Downthrust in a stunt plane, any bad effects?

[frank mccune]

     Hello all:


     Before I had a chance to fly my "new" stunt plane, a mate of mine pointed out that the engine had a bit of downthrust.  What will be the effects of this downthrust on the flight characteristics? 

     Would it be worth tearing up the nose and remounting the engine? This is a lot of work! Oh yes, this is a sport plane with a built up body, I do not think that I could do the AMA pattern with this plane.  It has about 3-5 degrees of down thrust.

     Any thoughts or suggestions?

                                                                                                                        Tia,

                                                                                                                        Frank

[Brett Buck]

     Hello all:


     Before I had a chance to fly my "new" stunt plane, a mate of mine pointed out that the engine had a bit of downthrust.  What will be the effects of this downthrust on the flight characteristics? 

     Would it be worth tearing up the nose and remounting the engine? This is a lot of work! Oh yes, this is a sport plane with a built up body, I do not think that I could do the AMA pattern with this plane.  It has about 3-5 degrees of down thrust.

     Any thoughts or suggestions?

     Many people put downthrust in full-tilt stunt planes on purpose. 3-5 degrees is A LOT more than is usually used, but if it's not a stunt plane, about the only negative effect is likely to be "cycling" in the wind. That's what the elevator is for.


     Brett

[Jim Roselle]

Is the engine mounted upright? If so you could use side thrust shims to level it.

Jim

[William DeMauro]

You should be able to shim it in most cases. Take the easy way out if its at all possible.

[Dennis Moritz]

You might have to haul back to get it to loop inside. I have a Jr Streak powered by an FP15. There's a little wiggle room in the engine mount. Flown for years. Recently I removed the engine, cleaned stuff up, remounted the tank, etc. Barely could get it to turn outside. Checked the engine. Seemed to have a smidgeon of up thrust. I loosened the bolts pushed the engine nose up a bit until it looked neutral. Tightened the bolts. Flew ok after that.

[frank mccune]

     Hi Guys:     The plane in question is a Veco Tom Tom with a MVVS D7 engine installed.  The engine is upright and I did not use blind nuts when I installed the engine.  To get at the mounting bolts, I must remove the bottom of the fuselage.  Wether to replace the bottom of the fuselage is worth the effort is the question.  The plane used to fly really great!

     What I think hapened was that when I fuel proofed the engine bay with epoxy, some of it must have run down onto the mounts in the back near the fire wall.  I could remove the excess epoxy and bed the engine mounts in metal filled epoxy to ensure a "perfect" fit for the engine.  The only problem that I can see with this method as heat from the engine may soften the epoxy and ruin the bedding.  Of course, there is always the use of aluminum
engine plates that are bedded to the hardwood mounts.  Perhaps they will not get soft from engine heat.

     I have been advisd to go ahead and fly the plane as it is and if not satisfied, make the changes. 


                                                                                                               Thanks for the replies,

                                                                                                               Frank
 

[john e. holliday]

If there is down thrust in the engine and it turns better outsides than insides you will not get equal loops.   Fly it and see if it flies okay or good.   If not use shims. 

[Bob Reeves]

     Hi Guys:     The plane in question is a Veco Tom Tom with a MVVS D7 engine installed.  The engine is upright and I did not use blind nuts when I installed the engine.  To get at the mounting bolts, I must remove the bottom of the fuselage.  Wether to replace the bottom of the fuselage is worth the effort is the question.  The plane used to fly really great!

     What I think hapened was that when I fuel proofed the engine bay with epoxy, some of it must have run down onto the mounts in the back near the fire wall.  I could remove the excess epoxy and bed the engine mounts in metal filled epoxy to ensure a "perfect" fit for the engine.  The only problem that I can see with this method as heat from the engine may soften the epoxy and ruin the bedding.  Of course, there is always the use of aluminum
engine plates that are bedded to the hardwood mounts.  Perhaps they will not get soft from engine heat.

     I have been advisd to go ahead and fly the plane as it is and if not satisfied, make the changes. 


                                                                                                               Thanks for the replies,

                                                                                                               Frank
 

Are you saying you built an airplane you can't remove the engine from without surgery?

[john e. holliday]

Hey Bob, it has been done before.   I even accidently did it a couple of years ago with a new so called fuel proofer.   I waited q week before mounting the engine.   Later when I wanted to change the engine for another one, found out it was glued in.   Still have some of that finish on the engine.

[Jim Kraft]

JB Weld will stand lots of heat. I have put my Weller soldering gun to it and it won't even make a mark.

[Bob Whitely]

Good question. A little downthrust and right thrust works every time on the
normal stunter that flies counter clockwise and uses a normal tractor engine.
We build it in as a matter of course cause we know that it will always be needed
later anyway.  Also there are no detrimental effects to doing this. Period. RJ

[Don Hutchinson AMA5402]

Fly it first! You may find that the model flies just fine the way it is.

[John Leidle]

   One of the local Experts told me a while back  that he always put 1 degree of downthrust in the engine..

[john e. holliday]

JB Weld will stand lots of heat. I have put my Weller soldering gun to it and it won't even make a mark.

J-B Weld is what is holding the muffler on my B-40.   Have at least a dozen flights on it.

[Brett Buck]

J-B Weld is what is holding the muffler on my B-40.   Have at least a dozen flights on it.

   I have early 40-sized tuned pipes that appear to have been constructed entirely of JB Weld.

   Brett

[Jim Thomerson]

My first Agile Arrow would turn 10 ft diameter outside loops, and maybe 60 degree insides.  I put in  3 degree plastic shims to cure the downthrust and that did the trick.

[PJ Rowland]

A little downthrust and right thrust works every time on the
normal stunter that flies counter clockwise and uses a normal tractor engine.
We build it in as a matter of course cause we know that it will always be needed
later anyway.  Also there are no detrimental effects to doing this. Period

Spot on - I read this YEARS ago this exact quote, from that EXACT same stunt flier.
Every model I've built in the last recent 10 have included this as part of the design. Never had anything
difficult to trim, model locks in and performs well every time.

Perhaps electrics are different with more Right thrust required seems to be the way many are moving.