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Old Sep 30, 2015, 04:01 PM
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Question
Calculating engine offset anf thrust angles

I came across an old post on FG that describes very well how to use trigonometry (SIN and TAN) to calculate engine offset and thrust angles.

http://www.flyinggiants.com/forums/s...22&postcount=6

Engine offset

The formula for measuring the engine offset is: Engine Offset = (engine length) * sin(thrust angle)
Here is an example for the DA 100. The length of the engine from the back of the mounting plate to the back of the spinner is 6.5". If I want to have 2.5 degrees of thrust built in then: Offset = 6.5" * sin(2.5) = 0.2835". This is approximately 9/32. Which means that to have 2.5 degrees of right thrust, you will need to move the centerline of the engine 9/32 to the RIGHT (when looking at the firewall) for mounting so that the center of the prop is in line with the center of the firewall (or the manufacturer stated centerline).


Thrust angle

How do you find out how many washers you need, or how much longer to make the left spacers versus the right spacer to get the right thrust you want? In other words, if the firewall has no thrust built in, then you will need space out the left mounting holes of the engine more than the right to put in right thrust.The equation to do this is: Extra Length = (distance between mounting holes) * tan(thrust angle)
To give an example, the distance between the mounting holes on the DA 100 is 2.75". If you want a 2.5 degree right thrust angle, then: Extra Length = 2.75" * tan(2.5) = 0.120". The left side of the engine will need to be extended 0.120" farther than the right to have a thrust angle of 2.5 degrees.
This is great stuff and just what I was looking for.

I am confused, however, on a couple of points about the combination of down and right thrust. Let us say for the sake of argument that the manufacturer recommends 2º right and 3º down and that we have found washers that are just the right thickness to provide 1º of angle.

Here's a diagram numbering the standoffs.



We add two washers between the firewall and standoffs on the port side of the plane which gives us 2º right thrust. So far, so good. Here are top and port views:





But how do we add the down thrust? Do we place three washers between the top standoffs and the firewall, like so?





Assuming this is correct, here's where I'm stuck.

POINT OF CONFUSION #1

Don't the washers behind standoff #1 cancel the right thrust? And don't the washers under standoff #4 cancel the down thrust?


POINT OF CONFUSION #1

Now the flanges (ears?) on the engine will not be flush with the faces of the standoffs because they are effectively three different lengths. Wouldn't this cause unnecessary strain on the flanges, leading to breakage?

Clarification would be very helpful. Thanks in advance for your help!
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Old Sep 30, 2015, 05:20 PM
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Lets say you add shims at the top to give you the 3 deg. down thrust, you will still have an equal plane for the engine to set on, you can test this with a simple mock up to set the mounts on. Now you need some right thrust add a washer or two to both mounts on right side and set your engine on them, all 4 legs should sit flat at this point, if it doesn't there may be a twist in the fire wall , if that's the case, hold the two left stand offs firmly down and use a feeler gauge to find any difference in the right stand offs and shim it out. you can also put the 4 legs against the fire wall and see if it rocks any at all and make sure the washers are the same thickness.

Different prop lengths will mandate the need of varying off sets, some planes need it others don't. you can trim out down thrust easier than right thrust. A correct incidence of the stabilizer and alignment of vertical fin is also something to look at if it requires excessive trimming.
There is a tapered shim kit out there to do this, I can't remember where to get it, it allows a flatter surface to mount to taking the stress off the stand offs and engine ears.

Leroy
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Last edited by Leroy G; Sep 30, 2015 at 05:26 PM.
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Old Sep 30, 2015, 06:05 PM
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You are basically right on the stand-offs being uneven, but, the angles and distance is so small one would never know it. One can also add the fact that the firewall is wood and will absorb this very minute difference, hence, one will never know it.
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Old Sep 30, 2015, 08:12 PM
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I agree with Ace. Ive never hax the taper be an issue. One thing I will add though is that thrust angles is very much the same as CG in regards that I dont spend a bunch of time doing calculations based on the fact that it will need to be changed after some test flights.
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Old Sep 30, 2015, 10:45 PM
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grosbeak this is a good subject and I'm glad you brought it up, others have had problems with this very thing.

For illustration purposes I did a case of all things being equal in this case. First I made some spacers you can see, they are 1/32' ply, two each on the top stand offs for down thrust, all legs sit firmly on the mount. Then I put one more on the top right stand off making it 3 and one on the lower right for right thrust and all legs sit firmly on the mount, none are longer or shorter because all things are equal.

# 1 top left has 2 shims, #two top right has 3 shims # 3 bottom has no shims and # 4 bottom has one shim, that might give the impression that equal lengths could not sit flush on those shims but nothing about the geometry has changed, only the down and right angles have.

What else has changed is the legs do not sit flush on the mount because they are at an angle and forcing the difference, washers, into the mount is not the best way to make them flush but it will do the trick also, easy enough to do if your using bolts screwed into the stand offs, they will also be at the same thrust angels, In the case of blind nuts they should be installed at the same angles also to take the side load off the through bolts if used. Shims can be made from aircraft ply at the same angles the thrust angles are.

Leroy G
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Old Oct 01, 2015, 04:51 AM
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Quote:
Originally Posted by Leroy G View Post
Lets say you add shims at the top to give you the 3 deg. down thrust, you will still have an equal plane for the engine to set on, you can test this with a simple mock up to set the mounts on. Now you need some right thrust add a washer or two to both mounts on right side and set your engine on them, all 4 legs should sit flat at this point, if it doesn't there may be a twist in the fire wall , if that's the case, hold the two left stand offs firmly down and use a feeler gauge to find any difference in the right stand offs and shim it out. you can also put the 4 legs against the fire wall and see if it rocks any at all and make sure the washers are the same thickness.

Different prop lengths will mandate the need of varying off sets, some planes need it others don't. you can trim out down thrust easier than right thrust. A correct incidence of the stabilizer and alignment of vertical fin is also something to look at if it requires excessive trimming.
There is a tapered shim kit out there to do this, I can't remember where to get it, it allows a flatter surface to mount to taking the stress off the stand offs and engine ears.

Leroy
Quote:
Originally Posted by ace4rc View Post
You are basically right on the stand-offs being uneven, but, the angles and distance is so small one would never know it. One can also add the fact that the firewall is wood and will absorb this very minute difference, hence, one will never know it.
Quote:
Originally Posted by exf3bguy View Post
I agree with Ace. Ive never hax the taper be an issue. One thing I will add though is that thrust angles is very much the same as CG in regards that I dont spend a bunch of time doing calculations based on the fact that it will need to be changed after some test flights.
Quote:
Originally Posted by Leroy G View Post
grosbeak this is a good subject and I'm glad you brought it up, others have had problems with this very thing.

For illustration purposes I did a case of all things being equal in this case. First I made some spacers you can see, they are 1/32' ply, two each on the top stand offs for down thrust, all legs sit firmly on the mount. Then I put one more on the top right stand off making it 3 and one on the lower right for right thrust and all legs sit firmly on the mount, none are longer or shorter because all things are equal.

# 1 top left has 2 shims, #two top right has 3 shims # 3 bottom has no shims and # 4 bottom has one shim, that might give the impression that equal lengths could not sit flush on those shims but nothing about the geometry has changed, only the down and right angles have.

What else has changed is the legs do not sit flush on the mount because they are at an angle and forcing the difference, washers, into the mount is not the best way to make them flush but it will do the trick also, easy enough to do if your using bolts screwed into the stand offs, they will also be at the same thrust angels, In the case of blind nuts they should be installed at the same angles also to take the side load off the through bolts if used. Shims can be made from aircraft ply at the same angles the thrust angles are.

Leroy G
Fellas, thanks for the feedback. Leroy, thanks especially for the pictures. I understand that the thrust angle may have to change after the maiden... I'm just trying to get it to match the specifications first.
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Old Oct 02, 2015, 03:21 PM
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My first kick at the can will be to mill a tapered plate - I have added 1/8" to the plate to push the engine out to where it needs to be. Here are a few drawings.

Top view




Port view




Starboard view




Offset position

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Old Oct 02, 2015, 04:14 PM
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The wedge is a great idea. I did pretty much the same with my Extra. Another thing I did was to cut 4 1" circles with a hole saw out of 1/4" ply and tap the blind nuts into them. Then I sanded a slight angle in the ply then epoxied them to the back of the firewall.
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Old Oct 02, 2015, 09:37 PM
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Quote:
Originally Posted by exf3bguy View Post
The wedge is a great idea. I did pretty much the same with my Extra. Another thing I did was to cut 4 1" circles with a hole saw out of 1/4" ply and tap the blind nuts into them. Then I sanded a slight angle in the ply then epoxied them to the back of the firewall.
Great idea!
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