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Kolb-List Digest: 7 Msgs - 10/03/10

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tc1917(at)bellsouth.net
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Posted: Mon Oct 04, 2010 2:23 am Post subject: Kolb-List Digest: 7 Msgs - 10/03/10

I have been listening to all of you talk about thrust line, etc on your 

kolbs.  Some Slingshot talk also.  I am wondering at this point if the 

slingshot people have raised the rear of their engine.  If you do not, the 

thrust will literally push you over.  I learned this over five hundred hours 

ago.  Mike Highsmith put me up to it and he was very correct.  I have the 

912 ul raised in the rear off the mounts 7/8s of an inch higher than the 

front mount.  You see, the angle of attack of the wing on the slingshot is 

only 9 degrees rather than the 12 degrees of the average other kolbs.  The 

boom of the slingshot flies mostly straight and level at most speeds, 

including very fast.  It does not continually rise as do the MkIIIs and 

such.  If you do not raise the engine in the rear it will push you over and 

the faster you go the worse it gets.  It will work against itself.  On 

take-off I learned very quickly (thanks to Hauck) you only use 75 to 80 

percent power until you wheels are clear and you are climbing.  Relieves the 

torque both to the left and nosing over.  If you need further information or 

pics, let me know.  I would think 60 hp from an HK would be rather slow for 

a slingshot.  It has such a short wing it needs power to sustain flight.  I 

found a 582 was good but a 912 put some of the weight forward and worked 

wonders.  Actually flew better than the 582.  I would not want to fly it 

with less than  65 hps.  Of course, I only burn 3 gals an hour at 80/85 mph 

cruise compared to the 4/5 gals an hour with the 582.  I also went to a 

longer three blade taper point from warp.  Too bad I still have the rom, rom 

in the engine but thanks to a better muffler system, it is tolerable now. 

Trial and error brought me to the answers and I hope you have good luck on 

your experimentation away from a workable solution.  Ted Cowan, Alabama 

912UL zoom zoom.

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tc1917(at)bellsouth.net
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Posted: Mon Oct 04, 2010 4:26 am Post subject: Kolb-List Digest: 7 Msgs - 10/03/10

I decided to send a pic of my little slingshot idling, waiting to take off. 

Note the angle of the blade in relation to the boom.  Ted Cowan

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Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Tue Oct 05, 2010 3:38 am Post subject: Re: Kolb-List Digest: 7 Msgs - 10/03/10

Ted,



My slingshot hits a wall at 95 mph and that is its top speed. Anything above about 85 mph requires more and more nose down elevator. I believe that this is due to the thrust line angle being way too high, as you mentioned. I suspected this but could not get Travis to tell me what angle it was supposed to be so I've left it alone. The drawings and plans that I have don't say. That is probably why Travis could not tell me.



Now that I know your aft end of the mount is 7/8" higher than the forward end, I can start thinking seriously about fixing mine. Since the motor mount for the Jabiru is way different from the Rotax, 7/8" may not be the right answer for me. 



At your convenience, if you could measure the thrust line angle relative to the bottom chord of the wing and let me know what that is, I can adjust the Jabiru motor mount for that angle. Right now, my thrust line is pointing UP by 3.5 degrees relative to the bottom chord of the wing and it should be pointing down somewhat relative to the bottom chord, based on your photo and what you said.



Thanks for your help.

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_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
- Anonymous

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