Horizontal Thrust line

[nickrage]

Where is this on planes. let's use a wing on top scenario with a cub like body. If you need up or down thrust what and where is the line u use for reference.?/?

[Jim Walker]

I'm pretty sure up or down thrust is in relation to the main lifting surface which is the wing. So draw a line on the plans paralell with the wing's camber line through the motor. This will tell you the incidence of the horizontal stablizer and the degrees of up or down thrust.

[DLC]

nickrage
On the type of plane you describe, the wing is typically set at a positive angle(incidence) to the zero reference line. So if you align the motor center line with the wing, you will have up thust. If you have a plan for the plane, it will probably show the zero reference line. If not, I would suggest using a line parallel to the horizontal tail as the reference line, since the horizontal tail usually is at zero incidence.

[Jim Walker]

Hmmm.... First a little background.

The average model airplane's horizontal stabilizer is a flat plate and provides no lift other than what it generates by it's angle of attack or "incidence". The primary lifting surface, the wing, generates it's lift by pressure differential (bernouli principle), and forcing air downward. The second lifting action is the one we are interested in for this discussion. When a wing creates lift by redirecting air downward it also generates a foward rotational force. Now if this rotational force was constant we could balance the plane to compensate for it. Unfortunately as the angle of attack and the airspeed changes for the wing, so does it's center of pressure, how much lift it's generating, and finally the rotational force on the wing. So plane designers came up with a horizontal stabilizer on a stick (fuselage) out behind the wing. Then they put some negative incidence in the horizontal stab in relation to the wing. This made the horizontal stab push downward out behind the wing and compensate for that annoying forward rotation. Finally they put a movable surface behind the horizontal stab called an elevator to make adjustments during flight. This was needed for attitude control and to make fine adjustments to the aerodynamic balance of the plane in response to the constantly moving center of pressure on the wing.

Now we can talk about incidence and thrust lines. Unless the wing is fully symmetrical, the horizontal stab will never be at zero incidence. A symmetrical section doesn't generate any rotational force and so needs no incidence in the horizontal stab. Both surfaces will have zero incidence in relation to each other. However, the more camber a wing has, the more lift it generates, the more rotational force it generates, and finally the more incidence the horizontal stab needs to balance the airplane.

So let's say we have a nicely balanced plane flying straight and level and we slap a motor on the front of it. We just changed the aerodynamic balance of the plane. The prop blast moves the center of pressure on the wing just like airspeed and angle of attack. The faster the prop moves air over the wing, the more lift the wing generates, the more rotational force is created, (does this sound familiar?), and so finally the more something has to compensate. You could just hold in ever increasing amounts of down elevator as you advance the throttle. Then release some of the held in elevator input as you back off the throttle. But designers realized really quickly that it's much easier to just point the engine down in relation to the wing. This brings the airplane back to an aerodynamic balance again even with the engine changing things. The stronger prop blast that creates more rotational force is balanced now by the motor pulling the nose down. The faster the motor moves air over the wing, the harder it pulls down and you don't have to fiddle with the elevator anymore. You get straight and level no matter what the throttle setting.

I'm sure there's lots of formulas out there for true aerodynamicists to figure this out before their creations ever take to the air for the first time. For most modelers it's a trial and error process through several prototypes until that great flying kit reaches your workbench. There are rules of thumb to start with like this one. For flat bottomed airfoils you usually need around 3 degrees of down thrust. This three degrees is calculated by drawing a line through the motor paralell to the wing's camber line as I stated in my first post. Now draw a line through the engine paralell to it's output shaft. Compare the lines with a protractor and you have the degrees of up or down thrust. Remember though, getting a plane aerodynamically balanced is a combination of several factors. They are.....

Static weight balance
Nose and tail moments
Thrust line
Stabilizer incidence

Hope this helps,

Enjoy!!

[DLC]

I suspect this discussion could go on and on because unless standard conventions were set up during the many years I was out of the hobby (but not out of aerodynamics), the zero reference line is somewhat arbitrary. Perhaps a good answer to the question is that if you look at the side view with the nose facing left, a counter clockwise rotation of the motor will provide down thrust and a clockwise rotation will provide up thrust.

[steve lewin]

Jim, unless I have my terminology all wrong I think you probably mean either the airfoil's chord line or zero lift line rather than camber line. Zero lift line, i.e. chord line at zero lift, would make most sense and by a bit of good luck happens to closely equate to the bottom of many so-called flat bottom foils like the Clark Y series.

The camber line for anything other than pure symmetrical airfoil is a curve. It's hard drawing a thrust line parallel to a curve.

Steve

[Jim Walker]

You got me Steve, I meant the straight line you get if you draw a line from the point of the trailing edge to the place on the leading edge that's farthest forward. Thanks for correcting me and clearing up any confusion it may have caused.


DLC, you're exactly right and I almost put that in my post. The reference line can be where ever the guy who draws up the plans wants to put it. The relationships won't change in relation to each other, just to the reference line. This is the exact reason I went through that long discussion about incidence and such. It took me a long time to figure all that stuff out because the reference line is arbitrary as you said. What's really important is the relationship of everything on the plane in reference to the wing's position at zero angle of attack. I just wanted to save someone all the heartache I went through..

Jim

[feihu]

nickrage:

You don't really need a horizontal reference "thrust" line. All you need is the location of the engine and the "vertical" CG. Adjust the engine thrust line up or down as required so that the line passes thru or slightly above the vertical CG. This will minimize the pitching moment caused by power changes which is the main purpose for adjusting the thrust line.

feihu :)

[Jim Walker]

What's the "vertical CG" and how do you find it? I've never heard that term before.

Thanks, Jim

[Sparky Paul]

What's the "vertical CG" and how do you find it? I've never heard that term before.

Thanks, Jim
.
Everyone has one... :)
It's right here!!!
This is THE c.g. position both longitudinally and vertically.