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Old Aug 10, 2006, 08:25 PM
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Mounting the engine at an angle?

Okay i have been looking around at another plane but i keep reading how people have to angle 2 degrees here 5 degrees there, what that all about?
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Old Aug 10, 2006, 08:50 PM
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Those angles refer to what amount of downthrust or sidethrust the particular airplane design might need.
Usually if any is, it's automatically established in the assembly/building process.
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Old Aug 10, 2006, 09:42 PM
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Some more clarification ....

In a good kit, when you assemble the fuse, you can see a cross on the front of the firewall. This is where you line up the center of the mounting rails of the engine mounts.

If the firewall is built "at some angle", then the engine off set thrustlines are built in.
Sometimes the cross lines are also slightly offset from the center.

The reason for this is that offset of the prop has to be on the centre of the thrustline, so the engine mounts a bit off the centerline.

Note that Sparky said : "downthrust or sidethrust the particular airplane design might need". Not all plane designs require this configuration.
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Old Aug 11, 2006, 08:24 AM
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Most ARFs have the angles built into the firewall. You just bolt on the motor mount where it's supposed to be bolted, and the mount points where it should.

But as with all ARFs, it pays to verify that they did actually do what the manual says, and that they did it right. And sometimes the manual is wrong and the model is right.

The Phoenix Sukhoi is a perfect (did I say, "perfect" ?) example of ARF's at their best (did I say, "best"). The manual says that there is no down or right thrust. The firewall on the airplane had both. The manual shows where to mount the motor mount. The location was appropriate for UP thrust and right thrust. The markings on the airplane were equally wrong. It makes you wonder if anyone over there actually flys their products.
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Old Aug 11, 2006, 09:15 AM
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Actually I do fly a Phoenix Sukhoi Su-31 and it's a nice flying plane (when powered and setup properly). I'll have to check the manual again, but as far as I remember I didn't have any trouble installing the engine. To be honest, I didn't pay much attention to the manual when I was assembling it - it is not a beginner's ARF anyway.

Ivan
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Old Aug 11, 2006, 01:34 PM
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soooo..... how do you know when you have to mount them at an angle, and how do you make it so percise?

Like when you look at the plane from the front, should the prop be inline or pointed down and right (or what ever the manual specifies)

sorry i am just not really understanding this, and i just want to know for when the time comes on my next model if it req.'s it
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Old Aug 11, 2006, 02:20 PM
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The only thing you can do at this point is build and fly the plane.
If you have the flight experience to note that the engine needs to be offset, it will make itself known.
Right now, it's more information than you need.
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Old Aug 11, 2006, 06:29 PM
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right
If you're assembling an ARF, or building a trainer, don't worry about it.

However, to answer a couple of your questions.............
Plans and kits are usually created after an original model proves to be a good flyer. Most have been sweated over and an effort made to adjust them to fly good. Most plans and kits will have the correct amounts in them. ARFs sometimes are done the same. But don't bet on it.

How exact does it have to be? Unfortunately, each model is going to be enough different than others of it's kind that having exact thrusts actually isn't so great. But the good news is that the thrusts don't work exact anyway.

There have been entire magazine articles on each of those thrusts. So the best answer about what they do is to suggest that you do a bit of searching for the info. It's out there. And it ain't simple.

How do you actually work it out when you think you need it? Shim the engine where it bolts to "permanent" engine mounts or shim bolt-on motor mounts. That is simple, at least. All the rest isn't.
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Old Aug 11, 2006, 09:43 PM
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Even full scale single engine prop planes (front engine) will have thrust adjustment angles built in. The reason for right thrust is that the rotating mass of the prop has gyroscopic precession, a form of torque, meaning it tries to direct the plane to the left, with tendencies to roll left also. (Full scale light planes typically require some right rudder input on takeoff for the same reason). Therefore, the thrust line is offset to the right a few degrees. How much depends on the prop and the airframe itself. The reason for downthrust is that planes generally fly better when the airflow from the prop is directed slightly upward to get more air flowing across the chord of the wing, and therefore, more lift is generated.

The two full scale planes I have owned had offset thrust. Every kit I've built and the one ARF I own all have these thrust offsets.

I found out what happens when a RC plane has no offset thrust angles. The first RC plane I built was my own design, a high wing tricycle geared thing. I was sooo careful to make sure I had the engine mount perfectly straight ahead. On takeoff it rolled left, inverted, and proceeded to do an impression of a skipping hand grenade while I enthusiastically gave it all the "up" elevator I could. Very impressive, but not conducive to a second flight.
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Old Aug 11, 2006, 10:34 PM
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That wasn't "thrust offset, lack of", that was "tip stall".
Usually due to a combination of taking off too slowly, with too much elevator and an aft c.g.
Thrust problems are much subtler.
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Old Aug 12, 2006, 09:19 AM
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Quote:
Originally Posted by Sparky Paul
Thrust problems are much subtler.
Subtle is the description to use and then some.

When I built from scratch, kit bashing or my own completely new design, I built everything "at zero degrees", from the tail surfaces as the reference points.
Then fly her and if she can be reasonbly trimmed out for proper flight characteristics (I really do not have any planes that I use for "competition", I fly purely for my own amazement, and NOT for your amusement) then I am done.
But there have been cases where I needed to do something because for one reason or another she was just not doing what I wanted.
Then I go through a step by step programme that shows me which way I have to apply some engine mounting adjustments.

For Side Thrust check, I do the following:
Fly her straight into the wind at full throttle and pull up vertical.
If she:
1 - continues to climb vertically, the side thrust is correct.
2 - veers to the right, the engine has too much right thrust.
3 - veers to the left, the engine needs a little right thrust.

I have to do this over and over again until I finally "think" she is OK.

For Down Thrust check, I do the following:
Fly her straight into the wind, full throttle, level flight. Quickly close throttle. Open the throttle fully.
If she:
1 - after she slowly noses down (speed slows down first), then (throttle open, speed increases) she continues downwards, straight, the down thrust is correct.
2 - after she slowly noses down (speed slows down first), then (throttle open, speed increases) she continues downwards, more nose down, the engine has too much down thrust.
3 - after she slowly noses down (speed slows down first), then (throttle open, speed increases) she goes nose up, the engine needs more down thrust.

It takes me a long time to do this since I am not sure how many shims to add. After I "think" everything is OK, then I measure and determine if I need to offset the engine horizontally or vertically from the original thrustline.

LBNL, after all this, I wonder if it was worth it ....
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Old Aug 12, 2006, 10:07 AM
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Quote:
Originally Posted by indoruwet
Subtle is the description to use and then some.

...
LBNL, after all this, I wonder if it was worth it ....
.
If you're going to compete in Masters Pattern, it's mandatory!
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Old Aug 12, 2006, 12:45 PM
Flying RC since 1974
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Quote:
Originally Posted by CanDyRaVeR
soooo..... how do you know when you have to mount them at an angle, and how do you make it so percise?

Like when you look at the plane from the front, should the prop be inline or pointed down and right (or what ever the manual specifies)

sorry i am just not really understanding this, and i just want to know for when the time comes on my next model if it req.'s it
For sport flying, you are usually pretty safe with no side thrust.

If the plane points it's nose skyward when you goose the throttle, you need a bit of down thrust which you add by putting a couple of washers on the top firewall mounting bolts between the firewall and the mount.
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Old Aug 12, 2006, 03:22 PM
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Quote:
Originally Posted by Sparky Paul
.
If you're going to compete in Masters Pattern, it's mandatory!
You can say that again !!
Nah, don't say that again .....
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Old Aug 12, 2006, 08:02 PM
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thank you all for explaining this, i was very confused, now it seems very clear.

I like your system indoruwet, makes sense...

again thank you very very much, i will definately save this thread for future useage... this'll be a great refrence for anyone else that asks
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