Effect of Wing Slats on CG

[Philipp]

If L.E. slats are installed on the wing for low speed stability, what effect will this have on the desired CG? ie: Does the xx percent MAC move forward? Is the wing chord increased by the "depth" of the slats?
Something similar will probably apply with Fowler flaps, as well. Any theories or facts available on this?

[B717 Furloughee]

I don't know the direct answers to your questions, as I'm not an aerodynamicist. However, I have flown full-scale airplanes with slats, fowler flaps, as well as some with leading edge flaps. I have not flown any models with these devices.

For any given wing, there is a coefficient of lift which basically decribes that wing's ability to produce a pressure differential, i.e. lift. The only way to increase the coefficient of lift is by increasing AOA or changing the wing planform. Slats increase the camber and thereby increase the coefficient of lift for the wing. Slats do not improve low speed stability, they simply increase the wing's coefficient of lift (wing planform change) which in effect lowers the 1g stall speed primarily for the purpose of slower takeoff and landing.

When deployed in flight, (without trailing edge flap deployment) slats increase the pitch attitude for any given AOA (Note- this does not mean the airplane is climbing, just flying with the nose high). This is the opposite of trailing edge flaps which lower the nose. The increased pitch attitude is due to the change in chord line, i.e.- the leading edge of the wing extends and lowers with slats. It's like changing the angle of incidence mid-flight. The chord is a line between the leading edge and trailing edge, so yes the chord does increase and L.E. lowers with slat extension just as it does with trailing edge flap extension.

With regards to CG, if weight shifts forward, the CG must also move forward, if ever so slightly.

Hope this helps with what you're after.

[ChrisP]

Here's my 2 Eurocents :

Slats are added to increase the lift coefficient, not increase low speed stability. The aircraft can fly slower as it generates the same amount of lift at a lower speed.

If the slats move forward, the C.G. moves forward. If the flaps move rearwards, the C.G. moves rearward. However, the change in pitching moment due to the camber change of the aerofoil is significantly greater than that caused by a change of C.G. due to the mass of the parts moving.

The easiest trick on a model is to mix flaps with elevator to compensate for the pitch change when flaps are deployed.

[Philipp]

ChrisP - I believe that the Center of Lift (Rather than CG) would move foreward with the deployment of slats, or rearward with flaps deployed. Mass, as in cargo, would have to be moved to change the location of the CG. Of course the moving of flaps would move it very sligntly, only depending on the weight of the flaps.
I had always thought that if the slats had a gap between them and the leading edge of the wing, the slats would improve the flow over the wing and thus allow a higher angle of attack before the wing stalled. In this example, I am referring to fixed slats as in the Fiesler Storch.

[vintage1]

I had always thought that if the slats had a gap between them and the leading edge of the wing, the slats would improve the flow over the wing and thus allow a higher angle of attack before the wing stalled. In this example, I am referring to fixed slats as in the Fiesler Storch.

Every book I have ever read makes that the reason for slats - to delay turbulent flow at high AOA and low speeds.

Whether not stalling represents an aid to staibility is up to you to decide, dear readers :D

[hall woo]

I balance all my models using the " CROWN " or thickest part of the wing or the highest camber part of the wing as specified in the SPAD forum, flies whole lot better

SLATs should not affect the CG or balance point, only increases drag at hi-speeds or increase lift at low speeds

[Sparky Paul]

With the airplane on scales on the ground, the effect of extending/retracting slats/flaps on the c.g. is probably detectable.
With the aerodynamic responses to lift, drag and pitching moment in flight, the slight c.g. shift would affect nothing.

[bluesky123]

The mass of the parts being moved is negligibly small comparing to the total mass of the aircraft, so, movement of the C.G. with respect to some fixed location in the aircraft structure is negligible too.

I guess Philipp was referring to the change of CG % with respect to the MAC, which moves due to the changes in the wing planifom. In other words, CG stays where it was, w.r.t. the airframe, but MAC moves either forward or backward. I think the flaps must be might big to cause substantial change in MAC...

From what I remember from my aerodynamics class in college (this was 20 years ago, time flies!), extending flaps increases Cy of the wing without tilting the fuselage. We weren't told about any effect of MAC movement and resulting change in static stability of the aircraft. I would gess that the change of MAC location has much smaller effect on the aircraft stability than many other things taking place when the flaps are lowered--change in Z-axis moment, requiring the elevator input; airflow becoming more three-dimensional with strong tip vortices, etc.

Boris

[ChrisP]

ChrisP - I believe that the Center of Lift (Rather than CG) would move foreward with the deployment of slats, or rearward with flaps deployed. Mass, as in cargo, would have to be moved to change the location of the CG. Of course the moving of flaps would move it very sligntly, only depending on the weight of the flaps.
If you move mass forward, the C.G. moves forward. It you move mass rearwards, the C.G. moves rearwards. By definition. And that was the question you asked, I thought !
If your understanding of pitch change is based on centres of pressure which move back and forth, or centres of pressure which are always at 25% MAC, but subject to changes in pitching moment, is up to you ! As you will see from my comments, I favour the latter approach. :)

[Philipp]

Actually, what I was trying to ask was something like this: If (zero mass) slats are added in front of the leading edge, does this move the DESIRED LOCATION of CG forward? Or, as stated earlier, Does it move the 25 percent MAC location forward?
I believe that the answers that I am hearing say, YES. Therefore, if the actual CG stays where it was before adding the LE slats, the plane should pitch up.

[HELModels]

This probably wont help you much, but slats, flaps, doodads are flow control devices and pitch effects are result of Aerocenter changes. With the CG fixed and AC variable, then something has to give - pitch.

[vintage1]

I am not sure, but it seems to me that at the intended speed, slats would increase the airflow at the wing rear, leading to a nose down moment.

[ChrisP]

Actually, what I was trying to ask was something like this.......
Aaaahhhhh.........

Reminds me of a glider I once lost to the dreaded 'tuck in'. I gave UP elevator close to the ground, the model rotated 90 degrees nose DOWN and was totally destroyed.
The reason was a C.G. that was too far rearward. In your terms the centre of lift moved from in front of the C.G. to behind it causing a nose down pitching moment.

Yes, if you set the model up with a rear C.G. things like that can happen.

[Ollie]

The effects due to slats and/or flaps change the airfoil camber. The effect of changing camber changes the pitching moment around the aerodynamic center (AC). The position of the AC is not changed except for very, very small changes of AC position with large changes in pitching moment with slats and/or flaps.

With good stability margin, the CG should not be changed. With slats and/or flaps the decalage angle must be changed for pitch trim.