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Apr 22, 2019, 07:40 PM
It's time for me to fly
JimZinVT's Avatar
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Discussion

Wing to Stab incidence?


I'm mainly a slope flyer that occasionally throws around a DLG (and I don't do either as often or as well as I'd like). The elders of the slope forum state that the wing-to-horizontal stab incidence (or decalage?) needs to be zero or the glider will fly poorly. They are measuring in relation to the airfoil's chord line, not the zero lift angle so maybe that's not really accurate but close enough?
But on the DLG forum a year or two ago there was a thread that rather angrily shot down the zero decalage "myth". I didn't follow the thread too carefully after it quicky broke down into name calling and insults, so I can't comment on the explanation.
I've scratch built a few DLGs and slopers and have set the decalage to zero (mainly out of laziness) and they flew fine. Wondering what you Science Guys have to say about it?
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Apr 22, 2019, 08:46 PM
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There are two forces (or more accurately,moments) that need to cancel each other out.

For stability, center of gravity needs to be in front of the center of lift. How much, depends on plane's mission and pilot's personal preference. It is called static margin. This generates a nose-down moment.

To prevent a nose dive, the stab/elevator needs to generate a nose-up moment, and to do that it needs either some angle of attack (relative to airflow, not to be confused with decalage) or airfoil camber (in practical terms: elevator trim). How much is needed, depends on the static margin.

Most people seem to prefer that the elevator and stab are lined up (zero camber for the stab/elevator combo) while in level flight, and attempt to set a decalage to achieve this. Of course, the actual rigging angles depend on CofG, wing profile (its pitching moment, Cl/alpha, etc), stab profile and probably a bunch of other factors I'm not even aware of, and even then it would be correct at only one airspeed.

Tl:dr: Don't stress about it. Just set the decalage somewhere within half a dozen degrees of what looks about right and use elevator trim for fine tuning when you see how it behaves in flight.
Apr 22, 2019, 10:24 PM
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Quote:
Originally Posted by jkettu
For stability, center of gravity needs to be in front of the center of lift.
Actually, for stability the center of gravity needs to be in front of the "aerodynamic center" (AC). This is not the same as the "center of pressure" or "center of lift". Instead, think of it as the point where a CHANGE in lift appears in response to a CHANGE in angle of attack.

For an aft tail configuration, the AC of the aircraft will be somewhere between the AC's of the main wing and horizontal tail. These are (to a very good approximation) located at 25% of the main wing and horizontal tail "mean aerodynamic chord" (MAC). Which means the AC of the airplane will be behind the quarter chord of the main wing - how far behind depends mostly on the horizontal tail area and moment arm.

So it's possible to have a stable aircraft with an upload on the horizontal tail - provided the main wing camber (which causes a nose-down moment) isn't excessive and the static margin isn't too large.
Apr 23, 2019, 07:38 AM
Red Merle ALES VII SJ
Curtis Suter's Avatar
If balanced the same as in the same forward/aft CG location the only difference in decalage would be the angle the fuselage flies. A rough rule of thumb is to use a main wing leading edge up angle in degrees equal to the amount of camber the main wing has. I would surmise most slopers have a very low airfoil camber thus the zero incidence rule works.

Curtis
Apr 23, 2019, 09:10 AM
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Quote:
Originally Posted by Curtis Suter
If balanced the same as in the same forward/aft CG location the only difference in decalage would be the angle the fuselage flies.
The main wing incidence angle sets the angle the fuselage flies.

The decalage (difference between wing and tail incidence angles) sets the trim airspeed.

I haven't flown a DLG, or seen one fly, so I don't know what speeds are typical or how important a wide speed range is. A zero-zero setup would favor higher speeds and small wing camber.
Apr 23, 2019, 11:46 AM
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JimZinVT's Avatar
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I hadn't heard that rule of thumb before Curtis. So for my favorite AG04 airfoil I'd raise the leading edge 1.7° relative to the centerline of the fuse? And put the stab at 0° to the centerline?
Apr 23, 2019, 11:51 AM
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DLGs have pretty wide speed range. They can float around slow with some camber dialed in, or in speed mode with some reflex in the wing they cover a lot of ground quick.
Apr 23, 2019, 12:13 PM
Red Merle ALES VII SJ
Curtis Suter's Avatar
Quote:
Originally Posted by JimZinVT
I hadn't heard that rule of thumb before Curtis. So for my favorite AG04 airfoil I'd raise the leading edge 1.7° relative to the centerline of the fuse? And put the stab at 0° to the centerline?
Guess what the camber is form that airfoil? 1.7!
Apr 23, 2019, 12:59 PM
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Quote:
Originally Posted by JimZinVT
DLGs have pretty wide speed range. They can float around slow with some camber dialed in, or in speed mode with some reflex in the wing they cover a lot of ground quick.
Where do you guys usually set your CG? Near neutrally stable?
Apr 23, 2019, 01:37 PM
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Yes, it is I use it on my DLGs and light slopers.
Apr 23, 2019, 01:38 PM
B for Bruce
BMatthews's Avatar
DLG's would need to be set fairly close to neutrally stable or they would pitch upwards and maybe even over into a part of a loop. Either that or some manner of launch mode would be needed that provides a pre-set amount of down trim for the launch then is clicked out to glide trim after the transition.

Let's keep in mind too that even if we set 0-0 on the bench that as soon as we click in a bit of up or down trim during the first test flight we no longer have a true 0-0 wing to tail. Either that or some fine tweaking of the CG would be needed over the course of a few flights to allow us to restore the elevator trim back to a true 0-0 setting.
Apr 23, 2019, 01:44 PM
It's time for me to fly
JimZinVT's Avatar
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Quote:
Originally Posted by NC14310
Where do you guys usually set your CG? Near neutrally stable?
Different pilots seem to have different tastes there, but yes usually close to neutral.
I don't fly DLG competitions, just fun flying. I like the CG just barely positive.
Apr 23, 2019, 01:44 PM
Red Merle ALES VII SJ
Curtis Suter's Avatar
Quote:
Originally Posted by BMatthews
Let's keep in mind too that even if we set 0-0 on the bench that as soon as we click in a bit of up or down trim during the first test flight we no longer have a true 0-0 wing to tail. Either that or some fine tweaking of the CG would be needed over the course of a few flights to allow us to restore the elevator trim back to a true 0-0 setting.
Very well said and I would add....or main wing trailing edge camber changes.
Apr 23, 2019, 05:03 PM
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Quote:
Originally Posted by BMatthews
DLG's would need to be set fairly close to neutrally stable or they would pitch upwards and maybe even over into a part of a loop.
I agree. Only a neutrally stable airplane has the characteristic that trim doesn't change with AoA (and therefore airspeed).

This is probably more important than worrying too much about decalage. Get the CG right and fly with the decalage that makes it work. Zero-zero is probably a good place to start if the wing camber is small (or variable).
Apr 23, 2019, 05:52 PM
B for Bruce
BMatthews's Avatar
Quote:
Originally Posted by Curtis Suter
....and I would add....or main wing trailing edge camber changes.
Yep, that too. Dropping the flaps not only alters the camber but also the wing incidence across that section of the wing with flaps. So yeah, there goes the 0-0 setup right out the door for either change.


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