Howard Metcalfe has recently been flying on Salisbury Plain, see:
This was (for us Brits) a hotter-than-average day: 27o Celsius. Howard reports that his Rapier L-2 motors were giving more thrust than he expected:
"I was frustrated by poor flights caused (in my opinion) by the high temperatures. I‘ll try and explain. In the winter, the only way I could get a reasonable performance from the motors when flying on a snow covered field was to warm them up by keeping them next to my skin in my trouser pocket. In my experience cold definitely reduces thrust.
So I should not have been surprised by the much higher thrusts I was now getting in the heat. The Valiant and Comet performed high speed loops . . . and, in the case of the Comet, this was with an L-2LT! I eventually worked out what was going on and selected lower power motors and got some good flights with my profile Skyray, and, with old green L-1s, with my 1.6x oversized Bill Dean Spook. This climbed to a considerable height in a series of steps. [Oh wow! - Roger]. I used the same L-1s in my Depron BAe Hawk (designed for original L-1s). These gave long high flights, demonstrating once again that the power was well up - to around at least 80-90mN.
Is it only me or has anyone else experienced this phenomenon? It can’t be just the batch of motors from the last year or so as the L1’s were from way back."
Howard presents a plausible hypothesis, and if, on a hot day, my models start looping or cavorting about the sky unpredictably on a flying day's afternoon, when previously, in the cooler morning, they'd been flying steadily, I will suspect the motors are indeed getting more potent. If a day's flying continues into the evening I shall be very interest if the flight patterns of my models calm down!
To test, or prove, Howard's theory properly will entail the use of cooler bags, hand warmers and either my test rig or a very reliable model like the Jetex Wren or Keil Kraft MiG 15. Given the known variability of Rapiers, though, each cold/warm/hot data point will need to be at least in triplicate. A lot of work, a lot of motors. Personally, I'd rather be flying, and, as a day warms up, choose models from my "likes a lot of thrust" box. For example my power-hungry T-38 Talons.
Now Howard has alerted us to this phenomenon I look forward to other rocketeers' observations and feedback.
It is thought that Jetex motors, in general, give smoother flights. Last year I had some really excellent sorties with my profile F-80 KK Comet powered with a ribbed Jetex 50 and ICI pellets. I also had ome quite spectacular, if short lived, flights with a Wren powered by a Jetex 50C and Sebel pellets. So I decided to test the thrust these motors were actually giving.
First, an original ribbed 50:
Next a Jetex 50B, which proved rather more potent:
And then a 50C:
This gave a lot more thrust. Worryingly, the duplicates were not that similar, perhaps because the pellets came from a different tin. Hmmm .... this suggests yet another batch of experiments ... how have the pellets been stored, had they been dried well before use (perhaps in an oven); what was the diameter of the motors' orifices? Etc etc. But the 'bottom line' to be taken from the above graphs is: it is perfectly possible to fly genuine Jetex today with stuff bought from eBay!
The formal experiments with Jetex motors and pellets will not be done. At least, not by me -- I'm quite content to take 'pot luck' when flying my small jets, and accept we will never, ever, pin down all the variables. A good flight and, better yet, the occasional outstanding flight, more than makes up for the frustrations. Even Howard was, at the end of his day at Salisbury Plain, happy!
Above: Howard's Spook, Hawk, Skyray and Valiant at his recent trip to Salisbury Plain. They look good to me!