitate observation and timing, the cable must be equipped with a pennant, having rectangular shape of
a minimum area of 2,5 dm2 (about 39 square inches) and the smallest side of at least 5cm (1.97 inches), attached directly to the main cable.

So if you went with the minimum dimension, either a 2″ high by 20″ long would be required which would have quite a length to flap around, or 20″ high x 2″ long which would have a lot of profile drag and make it hard to wind up.

So we are talking:
2×20
3×13.5
4×10
5×8
6×6.8

So of the choices, 4×10 is pretty average. But is average the best?

Maybe a stiffer material like .003″ mylar would have less drag.

But getting back to the real question, how much does the line and penant type really have on launch height?

I only use thin spectra line on F1H, and prefer the 1mm Russian rod for F1A with the same 4×10 flags on both.

I think the Russian rod will stretch about 12 inches with a 40 pound pull, which means that you can make your line pass the pull test at 4 inches less than the 50 meters, and get an additional 8 inches of length at 40 pounds….kind of cheating, but within the rules. Probably the pull test should be increased in the rules to eliminate this type of cheating. A 40 lb pull is typical of a good launch. Another nice thing about the Russian rod (polyester chording material) is that it returns to the original length after being pulled to this value. Spectra has less stretch and is a pretty dead material. Polyproplylene thins and does not return to the original length.

[Anonymous]

Found this :-

http://pdf.aiaa.org/preview/CDReadyMASM05_666/PV2005_856.pdf

Looks like Carruthers may have some of the answer ! Also found elsewhere that material is a big influence on drag. What I didn’t find was anything that discussed triangular versus rectangular shapes; I would have throught a triangular shape would have been better.

Comments ?

[Anonymous]

Hello CHE

I think you are right about triangular. However if you read the rules above, rectangular is only allowed. If you do triangular, or some form of triangular, the smallest dimension needs to be 5 cm, so you may need to go bigger to get the area, and bigger is BAD in this case.

[Roger Morrell]

I was on the jury in a World Cup event in the Ukraine and a contestant had a pennant that was well oversize but did not enclose a square of sufficent area. This person had flown with that towline in many contests and was not aware that he was not in compliance. He was only out of compliance by a very small amount. The jury decided to let the results of the contest stand and not DQ the contestant. The contestant indicated that he would fix it for the future – seeing some of the jury members knew him and would notice if he did not, I assume he did. In this case there was no clear advantage gained as the pennant was quite large and very visible.

Roger

[Anonymous]

I now have recieved a copy of the Carruthers paper and gleaned some info from elsewhere.

The optimum shape of a penant is given by the highest aspect ratio and by it being streamed, ie not looped back onto the latch. The better material of two (no data on other materials) is Nylon with Polyester (ie Icarex) giving 40% more drag force apparently.

So my idea of a looped penant shaped like a Blue-Fin Tuna looks like a dead end. Needless to say the Haggis-shaped idea is also naff.

[Author]

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