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Gruse BG15/1 1930s motor glider prototype

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Mike Roach
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Joined: 21 Mar 2020, 13:44
Location: Christchurch

Gruse BG15/1 1930s motor glider prototype

Post by Mike Roach »

This attractive little plane was featured as a 48" span free flight design by Kurt Schnittke in the 1958 Aeromodeller Annual (the one with a Fokker DR1 on the cover, and yes, I did buy it when it was published). He called it the Rossiten AS but the correct designation for the aircraft was Gruse BG15/1 and I gather only one was made by Herr Gruse. Back in 2005 I modified it for RC rudder/elevator/motor indoor and light wind outdoor flying and had the plan published in Aviation Modeller International (AMI). The plan was taken up by a number of modellers and there is a short build discussion https://www.rcgroups.com/forums/showthr ... 28Gruse%29. Whether Charlie (portablevcb) still has the cut flies I don't know!

The scale of the drawings seems to be about 1:8. The 1930s picture with the man (Herr Gruse?) puts the wingtip about 40" off the ground (6" on the model), which puts the span at between 26 and 30 ft but documentation puts it at 10.8 metres, 33'6". A quarter scale model would be about 96” span.

There is a 2014 German build log for this aircraft, with at least four versions in different colour schemes. It seems certain that the registration D-YGYF was carried and the colour scheme was red over white, with a couple of detail variations and engine types documented on page 2 of the log.

Last year I built a new version at 68” span with ailerons and no dihedral, as scale. I've flown it a number of times and found that it flies best with a 3-cell 1300 battery placed as far back as you can under the pilot's seat. In this way she climbs strongly (for a 1930s motor-glider) and has a very reasonable glide, even tipping a wing in the right thermal conditions. She's a lot of fun to fly in tight spaces and an interesting shape.
The original free plan and lots more details are on my RC Groups thread here https://www.rcgroups.com/forums/showpos ... stcount=30
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VinceC
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Re: Gruse BG15/1 1930s motor glider prototype

Post by VinceC »

Interesting project, but to be correct, it was a Bo 15/1. Here are the specs from an old book of the period

Addendum
" but to be correct, it was a Bo 15/1" - I may have jumped the gun as in Flugsport it refers to a BG 15/1, more shortly
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VinceC
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Re: Gruse BG15/1 1930s motor glider prototype

Post by VinceC »

Here is an extract from Flugsport with a 3-view which I have also provided separately
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Mike Roach
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Re: Gruse BG15/1 1930s motor glider prototype

Post by Mike Roach »

Thanks Vince. The Flugsport 3-view shows the pylon as being uncovered, which is why my first model was like that. But the photos show it as covered - in ply I presume, like the rest of the fuselage. It's a pretty little thing. Kurt Schnittke's free flight model had the boom made from a 1/32" balsa box: it can't have lasted long! Mine is a depron laminate core with 1/16th balsa covering. The engine on the real thing was a horizontally opposed twin, but later changed to a very ugly 4-cylinder upright. starting it must have been fun!
Paul_Williams
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Re: Gruse BG15/1 1930s motor glider prototype

Post by Paul_Williams »

I somehow missed this thread first time around.

The Gruse is basically a development of the little known Rossitten AS primary glider with the addition of a fuselage pod and a motor.
At least four and maybe more, Rossitten AS orimaries were built and flown on the sand dunes of the Baltic coast.

The Gruse (and coincidentally several of my favourite types) is powered by the 18hp Kroeber M4 flat twin - dspite looking for some time, I have ben unable to find drawing of the engine - can anyone help ?

Paul
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chris williams
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Re: Gruse BG15/1 1930s motor glider prototype

Post by chris williams »

Here's Mike's pretty little machine in action @ CMFC...
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GeeW
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Re: Gruse BG15/1 1930s motor glider prototype

Post by GeeW »

Quite a few years ago I did some research into the Gruse, it appeared that it was a development of a 2 seat pure glider. There were only a couple of images, the one showing the whole a/c was very grainy. The other image which was usable showed an instructor boarding.
The main differences (apart from having no oily/noisy thing strapped to it) was that there was no sweepback and the instructor sat under the wing (a la T-31), between the wing supports, which may explain why the motor glider initially had no covering between the wing supports?


Gordon
Paul_Williams
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Re: Gruse BG15/1 1930s motor glider prototype

Post by Paul_Williams »

Hi Gordon, what was the two seater called ?
The Rossitten Ass has the same sweepback, tailboom and tailplane asthe Gruse - maybe the two seat is separate offshoot ? Always difficult to establish the line of development in that era.
Paul
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Mike Roach
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Re: Gruse BG15/1 1930s motor glider prototype

Post by Mike Roach »

I have been asked by Tony Dowdeswell (ed Flying Scale Models) if he can reprint my 2005 AMI free plan of the Gruse at 48", When I said I had built a 72" version he was keen to have the plan and photos for the article, so I've been trying to reconstruct my bigger model and draw up the plan.

So thanks to Chris Williams, I am now in steady employment drawing up plans and building for a magazine once more. The "It'll take 12 months" model will have eight motors, and no, it's not what you might think.

And thanks too to contributors to this thread. I'm getting the German text translated by the Hun in the Sun next week.
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Mike Roach
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Location: Christchurch

Re: Gruse BG15/1 1930s motor glider prototype

Post by Mike Roach »

Well, Tony D has published the Gruse in the latest (October) FSM and very pretty it looks too. I like the way that the 72" construction sketches are linked to my Free Plans thread in RC Groups. I think Tony no longer has a draughtsman on his team, so getting the plan details to him was going to be impossible. This solution has worked really well.

Mike
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