ENGINE DOWN THRUST ANGLE QUESTION???  
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ENGINE DOWN THRUST ANGLE QUESTION??? - 6/10/2009 12:52 AM   
airbusdrvr


 

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Status: offline 		I was thinking of posting this question in Kit Building, but maybe aerodynamics might be a better choice.   I am getting ready to slightly kit bash a Sig Kadet Senior so as to use a larger engine than the original design.  The plans show 6 degrees down thrust and 2 degrees right thrust to be built into the forward bulkhead/firewall.  I am just curious if this amount of engine down thrust  would be applicable as the engine size is increased?  Two degrees right thrust is fairly common but 6 degrees seems like a lot for downthrust.

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RE: ENGINE DOWN THRUST ANGLE QUESTION??? - 6/10/2009 1:59 AM   
dick Hanson



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Status: offline 		The  "theories "on down thrust  are  pretty darn subjective
 A  LOT depends on the  speeds yo like to  fly.
   Why the larger engine?
 A whim?
 essentially th model is  of  a size  and the  wing setup makes for  a lot of  climb under power
 The  huge down thrust  was likely done to  increase speed yet   hopefully provide  some force to  keep the  nose  down
You can screw around with the  speed n downthrust  and the wing saddle position till the  cows come home 
 BUT the  stock setup makes for  a gentle very ,very easy model to fly.
 My old Seniorita can quickly be trimmed  for  complete hands off  flying - it  will  do huge circles  with a gentle undulating  flight path
 meaning the model will slightly climb till speed drops back  then dive slightly  picking up speed  and  the  nose will gradually  come up  -repeat etc.. lovely  free flight  flying
 If you are after an aerobatic setup , the  basic model arrangement is  wrong  -so once you dig into  more  power for more speed  you wil end up having to  change  angles of  wing and thrust  -or just accept it is what it is.


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RE: ENGINE DOWN THRUST ANGLE QUESTION??? - 6/10/2009 3:11 AM   
airbusdrvr


 

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Status: offline 		Why the larger engine?
Whim? 

Well actually I was looking for something in the 75 to 80 inch range that I could hopefully keep at around 9lbs or so to put a larger four-stroke on. I wanted something similar to J-3 Cub  performance.  Mild acro and slow flighting in the traffic pattern.  Just a plane to play with.  Also, the forward fuse needed to allow the fuel tank to be placed below the thrust line by about 2 1/2 inches.  One of the guys in my club was flying his Kadet Senior ARF today.  The Kadet Senior was one plane I had viewed with some interest while on the SIG site.  Over the years I have seen several Seniors bashed into  fairly interesting and fun planes.  Take out most of the dihedral, add ailerons, make the wing mount more modern, perhaps lift struts,etc.  During today's conversation with the Senior  flyer, he mentioned he had a kit, NIB that he'd sell for $60.  So now you know how all this got started. 

Being a kit, I Will be able to more easily move equip, batt, and such so as to keep the CG right without adding weight.  I can strengthen the forward fuse and use thicker firewall plywood to accommodate the larger engine.  Anyway, that is the present plan.

dick Hanson, I checked the plans and the flat wing bottom and stab are at 0 degrees incidence to the fuse datum line.  The chord line of the wing is probably pretty close to 5 or 6 degrees up.  So I guess that is why the motor is 6 degrees down.  What would happen if the wing incidence and motor thrust angle were both at or near zero?  Since this is going to be a taildragger, negative lift at takeoff would not be an issue. 

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RE: ENGINE DOWN THRUST ANGLE QUESTION??? - 6/10/2009 3:28 AM   
dick Hanson



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Status: offline 		You could easily raise the trailing edge 1/4" for starters  - make allowances to raise it more
 the  flat bottom of the wing is just a referrence line.
the true zero lift line is up to at least the  center of the  leading edge 
with good ailerons - take out all the  dihedral  THEN use a slightly raised  position for  "center "of ailerons.
 keep the  weight down -big chunks of wood do no good strength wise - add some beef to  the  fuselage sides which take th  engine loads



< Message edited by dick Hanson -- 6/10/2009 3:30 AM >

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RE: ENGINE DOWN THRUST ANGLE QUESTION??? - 6/12/2009 4:44 AM   
Doc.316


 

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Hello;
Piped .60? Mine has one on it and it fly's good on 0 down thrust.  (set it up as a pattern ship...lol)...what ya have to be careful of  is the wing blowing...(it took a couple of wings to get the wing strong enough).   She will do OK if you just keep it slow and don't go crazy with her...
Steve



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RE: ENGINE DOWN THRUST ANGLE QUESTION??? - 8/4/2009 10:46 PM   
DBono99



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Status: offline 		Hi All
I have a Sig LT 25 that I just converted to e flight..It use to fly with an OS 25FX.
Now with the electric motor it wants to climb..I added some washers behind the mount for added down thrust and it still wants to climb. I check the engine down thrust agaisnt the plans and motor seems prefect.
Could IT be a matter of the Cg being tail heavy? I balanced It on the main wing spar as per the building instructions.

THanks Dan
PS the wing chord is 12" and the wing spar is 3 1/2" form the leading edge.Thats what wasa called for in the instructions.

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RE: ENGINE DOWN THRUST ANGLE QUESTION??? - 8/6/2009 10:10 PM   
DBono99



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Status: offline 		Hi All Where did all the help go?? I finally got the LT 25 dialed in.. It was more of CG problem then a dwon thrust problem.

Dan

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RE: ENGINE DOWN THRUST ANGLE QUESTION??? - 8/7/2009 12:29 AM   
BMatthews



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Status: offline 		It's summer, we were probably all out enjoying it....

Sounds like you figured it out for yourself. As you found the amount of downthrust required is actually linked to the CG location and the wing to stabilizer angle. The further back the CG and less the angle the lower the tendency is for the design to pitch up with a speed increase. Taken to major extremes you'll find that a 0-0-0 model with the CG located right on the neutral point does not need any downthrust and won't nose up when the speed increases. Trainers with forward'ish CG's and strong wing to stab decalage angles greatly benefit from having some downthrust.

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