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Old Mar 05, 2012, 12:52 PM
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ASK 21 Jet glider

Hello,
I have buided a ful GFK ask 21 with a span of 5m66.
To power it I installed a Jetcat P160 on its back and mounted a piece of thin inox metal plate on the tail to absorb the heat a bit.
Yesterday I have made a few testruns with the engine and it seems that the tail is getting realy hot, I measured 92C with the engine running on idle for less than one minute.
I have seen already a few big scale gliders with a jet on their back, how come they do not get to hot?
What can I do?
I added a few pictures to show what I mean.
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Best regards Johan
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Old Mar 12, 2012, 09:42 PM
tjp
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United States, NY, Syracuse
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I have seen gliders that big fly with just a P60. I think you have way to much engine.
Search RCU's jet section and you will find info.

Tom
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Old Mar 13, 2012, 01:49 AM
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Sorry my friend but it's simple common sense not to place your power unit where it is! Try to fit it to some twin boom design unless you like to melt Rudd. fin. Few years back at Avalon Air Show was a pilot with full size glider having twin 160's or 200's in the same position but lot higher and did some spectacular demo flying. Obviously the distance from his units to the fin was lot larger to worry about the heat, and having the twin the exhaust was not directly at the fin.
I built two large " foamies" powered by jet turbines and managed to prove to all sceptics it can be done but the design is totally different then your scenario, Joe.
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Old Mar 14, 2012, 12:45 PM
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Maybe you can ask Ali Machinchi about it. He's flown them before.

hmarmaizmd717
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Old Mar 14, 2012, 05:32 PM
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Quote:
Originally Posted by hmarmaizmd717 View Post
Maybe you can ask Ali Machinchi about it. He's flown them before.

hmarmaizmd717
Next time you are in the kitchen put your hand on 92deg.C hot plate and see how long you can keep it there? It's only an idle! Joe.
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Old Mar 14, 2012, 08:13 PM
THAT'S WHY
causeitflys's Avatar
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I think that most angle the engine slightly to avoid the vertical damage. A slight angle wouldn't be noticed in the flying.
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Old Mar 17, 2012, 10:38 AM
still trying to get it right
yakhakker's Avatar
benelux
Joined Feb 2009
680 Posts
Yep I think you are way over powered there, be careful with the throttle if you are going to fly it also check out the structural integrity of the plane to be sure it can handle the forces being put upon it.
It is possible to put 3 or 4 degrees of side thrust without it having much of an effect
Good luck.
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Old Mar 17, 2012, 11:18 AM
THAT'S WHY
causeitflys's Avatar
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Quote:
Originally Posted by tjp View Post
I have seen gliders that big fly with just a P60. I think you have way to much engine.
Search RCU's jet section and you will find info.

Tom
I don't think a P-60 would do on a 17ft glider
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Old Mar 17, 2012, 03:19 PM
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I wish I had that much money.
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Old Mar 17, 2012, 07:00 PM
I can 3 point
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Mount Dora Florida
Joined Aug 2002
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Most of the 7metre 50pound avg flying weight fly on a jetcat 120 / 26lb thrust that I have seen. That means if yours is 5.66 m , then about 13 0r 14 pounds thrust such as a jetcat p60 sounds about right. . The 160 is the same size and weight as the 120 but has a higher egt.
You do however seem to have the engine mounted in the correct position, just too much of an engine, that is all.
good luck
Paul
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Old Mar 17, 2012, 07:07 PM
I can 3 point
Paul Wilson's Avatar
Mount Dora Florida
Joined Aug 2002
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This is Als 7 metre ask21 with jetcat 120, perhaps Jofro hasn't seen this configuration before, the central mount is quite common in fact. It is much bigger than yours and the lower egt and larger distance to the tail will keep heat down. As I said earlier, the p60 has a lower idle egt than the 160, 580C vs 650C and a lower idle thrust, .8 lbs vs 1.0lb. That would reduce the heat substantially.

http://www.alsgallery.co.uk/view_alb...lbumName=ask21
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Old Mar 22, 2012, 05:42 AM
always looking for clouds
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how about making your own jet exhaust....divide into 2 exits one either side with a slight outward direction?
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Old Mar 22, 2012, 07:06 AM
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Quote:
Originally Posted by socommk23 View Post
how about making your own jet exhaust....divide into 2 exits one either side with a slight outward direction?
Because they are loud, and you loose thrusts.

There is nothing wrong with the way it is. A few degrees on the engine mount will keep the tail from burning. (or a little foil tape)
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Old Mar 22, 2012, 02:26 PM
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Colchester UK
Joined May 2004
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If you want a better material than the stainless sheet you have used then this is what you need for the fin.

http://www.zircotecshop.com/

Colin Straus used it on his Paritech 7m 1:2 scale Fox glider that is powered by a P120.

It is now common to have the engine offset slightly so the engine airstream does not hit the fin and so reduces temperature.

I have used the ceramic aluminium foil on my glider and have no problems with the heat.
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Old Mar 24, 2012, 03:10 PM
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I assume you mean static test runs and not actual flights since you said the engine was idling. In flight you would have cold air mixing with exhaust going over the tail. Static you only hot exhaust... or almost all host exhaust. Likely the tail won't get as hot in flight. But that's all theoretical.
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