Author Topic: Design Lift Coefficient and Airfoil Data (Read 13959 times)

Stoney74 · « **Reply #30 on:** June 23, 2007, 06:03:12 PM »

Apparently, wetted area is a major factor (assuming I did my math right).

I had to estimate some of the wetted area for the top because I couldn't get my CAD software to draw the contours I was looking for. As a substitute, I computed the area by approximation, using the areas of the triangle shapes that encompased the approximate shape I wanted.

Here's the equations and then the numbers I used:

Flat Plate Area (f), Fineness Ratio (fr) and Form Factor (FF) per the equations Tango posted

Wetted Area: 3.142(pi)*[Atop+Aside/2]

where Atop = the 2D area of the top of the drawing and Aside = the 2D area of the side. Got this equation from Raymer's book.

I iterated 6 basic sizes. All were 10 inches high and 10 inches wide of varying lengths: 3 ft long, 4 ft, 4.5 ft, 5.0 ft, 5.5 ft, 7.0 ft. The 7 foot length is approximately the length the assembly would be if the canopy was a fastback type configuration. The 10 inch height of the canopy occurs at a distance of two feet on all 6 sizes. The front and back slope away at fairly smooth contours, with the front more blunt, and back more shallow (i.e. half-teardrop shape).

3' f = 1691 in.^2
4' f = 1395 in.^2
4.5' f = 1404 in.^2
5.0' f = 1446 in.^2
5.5' f = 1461 in.^2
7' f = 1596 in.^2

Obviously the least drag is created somewhere close to 4' rather than at the longer lengths. Took me a couple hours, but interesting none-the-less. Next up is drawing the fuselage out. After that I'll put a 3D model together and post it.

Stoney74 · « **Reply #31 on:** August 08, 2007, 08:44:05 PM »

Raymer posts this simpler pitch stability equation is his RDV design book:

Xnp = ((Cla)Xwing - Kfuselageterm + Ktailterm(Xtail)) / (Cla + Ktailterm)

where Xwing = "Location of 1/4 chord Wing MAC"

and

where Xtail = "Location of 1/4 chord Horiz Stab MAC"

Do the units for this location of 1/4 chord matter? Also, what location? Is it the location along the MAC directly, or from the CG datum?

I'm using this equation for some simple comparisons between two drawings. He's got a much more complicated equation in his textbook that I'll use once I start narrowing down my options...

Thanks,

dtango · « **Reply #32 on:** August 08, 2007, 11:06:10 PM »

Hiya Stoney:

I'm out of town at the moment (once again) with no way to reference my books so don't know how Raymer derived that equation hence the significance of some of those terms in the equation.

That being said to answer your questions:

(1) I'm not sure about the units. I'm guessing the units don't matter as long you make sure you're using the same units for Xwing and Xtail. Pitching moment (Cm) is dimensionless so if the equation is pitching moment then the units mathematically drop out. Not knowing what the other terms in Raymer's equation are I don't know how the units drop out.

(2) 1/4 chord wing/h-stab MAC - that's the distance from the leading edge for each lifiting surface in question, wing or tail h-stab. 1/4 chord point (or c/4) is essentially very near the mean aerodynamic center (MAC) of the lifting surface at subsonic speeds. This has been demonstrated experimentally for cambered airfoils.

(I made an earlier statement in this thread that Cm_c/4 varies with aoa. This is only true at high aoa's where separation of airflow with the airfoil occurs. Otherwise the pitching moment at c/4 is nearly constant since it's close to the aerodynamic center of the airfoil.)

The key property of 1/4 chord point (or airfoil aerodynamic center) is that pitching moment at this point is constant and doesn't change with aoa. This makes it a very useful reference point because it is a fixed position and simplifies the math for stability.

Here's some more helpful information on the topic: (be sure to read further down)
http://adamone.rchomepage.com/index5.htm

Hope that helps!

Tango, XO
412th FS Braunco Mustangs

Stoney74 · « **Reply #33 on:** August 09, 2007, 12:51:56 AM »

Sorry, I didn't mention it...

Xnp for Raymer is the equation for neutral point, and then used for the static margin.

dtango · « **Reply #34 on:** August 09, 2007, 09:35:09 AM »

Ah, should have been more obvious to me

- Xnp.

Raymer's equation doesn't appear to be expressing location of neutral point (Xnp) as a % so that means the unit of length remain so whatever units you are using for Xwing and Xtail need to be the same and are carried through the calculation.

Tango, XO
412th FS Braunco Mustangs

evenhaim · « **Reply #35 on:** August 09, 2007, 11:13:21 AM »

tango check pms bud;)

Stoney74 · « **Reply #36 on:** November 15, 2007, 10:04:47 AM »

For those interested, my first concept drawing.

Length is 16.5 ft. Span is 24 ft.

SgtPappy · « **Reply #37 on:** November 15, 2007, 10:39:53 PM »

While we're on the topic, I'd like to know if there's anything I could use against the WWII Aircraft Forums guys who state that the 190 split flap is more efficient than the F4U's slotted flaps.

In addition, he states that the Spit14 can outturn a 'flapped' Corsair.

Stoney74 · « **Reply #38 on:** November 16, 2007, 08:37:14 AM »

Wrong thread Buddy.

Kweassa · « **Reply #39 on:** November 16, 2007, 12:42:48 PM »

Something's eerily familiar with that concept...

Mig-3?

Maybe you had similar objectives in mind with the Mig-3 when designing that concept?

Stoney74 · « **Reply #40 on:** November 17, 2007, 01:24:42 AM »

Actually, I started out with a 16.5 foot line. Added a 10 in. long spinner, then an 8 inch long prop extension, the engine, the fuel tank, the cockpit, the empenage, etc. That gave me the side profile. From there positioned the wing (the geometry was done prior to) on the fuselage at an approximate point to get the 1/4 chord point of the wing at about the CG of the aircraft. Once all those box-outs were placed, it was a matter of smoothing out the transitions, minimizing wetted area, and conforming some of the geometry to the International Formula One design formula. The cockpit interior is 18 inches wide just to give you a sense of scale.

Biggest influences were the Lancair Legacy, John Sharp's Nemesis, and the Arnold AR-6.