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Author Topic: Stabilizer Location Question  (Read 2561 times)

Offline Harleyman

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Stabilizer Location Question
« on: March 01, 2006, 10:49:55 AM »
Is there a rule of thumb for Horizontal Stabilizer location related to the wing centerline?
I am building my own profile, semi scale stunt T-38 Talon, .35-.40 size.  The full scale Jet has a very low stabilizer location; it’s pretty much mounted on the underside of the Fuse.  I have only seen and built airplanes with the stab mounted higher than the enterline of the wing.  Is this done for pushrod clearance issues?  Can the stab be mounted in line with the wing?  How does engine Thrust line influence stabilizer position?  Also, please assume typical nose and tail moments, similar to Nobler numbers.

Thanks.
« Last Edit: March 01, 2006, 04:05:08 PM by Harleyman »
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Offline RC Storick

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Re: Stabilizer Location Question
« Reply #1 on: March 01, 2006, 05:42:44 PM »
I will try a "stab" at your question. The stab will work in any relation to the thrust line or center line of the wing. However if you loft out the push rod you should try to keep it parallel to the TL. The reason for this is the mechanical advantage or swing of the control horns cause UN equal throw in one direction or the other. Hence it would cause unsurmountable trimming problems.
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Offline Ironbomb

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Re: Stabilizer Location Question
« Reply #2 on: March 02, 2006, 10:26:35 AM »
Would the same trimming issues be present if the elevator horn were above the surface, and the flap horn below the flap surface? Could the difference in throw be canceled like that?

The elevator pushrod could then be kept parallel to the thrust line, keeping a T-38 stab/elevator configuration more to a scale appearance. Yes?

Just wondering

 ???

Greg
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Offline Bill Little

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Re: Stabilizer Location Question
« Reply #3 on: March 02, 2006, 10:50:04 AM »
Hi Greg,
There have been a few ships set up with the horns as you describe.  Vic Macaluso did 2 versions of the F-8 Crusader with the elev. horn on top.  In fact, you will not need to reverse the bellcrank to get the up line in front.   Both flew great!
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Offline Ted Fancher

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Re: Stabilizer Location Question
« Reply #4 on: March 02, 2006, 05:35:16 PM »
Besides the mechanical and alignment issues already addressed, you do need to be aware of the effect of a very high or low location and the effect on the pitch axis caused by the drag of the surface.  The position of the stab will effect the vertical location of the center of drag of the ship and, when producing lift for maneuvering that drag will increase dramatically.  Most successful ships that have been designed with "high" or "low" tails have been quite modest in the degree of vertical offset from the wing and centerline of the fuse.

Ted

Offline Bill Little

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Re: Stabilizer Location Question
« Reply #5 on: March 03, 2006, 06:54:16 AM »
Besides the mechanical and alignment issues already addressed, you do need to be aware of the effect of a very high or low location and the effect on the pitch axis caused by the drag of the surface.  The position of the stab will effect the vertical location of the center of drag of the ship and, when producing lift for maneuvering that drag will increase dramatically.  Most successful ships that have been designed with "high" or "low" tails have been quite modest in the degree of vertical offset from the wing and centerline of the fuse.

Ted

Hi Ted,

As a "neophyte" when it comes to stunt ship design (I just build others planes!0 I have become aware, however, of the vertical CG effect on our planes.

When looking at the pic of the real Talon, I see a lot of mass above the wing AND stab.  On a low wing plane we get "rolling" iin the manuevers if the V-CG gets too high, what about the stab?  What effects would show up?

Thanks,
Bill <><
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Offline Rick Campbell

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Re: Stabilizer Location Question
« Reply #6 on: March 03, 2006, 06:27:47 PM »
Harleyman:

Have you looked at Dave Reese'  T38 stunter?  I think it was in FM sometime back in to 70's. Preceeded his unusual Dazzler design.

RC

Offline Harleyman

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Re: Stabilizer Location Question
« Reply #7 on: March 04, 2006, 12:25:56 PM »
Yes I have that issue.  Besides the beautiful lines of the full scale jet, that article was the inspiration for my profile.  I just have not seen many Stabilizers mounted below the wing centerline.
« Last Edit: March 17, 2006, 08:46:44 AM by Harleyman »
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Offline Ted Fancher

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Re: Stabilizer Location Question
« Reply #8 on: March 06, 2006, 02:08:26 PM »
Hi Ted,

As a "neophyte" when it comes to stunt ship design (I just build others planes!0 I have become aware, however, of the vertical CG effect on our planes.

When looking at the pic of the real Talon, I see a lot of mass above the wing AND stab.  On a low wing plane we get "rolling" iin the manuevers if the V-CG gets too high, what about the stab?  What effects would show up?

Thanks,
Bill <><

It's really pretty simple, Bill.  Let me use one of my favorite techniques ... envisioning the effect in the extreme.

Let's assume a more or less standard 60" span stunter with a very high T-tail six inches above the thrustline.

You're flying along straight and level and while passing under a "limbo stick" (bear with me here, we're talking theoretical examples) you're just a touch too high and the tall stab hits the stick.  What's going to happen (assuming everything stays in one piece)?

Clearly, hitting that stick is a big "drag" on the direction of flight and the ship is going to pitch up in response to that drag well above it's CG.

While this example is extreme it is important to remember that "any" amount of drag at that raised location is going to try to pitch the airplane nose up commensurate with the amount of drag and the deviation of that drag from the "aerodynamic centerline" of the airplane.

In level flight you would simply trim it out with some downthrust and/or a little down elevator or even build in a little extra negative incidence.

Where it might prove to be a problem will be when maneuvering.  As the tail produces the lift required to change the attitude of the ship it will also be producing a lot more drag (induced drag from the production of the required lift) than when in level flight.  (Granted, in some proportion to the overall induced drag increase from the wing).  That increased drag at the tail's location will act sort of like hitting the limbo stick.  It is going to accelerate inside maneuvers but will resist outsides.  It is likely going to result in uneven inside versus outside response rates.  The difference may or may not be something that can effectively be trimmed out.

Ted

Offline Bill Little

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Re: Stabilizer Location Question
« Reply #9 on: March 08, 2006, 07:42:04 PM »
Thanks, Ted, I can understand that explanation.  Not "too" technical! ;)
So if the stab was too "low", we get the inverse effect, right?
Thanks, again.
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Offline ash

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Re: Stabilizer Location Question
« Reply #10 on: March 10, 2006, 01:47:14 AM »
And a simple way to apply Ted's explanation:

The engine thrustline is your datum, or centreline for the side view. You want to balance the drag above the thrustline with the drag below the thrustline.

If the wing is below, put the tail above the line. If the wing is above, put the tail below. Or you can put them all in a straight line as Owen Rogers' Pizzaz and a number of current European designs do.

If you want to add a twist, you also have two to four other notable drag sources to look at:
Above you have a fin/rudder and sometimes a prominent canopy.
Below you have the undercarriage (and sometimes cowlings and air intakes are big enough to matter)

These things usually balance out just fine as they usually are. But if you want a flush jet canopy and a small fin, you might want to reconsider unless you can lessen the undercarriage drag to keep things balanced.
Adrian Hamilton - Auckland, NZ.

Offline Bill Little

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Re: Stabilizer Location Question
« Reply #11 on: March 10, 2006, 10:40:08 PM »
Hi Adrian,
I have flown two "inline" planes so far over the last ten years.  My "little" Junar, and my son's Geo (bolt) XL.  Both by Billy Werwage, and they are interesting as to their characteristics.  Equal turn rates took very little to no trimming.  Some bad air cost me my Junar, and it needs a new fuselage.  I really liked it, and need to get it rebuilt!
Strange, but "inline" never has seemed to catch on in the US as much as it has in other parts of the world.
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Offline Harleyman

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Re: Stabilizer Location Question
« Reply #12 on: March 11, 2006, 08:46:49 AM »
OK so it should be fine placed above, below, in-line as long as it's not too extream, keeping in mind an overall upper and lower balance. 

I have decided to mount the stab low because I have the engine thrustline about a 3/8" lower than the wing centerline. 

Puzzle: I have the Flap pushrod exiting the top of the wing.  and the elevator horn is mounted on the topside.  Instead of fabricating a very contorted pushrod; I'm thinking of adding a second horn to the bottom of the wing flap to connect to the top mounted Elevator horn, to get the correct "direction of deflection". 

Has anybody tried this setup?  It would be cool if I could find a double elevator horn, one for top one for bottom.
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Offline Ted Fancher

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Re: Stabilizer Location Question
« Reply #13 on: March 25, 2006, 09:34:20 AM »
OK so it should be fine placed above, below, in-line as long as it's not too extream, keeping in mind an overall upper and lower balance. 

I have decided to mount the stab low because I have the engine thrustline about a 3/8" lower than the wing centerline. 

Puzzle: I have the Flap pushrod exiting the top of the wing.  and the elevator horn is mounted on the topside.  Instead of fabricating a very contorted pushrod; I'm thinking of adding a second horn to the bottom of the wing flap to connect to the top mounted Elevator horn, to get the correct "direction of deflection". 

Has anybody tried this setup?  It would be cool if I could find a double elevator horn, one for top one for bottom.

Harleyman,  No reason that shouldn't work just fine.  The important aspect about control alignment in unusual configurations like this is that you need to bias the angle of the horns so that the relationship of the pushrod to the horns axis of rotation is 90 degrees at neutral.  This is to prevent control deflection to be more rapid for a given amount of handle movement in one direction or the other.

Plans for several ships including my Trivial Pursuit design show modest versions of such horns with one set of holes on the flap horn angled toward the Bellcrank to obtain that 90 degree relationship and a second set of holes angled aft to allow the flap to elevator pushrod to achieve a 90 degree relationship.  Finally, the elevator horn is slanted slight forward for th same reason.  The greater the angle the pushrod must be placed at neutral to attack to the horns the greater the offset must be ... and the more important it becomes that you make the effort to avoid disparate control inputs up vice down.

Ted

Online Paul Smith

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Re: Stabilizer Location Question
« Reply #14 on: July 13, 2006, 05:02:45 AM »
Take a look at some taildraggers at local airport.

The stab is elevated so the flippers don't hit the ground on full down elevator.  No other reason.

Imagine a Piper Cub or Citabria with the tailplane on centreline. The flippers would be 2 feet into the runway.

Note some real jets with tricycle gears.  Many had the tails on or below the centreline.

Simple mechanical clearance issues, not mystical aerodynamics.
Paul Smith


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