D.L.R.

Not sure this is the right place but I've seen some sensible/understandable answers so here goes. How is the engine "thrust line" determined ? I have built a few kit planes and stuck together a few ARF's in my day, but I can't say that other than the fact they all say "the engine goes here and that makes this the thrust line" I have no idea (other than being sensible about fuel tank/carb position) where you would mount the engine. What prompts the question is a rebuild I'm doing using a Telemaster 40 wing and tail feathers (the rest went to the dump ;0( The fuse is a simple slab side with wing and horizontal stab mounted on the same plane (I don't MEAN the same aircraft, even tho it is) which is the top of the fuselage. I HAVE flown the the plane with a TT .36 Pro and it is GREAT, just underpowered, the weight was 7 pounds. Because of the flat topped fuse the thrust line must be BELOW the wing and horizontal stab. There is a 3 inch high firewall for mounting the engine, but as noted before some consideration MUST be given the fuel tank/carb position. I'm putting a Magnum .46 XLS on. Maybe my question SHOULD BE do I put the engine as high as practical, or does it even matter ?????
Thanks in advance for any help.
Dan

Gray Beard

My Feedback: (-1)

With a kit or plans the engine thrust line is the line shown that runs through the center of the crank. If you run a straight edge along this line it is usually straight along the datum line. On planes that show no thrust line I find the center of the fire wall and say that looks about right and use the center. On planes like the Telemaster it doesn't seem to mater a lot.

pimmnz

It really won't matter, put the engine where access to the tank, and tank height, are both easily adjustable and so that the centre of the tank is aroung the line of the main needle. Where you point the engines thrust, ie the crankshaft axis, will depend on how far below the wing the engine is. This axis is generally referred to as the 'thrust line' and will be related to both the main fuselage datum, and the wing chord line. For your purposes a bit (2 deg) of 'down thrust' and a bit of (2deg) 'right thrust' will be about right, related to the fuselage datum.
Evan, WB #12.

D.L.R.

OK thanks, looks like placing engine where it needs to be relative to fuel tank should be my main concern. I'm not overly concerned with right and/or down thrust, I've always had pretty good luck with flat bottom wings and a lifting horizontal stab by assuring 0 incidence. My goal is to be able to run down the runway at full throttle but no lift off until an up elevator command is given. This way, the plane requires a command to do everything (almost), and stays at whatever attitude you choose until you give it another command. Of course straight up is short lived and inverted requires down elevator input.
Again, thank you very much for your opinions.
Dan

pimmnz

If that is what you want then you will have to arrange the wing to be at its 'zero lift' angle on the ground, that is, with the chord line of the wing probably at something like -4deg. Since the tailplane will be lifting both up and down during different phases of flight, you might do better with a symmetric tailplane section. FWIW.
Evan, WB #12.

Lnewqban

Dan,

I fully agree with the previous posts from Gray Beard and pimmnz; however, I feel certain contradiction in the quoted statement.

If you want to achieve a neutral response to changes in engine rpm's, you should allow yourself room for manipulating the angle of the thrust line.
The reason is that the named line must pass through the spatial location of the CG for neutral pitch reaction.

Al rotations in flight occur around the CG.
If the thrust line extends far from the CG, then the thrust force has an arm respect to that center of rotation and induces a pitch moment or torque that makes the nose go down or up with throttle command.

Reducing or eliminating that arm or lever is easier by angle than by height.
The sweet spot for angle of the thrust line is also easier to achieve by trimming than by design.

Gray Beard

My Feedback: (-1)

That is true but I use no engine thrust in my own planes until the maiden/trim flights then use shims as required if needed. 2 right and 2 down seems to be the industry norm but it isn't always needed or sometimes even more is better. After the first few flights I can pretty much tell then make up the correct shims for the pane. My own planes are mostly built from plans and thrust isn't mentioned very often. I do usually end up with some right thrust but not often down thrust. Go figure??

D.L.R.

As stated before I thank you all for your input, but now I believe you are over thinking the issue. I don't profess to be an aeronautical engineer, but I've been in RC since 1972 except for a brief period between 1983 and 1991 when I got a full scale license (learned in a Cessna 152) and put together and flew almost dailey (except in the winter, back to Cessna's then) a kit Quicksilver II, a glorified Ultralite which required a license because it was a 2 seater.

I guess the simplest way to restate my goal and what I consider successful attempts in the past, (and present) is that THE AIRCRAFT MAINTAINING AIRSPEED AT FULL THROTTLE DOES NOT CHANGE ATTITUDE (inverted won't work) UNTIL ANOTHER COMMAND IS GIVENanything less than full throttle, and all bets are off. The fun part of this is having a basic "trainer" with flaps that is at least somewhat aerobatic, but can land at almost 0 groundspeed in a 5 knot headwind. I forgot to mention the wing has only 2" diehedral.

Guess old timer's like me don't like to see major change in aircraft attitude (or anything else for that matter) unless I say it's ok !!!!!! Maybe I made it sound as tho I didn't feel right or down thrust was important, that wasn't my intent, I simply want to know if the HEIGHT of the THRUST LINE relative to the WING was of any importance. My best GUESS is that it is NOT when you consider low wing, high wing, tractor, pusher, monoplane, biplane, triplane etc.

Did any of this make sense ??????
Dan

D.L.R.

As stated before I thank you all for your input, but now I believe you are over thinking the issue. I don't profess to be an aeronautical engineer, but I've been in RC since 1972 except for a brief period between 1983 and 1991 when I got a full scale license (learned in a Cessna 152) and put together and flew almost dailey (except in the winter, back to Cessna's then) a kit Quicksilver II, a glorified Ultralite which required a license because it was a 2 seater.

I guess the simplest way to restate my goal and what I consider successful attempts in the past, (and present) is that THE AIRCRAFT MAINTAINING AIRSPEED AT FULL THROTTLE DOES NOT CHANGE ATTITUDE (inverted won't work) UNTIL ANOTHER COMMAND IS GIVENanything less than full throttle, and all bets are off. The fun part of this is having a basic "trainer" with flaps that is at least somewhat aerobatic, but can land at almost 0 groundspeed in a 5 knot headwind. I forgot to mention the wing has only 2" diehedral.

Guess old timer's like me don't like to see major change in aircraft attitude (or anything else for that matter) unless I say it's ok !!!!!! Maybe I made it sound as tho I didn't feel right or down thrust was important, that wasn't my intent, I simply want to know if the HEIGHT of the THRUST LINE relative to the WING was of any importance. My best GUESS is that it is NOT when you consider low wing, high wing, tractor, pusher, monoplane, biplane, triplane etc.

Did any of this make sense ??????
Dan

Lnewqban

Yes, Dan, your explanation makes sense.
I apologize if I did not express myself properly.

With the attached schematic, I try to explain that it is not worth sacrificing engine height against good fuel plumbing, because the desired effect of reducing the thrust leverage against the CG can be achieved simply with a change in the angle of thrust (positive or negative).

The real test is the actual flight; as you know, in some cases no deviation from the datum line is needed.

Other times, the effect on pitch of the drag from the landing gear and the pitching of the flat bottom wing is so high that some up-thrust is beneficial.

D.L.R.

Please, no apology needed nor expected, I guess in looking back if I weren't ME, I might not have understood what the devil I was talking about either !!!!! In the end I believe the answers I got were in fact mimicking my thoughts and bottom line is, place engine for best "fuel plumbing effect" and "shim engine thrust to achieve desired results". Guess I should have went with my gut and did it, but then I wouldn't have had the pleasure of the conversations with you gentlemen.
Thank you all again.
Dan