Letters to the Editor
An answer, another question and somebody whose folks definitely raised him right.
An Answer to Peter Scott’s Turbofan Question
Peter Scott, in his most recent Science for Model Flyers column, asks why model gas turbines use fuel so much faster than their IC counterparts. The answer is that they are very inefficient machines.
A modern full-size turbofan has a compression ratio above 40:1, a large fan to move air very efficiently and internal aerodynamics, fits and finishes that optimise airflow and minimise leakages. Our model turbine has, at best, a compression ratio of 3:1 or 4:1 and is designed to start and run reliably rather than being optimised for efficiency. The model turbine is much like the original Whittle engines from the 1940’s. Very simple devices that worked well but were a long way from the complex and ultra optimised aero engines of today.
Many thanks for a great magazine every month.
Mike Goulette
United Kingdom
Mike’s letter had a real ring of authority, and in a subsequent email chat he humbly offered that “I’m glad that my 45 years at Rolls-Royce have proven to be useful!” I don’t think Mike will mind if I also add his further comment: “Most of my career was UK based, however, I ran the R-R office in Seattle from 2000 to 2004 and flew with the Seattle Area Soaring Society at 60 Acres Park. I retired in 2010 as one of the senior engineering directors in the Company. I loved my time at R-R which took me all over the world but, looking back, it’s the people I miss the most!” Talking with people like Mike, alone, makes this job worth doing. — Ed.
A Question about a Graupner Classic
I was looking through some plans on Outerzone and came across a very interesting (and I daresay challenging!) subject for a scratch build: the Graupner Mosquito 2.5m span of 1975 (linked in Resources) with some modern electronics such as BLDC motor, micro servos and the like.
I might work with a foam wing, with a D-tube construction — 1.5 mm balsa sheeting from leading edge to spar, balsa leading and trailing edge, and some spars either in carbon fibre or bamboo, or a conventional balsa built-up wing.
I was wondering if anyone had any tips on a wing joiner arrangement. The one shown in the plans is just 3+3" long and about 3/16" (5mm) diameter of unspecified material. Lots of doubts there.
Thanks in advance for anybody able to help.
Sanjay
Gurgaon, Haryana, India
Okay, readers over to you. You did a bang up job with Norrie Kerr’s Anthem question a couple of issues ago. Who can help Sanjay out?! — Ed.
Speaking of Norrie Kerr
Just a quick thank you to all involved I now have the Anthem plans on a memory stick and will be going to the print shop very shortly.
Regards,
Norrie Kerr
On behalf of all who were involved, Norrie, you’re very welcome. Thanks for the opportunity to help you out — it has had the spin-off benefit of being able offer the Anthem plans to everybody. There is one condition, however: you have to provide regular updates on how that Anthem project! — Ed.
Resources
- Science for Model Flyers | Part III: Energy — Peter Scott’s article in which he posed his gas turbine question and for which Mike Goulette provided such an excellent, informative answer.
- The New RC Soaring Digest | January, 2023 | Vol. 38, №1 — The post to RC India which served as the impetus for Sanjay’s question above. RC India is a great resource you should check out!
- Mosquito from Outerzone. — “A very good model manufactured by Graupner. One of the first electric gliders…”
- Bob Dodgson’s Anthem (1.5MB PDF) — The plans as they were originally delivered with the kit. A local print shop should be able to transfer them to paper at a reasonable cost. These are also available in colour which can be used to distinguish the various types of annotations.
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