TGoodwin

I am building a .20 to .25 size FA18 pusher plane. I am to the point where I need to determine the general location of the installed equiptment. I will try to position as much of the equiptment forward of the the CG as I am able so as to offset the weight of the engine at the rear of the aircraft. The question that I have is with finding the Mean Aerodynamic Chord. I can find the MAC for the wing with no trouble but how should I consider the fuseloge fairings that blend into the wings on the sides of the fuse? They are equil to about 10-15% of the total wing area I think. I do not think I can just ignore then when locating the the CG for this aircraft but I have no idea how to figure them into the calculation of the MAC.

Any suggestion would be appreciated.

Thanks Ted

Ollie

Project the leading and trailing edges of the wing to the centerline of the fuselage to get the planform for calculating the MAC. The area of the wing inside the fuselage is included because the lift distribution of the wing extends across the fuselage.

TGoodwin

Yep that is exactly how I figures the MAC but this does not consider the large fairing along the sides of the fuse that run all the way up along the canopy. I think these will have at least some effect on the MAC given there surface area.

Thanks Ted

Ollie

The fairings you refer to may affect the location of the neutral point but need not be considered part of the wing. There are several effects that are hard to quantify like the wake of the fuselage reducing the effectiveness of the tail and the wake of the wing reducing the effectiveness of the tail. They are usually accounted for by a fudge factor and the fairings can be treated in the same way by increasing the fudge factor a bit. The MAC usually comes into play to estimate the CG location required to produce a given static stability. I am of the opinion that the presence of the fairings is destabalizing.

TGoodwin

Yeah I guess I will figure the CG as normal and then make it nose heavy when I balance it for the test flight.

I would think that the fairings along the sides of the fuse would help with the long nose moment on this aircraft. Adding extra lift at high angles of attack and taking some of the load off of the wing and tail.

Speaking of the tail surfaces. The f18 has a steep sweep angle on the tail feathers. I am making them up as elevons and am trying to determine the best pivit or fulcrum point. I know that if I place it too far aft then the leading edge will catch the air at high speed and rip the tail feather off or at least exceed there critical angle of attack and cause the tail to stall. If I place the pivit point too far foreward then they will put excessive loads on the servos. does anyone have any suggestion for the best percentage of the MAC for the the tail that would give the best fulcrum point? I am going to go look for PICS of the f18 with the elevator deflected and see if I can determine the percentage of the root chord for this.

Thanks again. Ted

Ollie

Assuming you are using a flat or symmetrical airfoil on the tail (no mean line camber) then the aerodynamic center of the tail will be near 25% of the MAC, the pitching moment will be zero and, with the pivot there, the forces on the servo and linkages will be near zero. The flutter speed should be beyond the maximum airspeed of the model. Keep the linkages stiff and slop free even though the forces are near zero.

TGoodwin

OK thanks Ill figure the 25% MAC and give it a try.

Thanks Ted

Rodney

Please correct me if I'm wrong but; isn't MAC the point on the span where the wing area outboard of that point is equal to the wing area inboard at that point? thanks, Rod

Ollie

The mean aerodynamic chord is the chord through the centroid of the area. If you cut the planform from a sheet of uniform density, the centroid is where the sheet balances or the center of gravity of the area.

TGoodwin

It is the average chord of the wing. I you had a straight accross leading edge and a straight accross trailing edge then the wing would have no sweep or taper. In that case the MAC would be the same as the root and or tip chords and you could figure your CG as a percentage of the root or tip chord.

If your wing is swept or tapered or both then you have to find the average or mean aerodynamic chord and figure your CG as a percentage of that. It sounds more complex than it really is. It is very simple.

Ted

Ollie

The Mean Aerodynamic Chord and average chord are the same for a constant chord wing but, the greater the taper the larger the MAC is compared to the average chord. See:
http://www.palosrc.com/instructors/mac.htm

haupt

Ted,

If you are going to an all-moving tail surface, make sure you not only PIVOT about it's own MAC, but also BALANCE about that point. A nice way to do that is to replace a portion of the leading edge near the root with lead needed for balance and glass over the lead, overlapping onto the rest of the surface. I first saw this mentioned on the plan for a sport-scale ducted fan F-15 from Ziroli.

BTW, anybody have one of the old RK-740 fan units?

Regards,

Mark