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Grob Egrett at 1/7.5 scale

Posted: 25 Mar 2020, 11:28
by Mike Roach
OK, the full size Egrett is not a motor-glider but I’m sure a model version could be classified as such. In addition, it has been used as a glider tug, towing a Perlan 2 to 40,000ft over the Andes. It has a span of 33m, so even at 4.25m the model would only be about 1/8th scale. Cheaper on model pilots, I suppose.

This fascinating aircraft has been on my list for many years. The main difficulty is building or sourcing a suitable scale wing but there are a number of attractions waiting for the builder including a tricycle undercarriage, four-bladed prop, huge exhaust and a bewildering number of variations on the basic planform. Three views, photos and a foamie model plan are here: https://www.rcgroups.com/forums/showpos ... stcount=90.

All the three-views are from the manufacturers, courtesy of a sailing club friend who worked for them when they had a servicing facility at Bournemouth Airport. The 1:20th scale drawings are of the Egrett DST twin seater and the G520 single seater and there are less good scans of the Agrett and the DST. I'm afraid the big scans miss off a section of the very tail of the plan view, but this can be interpolated from the other drawings. I don't have any fuselage cross sections but they are pretty easy to follow using the photographs and my model plan

The 60" span model plan was drawn a decade ago for a speed 400 and foam construction but my lack of skill with wing cutting proved to be the model's undoing. It did fly though. A 1/16th balsa chuck glider at 12" span has had a long and successful life in the garden and with the grandchildren, proving that the aerodynamic concept works well at small scales. You can see a picture of the model on my website here http://www.sopwithmike.org.uk/html/post_war_0.html.

So here's the question. I've drawn up a 148" span wing. The full size has an undercambered section a bit like a Quabeck HQ35, but all my wing-building experience has been with slightly easier sections such as E207, which to be honest I would prefer if I'm to take the plunge. As a complication, the wing is very narrow, only 250mm chord at the root and a mere 103mm at the tip. Are there any suggestions, comments, "barge pole" prohibitions, even?
Grob Strato 1.jpg

Re: Airfoil for 1/8th Grob Egrett?

Posted: 25 Mar 2020, 11:58
by Peter Balcombe
Mike,
I wouldn’t have thought a built up wing too much of a problem.
As an example I have designed & flown a built up wing design for a 3.75m Slingsby Vega which has a root chord of 226mm & tip chord of 108mm. This happens to use a Ritz 3-30-14 at the root, progressing to 3-30-13 at the taper break & same at tip.
These days, with CAD tools such as DevWing & DevCad, plus laser cutting of ribs, interlocking spar webs etc. it is not difficult to build most wing sections.
Peter

Re: Airfoil for 1/8th Grob Egrett?

Posted: 25 Mar 2020, 16:54
by Mike Roach
Hi Peter, thanks for the input. I'm actually
reasonably OK with the structure of the model. It's whether an undercambered section like HQ 35 or a flatter-bottomed one like E 207 would be more appropriate. I see that Chris W uses the former on his Fournier motor glider. Perhaps that's the way to go.

I fully intend to get the ribs laser cut.

Re: Airfoil for 1/8th Grob Egrett?

Posted: 25 Mar 2020, 16:58
by chris williams
Mike, I've never seen a subject more likely to fall prey to a nasty tip stall... I look forward to you proving me wrong! 8-)

Re: Airfoil for 1/8th Grob Egrett?

Posted: 26 Mar 2020, 18:43
by Mike Roach
:D I like a challenge, and a friend has agreed to laser cut all the wing ribs. I hope to get it in the air and check out the stall by the end of the year.

Re: Airfoil for 1/8th Grob Egrett?

Posted: 05 Sep 2021, 13:29
by Mike Roach
The cut ribs I got from Steve85 were not what was needed, so I've been in touch with Cliff Evans at Laser Cut Sailpianes with a pdf prepared by Steve. He's going to cut a set of ribs for me to the new spec, so "by the end of the year" might be pushing it a bit, but the next model on my board will be the Egret, the prototype aircraft with the very simple wing.

Re: 1/8th Grob Egrett?

Posted: 07 Dec 2021, 14:26
by Mike Roach
Here we go then on the model "Most likely to tip stall" (copyright Chris Williams). I got the ribs and spars cut by Cliff Evans and have started a thread on RC Groups. Because I am prone to the six Ps, I still have a lot of noodling to do before the melody emerges. So far I have got the centre section, which carries the main UC retract pods, sort of complete. The D Box has firmed up the structure and it's all looking OK. The scale of the 148" model will be 1:7.5.

Here https://www.rcgroups.com/forums/showthr ... rob-Egrett

Re: Airfoil for 1/8th Grob Egrett?

Posted: 07 Dec 2021, 23:08
by Cliff Evans
Thanks for the mention Mike, both here and on RCGroups.

Re: Airfoil for 1/8th Grob Egrett?

Posted: 07 Dec 2021, 23:48
by chris williams
Looking forward to the maiden already, Mike... ;)

Re: Airfoil for 1/7.5 Grob Egrett?

Posted: 09 Jan 2022, 15:14
by Mike Roach
The bare bones of this model have arrived at the point where they can be displayed. The wings have proved to be the easiest parts to build, thanks to the laser-cut ribs from Laser Cut Sailplanes. Unfortunately I didn't include servo lead holes in my design, so much use has been made of an old TX aerial, sharpened up at the cutting end.

There's loads to do, probably 75% more "work" but so far, so good.
14.jpg

Re: Grob Egrett at 1/9 scale

Posted: 09 Jan 2022, 21:50
by Cliff Evans
Looking good Mike.