Design Lift Coefficient and Airfoil Data

[F4UDOA]

JoeB!!

Hope you are well, I have been a passive reader on the boards lately. Always enjoy your post, keep'em coming.

Knegel,

I think their are many factors that contribute to the P-51's range performance. But keep in mind that the range chart in the manual is for a standard service aircraft with Wing racks attached so it was not in an optimal condition. The 109 was full of bumps, protrusions etc as was the Spit. The P-51 was designed to be a "clean" aircraft from the jump. Take a look at the results of the postwar Bendix Air Races where essentially stock P-51's raced from Cleveland to LA at speeds averaging over 400MPH. Frankly amazing for the time IMHO.

[Knegel]

Hi F4UDOA,

in no way i would disagree to this, thats why i wrote:
"The P51 was a masterpiece of a clean construction, from the Spinner to the tail, all is smooth(without wax) and the wheels are covered."


Hi joeblogs,

yes, the diefferent steps of development and knowledge are very interesting. Its sometimes suprising what "simple" unknown things turned to be a real wall.
I realy wonder how simple some "walls" from today can get cut down, if we only would know how.

Greetings,

Knegel

[Stoney74]

Originally posted by Knegel
I realy wonder how simple some "walls" from today can get cut down, if we only would know how.

I'd say these guys are doing it every time they complete a new project.  Case in point here

[dtango]

Stoney:

Sorry, I am late to the party as usual !  I was out of town last week on business and didn't browse the forums.  Just saw this thread and wanted to respond.

Firstly, best of luck on your endeavor to build that aircraft!  That's really exciting!  I don't think I would be ever bold enough to tackle something like that!  I don't design planes for a living so make sure you take anything I say with a grain of salt!

As to your question, it appears you've answered it yourself.  NACA 824 contains charts that give you airfoil pitching coefficient, Cm for airfoils at both c/4 (quarter chord-point: 1/4 of the chord distance from the leading edge) or AC (aerodynamic center).  CMc/4 varies with CL while CMac is the point along the airfoil where CM doesn't vary with changes in angle of attack.

I'm not sure which sets of equations you are referring to for your calculations in estimitating longitudinal stability.  I'm not at home right now so can't reference my books to look it over.  However here's one equation on the web from Stanford that's useful:



The equation and it's explanation can be found at the following webpage:
http://adg.stanford.edu/aa241/stability/staticstability.html

Basically the equation represents the entire CM of an airplane about it's center of gravity (cg).  A conceptual way to describe the equation is basically:

Airplane Pitching Moment = Moment(from wing lift) - Moment(from tail lift) + Moment(at Wing AC) + Moment(fuselage)

From this equation basically you just need the CMac of the wing, which NACA 824 gives you.  Of course like gripen said you'll have to account for 3D effects if you want a more accurate estimation.

Tango, XO
412th FS Braunco Mustangs

[gripen]

Originally posted by dtango
However here's one equation on the web from Stanford that's useful:



The equation and it's explanation can be found at the following webpage:
http://adg.stanford.edu/aa241/stability/staticstability.html

Basically the equation represents the entire CM of an airplane about it's center of gravity (cg).  A conceptual way to describe the equation is basically:

Airplane Pitching Moment = Moment(from wing lift) - Moment(from tail lift) + Moment(at Wing AC) + Moment(fuselage)

There is also a moment caused by powerplant(s) ie thrust line and also the propwash tends to change things.

I have been designing RC-planes since 80s and in practice we set the needed incidences (thrust line, wing, tail) by experience due the complicated nature of the issue. However, those formulas certainly give some clue. I think that in the case of the full scale airplanes, the fine tuning of the incidences is still done based on wind tunnel data and flight tests despite powerfull computer aided design tools available today.

[Stoney74]

Originally posted by dtango
Of course like gripen said you'll have to account for 3D effects if you want a more accurate estimation. [/B]

This is all extremely preliminary information I'm looking for here.  Right now I'm just basically looking at sizing.  I've got a long road of different steps to take before I even start drawing up plans, much less making parts.

It'll be a while before I have the potential to pull a John Denver...

[dtango]

Yep agreed gripen.  I thought about posting the nastier form of the longitudinal stability equation which accounts for thrust and other vertical offsets when I got home.  That lasted about 2 seconds .

Stoney- keep us posted!

Tango, XO
412th FS Braunco Mustangs

[Stoney74]

Any of you guys know a reference for the NLF series of airfoils?

[dtango]

Stoney:

Try the UIUC Airfoils Coordinate Database for starters:
http://www.ae.uiuc.edu/m-selig/ads/coord_database.html

Search for NLF.  There's an assortment of natural laminar flow airfoils there.  Hope that helps!

Tango, XO
412th FS Braunco Mustangs

[gripen]

Generally a good approach to the design process is to study the other designs for the same purpose first. I see developement of an aircraft as an evolution; you learn from your's and other's doings and a bit by bit you can improve the design. Radical approach tend lead to the enermous amount of problems which are impossible to see before the proto is built.

[Stoney74]

The original Nemesis, the most successful IF1 racer in history and now a resident of the Smithsonian, used a NLF airfoil.  That's why I asked.

[gripen]

Ah, you are in right track; you don't need to invent the wheel

[Stoney74]

Next topic for discussion:

Canopy profile...

Mariah, currently the perpetual champion in the Formula 1 Gold Class has a bubble type canopy profile.  Nemesis had a fast-back type configuration.   Hoerner states a fineness ratio of 3.7:1 is the most aerodynamic shape for a protrusion.  Given that, and my planned canopy height of 10 inches, a 3.7:1 fineness ratio supports a bubble type canopy approximately 37 inches long.  Kent Paser, author of Speed With Economy created a fast back canopy configuration for his plane.  Is there a way (without a wind tunnel) to determine which configuration produces the lowest drag?  Seems to me a fast back configuration gives a fineness ratio much higher than 3.7:1.  Personally, I'd prefer the bubble canopy since the visibility would be much better.  Anyone?

[dtango]

Stoney:

That's a tricky question with an equally tricky answer.  Here's my understanding:

You can use the component buildup method to estimate subsonic parasite drag of each component of the aircraft which you then build up to the total aircraft parasite drag.  With that in mind you may be able to use this method to assess the drag of different canopies.

The first key relationship is:

Dp = f*q

This equation basically says that parasite drag equals to the equivalent flat plate area of the aircraft multiplied by dynamic pressure (.5, air-density, velocity-squared).  

For the component buildup method you basically find equivalent flat plate area (f) for the entire plane by finding (f) for each of the components and sum them up.  Essentially the greater the total (f) the greater the parasite drag.  The equation for f for each component is:

f=Cf*FF*Q*Swet

where
Cf= skin friction coefficient (how smooth or rough the surface is)
FF= component form factor
Q= component interference factor
Swet = wetted area (total area exposed to the air)

Let's ignore Cf, Q, or Swet for now and assume they are they same for each canopy.  With this assumption you can see that form factor FF then determines parasite drag.  The larger the value of FF, the greater the drag.

Without getting into the details of the FF equations the relationship between form factor (FF) and fineness ratio is the following:


Diagram from Stanford:
http://adg.stanford.edu/aa241/drag/formfactor.html

In the diagram above K is the form factor.  You can see that the greater the fineness ratio, the lower the form factor.  The lower the form factor, the lower the parasite drag.  That's why the fast-back canopy is said to be less draggy than a bubble canopy because as you have said the fast-back canopy fineness ratio is greater than the bubble canopy.

So it appears you have your answer.  The fastback canopy is less draggy.  

Well... here's the tricky part.  Our assumption that we can ignore Q and Swet is not accurate because they may not be the same between the two canopies so you will need to factor these variables as well.

Interference factor, Q ranges between 1 to 1.5.  You'll have to to look up resources to helpy you estimate Q for the canopies in question.  Wetted area, Swet you'll have to calculate for each as well.

Oh, there are several different equations for FF for different parts of the airplane.  Also there are different rules of thumb to use on scaling each equation depending on the shape of each aircraft part.  Here's the FF equation for canopies and fuselages for your reference:

FF = (1 + 60/fr^3 + fr/400)

where fr (fineness ratio) is:

fr = l/d or fr= l/(sqrt[(4/pi)*Amax])

where
l= length
d= max cross-sectional diameter
Amax = max cross-sectional area for non-circular shapes

Tango, XO
412th FS Braunco Mustangs

[Stoney74]

Awesome Tango!

I'm setting up a spreadsheet to start running the numbers.  I'll be back with the results...