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Old Jun 06, 2012, 08:11 PM
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Rusty-Gunn's Avatar
Kotzebue, Alaska
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Down/right thrust... how do you measure the exact degrees?

I know we should add down and right thrust to the engine as used on a high wing plane. I hear some people say, for example, add three degrees down, and two degrees right thrust angle. How do you know you got the exact amount?

(To clarify ahead of time, I've always just guessed, using the TLAR method on my foamies. I have no idea how to make the measurements precise. Sometimes I got it good enough to fly right, sometimes I didn't.)

I've designed a trainer type plane, and got the firewall set squared up all round, and figure I'll add the down and right trust via adding washers to the engine mount, with the possibility of using wood shims if necessary. Yet, I don't know of the exact degrees are met.

How do you know if you got it right?
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Old Jun 06, 2012, 09:52 PM
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1/16" 6" from the firewall is one degree, or near as dammit.

howell
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Old Jun 06, 2012, 09:56 PM
Zor
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Rusty-Gunn

The right thrust angle and downthrust angle is best adjusted in dynamic conditions while flying.

You know it is correct when you can vary the power setting without major change in pitch and heading (yaw).

If you wish to measure it for curiosity of what it ends up to be an easy way is to split a straight drinking straw length wise and tape it to the engine (motor) shaft and measure the angles it makes with the lhe longitudinal axis of the fuselage.

Zor
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Old Jun 07, 2012, 03:15 AM
My plans are in my blog
Rusty-Gunn's Avatar
Kotzebue, Alaska
Joined May 2006
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Howell... I figure I might be able to do this idea, even though my engine ends at five engines.

Zor... So, maybe I should choose the angles, then fly the plane to see how it reacts, then make changes until I get it correct. This could take many flights, I'd think. I used to do this type of guessing when building foamies.
I was hoping for some sort of fancy trick, or tool.

Thank you guys for responding.
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Old Jun 07, 2012, 06:16 AM
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Usually flat bottom wing planes will have a bit of down thrust to counter the high lift of the wing. Right thrust you will have to fly and find out if it needs any. If it is a flat bottom wing I would set 1 to 2 degrees down and 0 right and adjust as necessary
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Old Jun 07, 2012, 10:00 AM
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Minneapolis
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Quote:
Originally Posted by Zor View Post
The right thrust angle and downthrust angle is best adjusted in dynamic conditions while flying.

Zor
Is this saying you are better off with the engine mounted true (zero down, zero right), and mixing a little elevator and rudder to your throttle via electronic mixing?

--Jeremy
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Old Jun 07, 2012, 10:33 AM
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Assuming the firewall is straight and perpendicular to the thrustline. You can then use a protractor to measure the engine offset.
Some planes came with a offset firewall that has the thrust angle and downthrust built into it too.

To really verify it, you need a use a straight edge and establish a reference straight line through the fuselage of the plane. Then you can use the protractor to verify the angles based on that reference line. Now it can get ugly as the vertical stab and horizontal stab make for a good reference from the rear of the plane, providing they were glued on straight too. Otherwise you have to use the rear tip or edge of the fuselage and create a reference line forward. If you haven't completely put the plane together you could hang the plane tail up and use a string on the inside, with a plumb bob weight on the end and use that to create your reference lines to base everything on.
I hope it makes sense.

Some planes actually handle better in the air with a little right and down thrust, it depends on the design though. Some planes need a lot and other planes do well without any too. Basically a little down thrust removes the need for some up elevator trim. A little right thrust counteracts the engine and prop torque reaction (why planes fly better making left turns than right turns) which can affect certain aerobatic maneuvers. Free flight planes tend to have a lot more right and down thrust in them. Flat bottom airfoil wing planes tend to have a fair amount of right and down thrust built in to help aleviate the need for lots of rudder and elevator trim. Fast planes like low wing aerobatic or sport planes or pylon racing planes tend to have no right and down thrust in them, but the plane design tends to allow for that.
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Old Jun 07, 2012, 12:38 PM
Zor
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Originally Posted by jrbackus View Post
Is this saying you are better off with the engine mounted true (zero down, zero right), and mixing a little elevator and rudder to your throttle via electronic mixing?

--Jeremy
--Jeremy,

I thought I was pretty clear in my previous posting.

Left / right thrust and up / down thrust are effective in flight; obviously not on the ground .

We normally want our model to fly straight and level at a specific power setting and with the elev and rudder being neutral. Neutral meaning in line with the stab and fin.

The CG location is also entering into the setup.

If no side or vertical angle is used then any tendency of the model to veer away from straight and level can be partly compensated with using side and verical thrust angles.

It can also reduce any required trim with changes in power settings.

So to find the angles needed can only be done while flying.
Of course you do not make the changes while flying. You fly to find out in which direction the angles are needed and how much.

Some mention of accuracy was written in previous postings. These side or vertical angles are a compromise and can only be accurate at only one specific speed. That speed will vary with the angles.

Experimentation is the only way for a flier to find what he likes best in terms of his model behavior and his style of flying.

Aerobatic models that are constantly controlled and that are expected to fly upside down and on edge as well as right side up do not use any thrust angles. The angles remain at zero.

We have to understand and visualize all the aerodynamic forces on the model to fully understand the necessity or the non-necessity of these side and vertical thrust angles.

I cannot go into all these details in a forum posting; sorry for that.

Zor
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Old Jun 07, 2012, 04:00 PM
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Thank you, Earl. Your post made sense to me.
Thank you all for the help.
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Old Jun 07, 2012, 04:28 PM
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Expanding a bit on earlwb's theory,I set the airplane up on it's gear,and turn the prop horizontal....measure from one prop tip to a spot on the rudder top,then the other tip.Right thrust will always have the left tip ahead of the right one,for a starting point.
Put the prop vertical to check downthrust-should be farther to the upper tip.

Washers work as well as anything for spacers.
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Old Jun 07, 2012, 09:10 PM
Zor
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Originally Posted by epoxyearl View Post
Expanding a bit on earlwb's theory,I set the airplane up on it's gear,and turn the prop horizontal....measure from one prop tip to a spot on the rudder top,then the other tip.Right thrust will always have the left tip ahead of the right one,for a starting point.
Put the prop vertical to check downthrust-should be farther to the upper tip.

Washers work as well as anything for spacers.
With this method the vertical angle (up or down thrust line) measured will vary depending where the spot on the rudder (or fin) is picked up.

It will work ok for the side thrust because the fin/rudder and the engine shaft are both on the vertical geometrical plane in which the longitudinal axis also exist.

Zor
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Old Jun 08, 2012, 02:24 AM
My plans are in my blog
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Over the years I see a lot of postings of people saying something like, "I go with two degrees down thrust and right thrust."
But how do they know it's two degrees, if they say it? Most likely they don't. They're just repeating what every body else says, I'm thinking.
I think it might be nice if a engine mount maker made them with built in thrust angles for various series of planes ie high wing, low wing, etc.
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Old Jun 08, 2012, 06:31 AM
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Well, many planes already come with the amount of down and right thrust built into the firewall. They angle the fire wall for it. The better more expensive ARF planes sometimes have markings on the firewall showing where the center thrust line is at, and it is offset from the actual firewall center.
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Old Jun 08, 2012, 10:21 AM
Zor
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Not practical nor economical

Quote:
Originally Posted by Rusty-Gunn View Post
Over the years I see a lot of postings of people saying something like, "I go with two degrees down thrust and right thrust."
But how do they know it's two degrees, if they say it? Most likely they don't. They're just repeating what every body else says, I'm thinking.
I think it might be nice if a engine mount maker made them with built in thrust angles for various series of planes ie high wing, low wing, etc.
The engine mount is the piece to which the engine is bolted (screwed) on. Then the engine mount is bolted to the firewall.

Just imagine engine mounts made for side and vertical thrust for every model on the market.

High wing monoplanes do not all need the same offsets.
Same for low wings or mid wings models.

It is not practical.
How could an engine mount manufacturer predict the quantity of each model being sold or built ?

The present practice is the most practical and allows the pilot to setup his model according to his style of flying.

Zor
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Old Jun 08, 2012, 10:32 AM
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As with most things,a generalisation will do.If folks would say"around 2deg. down and 2 deg.right thrust",the pressure would be removed from a relatively simple matter.
Logic tells us that a high wing airplane has a high drag concentration above the thrust line.This the effect of "dragging" the nose up.So we start at about two degrees down thrust,which theoretically,should help counteract the drag.
To test that,fly full throttle,level flight,trimmed ,and close the throttle quickly.Properly adjusted the airplane will continue for a distance,neither climbing,or immediately diving.Educate your self,after the test,add two washers at the place which will add more downthrust,and do it again. I.m betting you won't see a marked difference......a little but not something that'll crash your airplane.-For those of us to whom it's important,that's important...to those who won't try it,, your airplane will still fly well.

Right thrust is another ball game.The way most aircraft are designed,when the engine develops horse power,the airplane wants to turn left.So add right rudder.or if you add right thrust,it's like power steering..the harder the engine works,the more the left turn is cancelled,be cause the engine is now pulling to the right.
So the model test is to trim straight and level,carefully go into a vertical climb,and see which way the line goes,all this at full throttle.If it goes in a straight line,you're good,unless you had right rudder trim originally....then you'll have to back that off since it won't be needed,then readjust the right thrust to compensate.See how you have to chase things around?most guys don't bother!
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