Design Lift Coefficient and Airfoil Data

[Stoney74]

WW, F4UDOA, Tango, et al...

I was looking at NACA Report 824 and was trying to determine some values from the charts.  One item was design lift coefficient.  Another was pitching moment.  I see Cli on the charts but understand how to read it as presented.  And, I don't see the pitching moment at all.  Furthermore, I'm trying to determine what type of airfoil will be best for a certain Vmax and not Vcruise.  Raymer's book suggests designing the airfoil around Azerolift at Vcruise.

Just looking for some guidance I can't glean from the readings...

[Knegel]

Hi,

by far more important for the highspeed behaviour and at least as important for the lift coefficient is the wingshape(aspect ratio, trapezium or squary, the form of the wingtips, swept wings etc).

"Airfoil lift coefficient" is by far not the "wing lift coefficient".

In theory only on a perfect eliptical wing the "Airfoil lift coefficient" ís the same like "wing lift coefficient", other, more squary wings, with smal aspectratio, guns, not covered wheels, antennas, etc minimize the wing efficiency often down to factor 0,5.
As result two wings with exact the same airfoil can have a very different "wing lift coefficient", while the different of the "wing lift coefficient" due to Airfoils in WWII was mainly rather smal.
Though two exceptions are the Spitfire airfoil and the P51 airfoil, where the Spit airfoil was rather thin, while the P51 Airfoil didnt provide very high max AoA´s, so mainly the max lift was not that high.

For highspeed with "normal" asymetric WWII airfoils the aspectratio and swept-wings are most important as well. Although the (semi) laminar flow wing provide a smaler drag at some speeds, the also rather limited critical mach of the P51 show that also this airfoil dont have a that high influence.

The solution to gain higher critical mach´s are smaler aspectratios(He162, Me163, Spitfire), more thin airfoils(Spitfire) and/or swept-wings (Me163, Me262).

Since the wingform, providing the best wing efficiency regading the lift, is most bad for highspeed and the other way around, modern jets often use swing-wings. This provide a high aspectratio and so a high lift factor for slowspeed, but also a smal aspect ratio and swept-wings.

On a wing with a smaler aspect ratio the airmasses can get shifted sideward in a better way. Further more, while swinging the wing backward, the relative thickness of the airfoil decrease, while the swept wings provide a more stable flight(swept wings have a better result than the V position of not swept wings).

Greetings,

Knegel

[gripen]

Usually profile data is given 2D and there are some formulas to convert it to 3D. Some basic stuff here.

[F4UDOA]

Stoney74,

Funny thing about airfoils. Many WW2 fighters shared the same one. F4U, F6F, P-38, F7F and I believe the FW190 as well all used the 2300 series.

IMHO it is much easier to reverse engineer based on airframe than one section at a time. I like to read the range charts of various aricraft to determine which was most efficient. For instance look at the amount of fuel required to fly a given distance and compare several different aircraft types for instance P-51D, P-38L, F4U and F6F. You can look at fuel required, distance and HP and see which aircraft had the least drag at cruise speed and most efficient engine. The P-51D is truely a marvel in this regard because of the laminar wing.

Also take a look at the F4U document on my webpage. It has Cdo, airfoil type of many aircraft and a detailed drag breakdown of the F4U-1D.

http://mywebpages.comcast.net/markw4/MSWF4UDATA.pdf

FYI, I am not an expert on airfoil design

[joeblogs]

While it is certainly true the shape of the wing (looking down from the top) influences the ratio of lift to drag (higher aspect ratios are generally better) I don't think you can argue this is more important than the airfoil shape (the view of the wing from the side).

I am pretty sure the opposite is generally true. Why? Because the aerodynamic properties of wing shape (again looking down) can very nearly be determined from paper and pencil. The properties of airfoils however had to be measured in wind tunnels, that is until fast mainframe computers came along in the mid 1960s. Engineers tend to make fewer mistakes when there is less uncertainties about the calculations...

Getting back to the question asked, go to the website for Loftin's Quest for Performance (a NASA publication) I am prety sure he will point you to two variables - the zero lift coefficient and flat plate area, which is how you calculate static drag.

see http://www.hq.nasa.gov/pao/History/SP-468/ch1.htm

the equations are found in http://www.hq.nasa.gov/pao/History/SP-468/app-c.htm

As for pitching moments, they will tell you something about how the plane will manouver. It won't say much about Vmax or climb rates.

The reference to zero lift coefficient at Vcruise might be a statement about variations in the angle of attack of a plane driven by differences in thrust. Pitching moment might be relevant to that.

-Blogs

Originally posted by Knegel
Hi,

by far more important for the highspeed behaviour and at least as important for the lift coefficient is the wingshape(aspect ratio, trapezium or squary, the form of the wingtips, swept wings etc). ...

Knegel

[joeblogs]

greetings F4u - just up for my quarterly breathe of air..

-Blogs

[Stoney74]

First, I appreciate the responses so far...

Second, I should have preceded my post with a small bit of context.  I'm in the initial stages of coming up with a Formula 1 design for Reno.  I've taken a look at all of the most competitive planes--Nemesis (the original DR-9, not the Sport class NXT), Endeavour, Mariah, etc. and their sizes and shapes.  The craft that went into the original Nemesis is probably beyond my abilities due to the fact that the guy that designed it worked for Lockheed's skunk works and that it was basically assembled by parts made at Scaled Composites.  That being said...

Pitching moment of the airfoil has an effect on horizontal tail size and moment, so therefore, after selecting an airfoil, you make the tail have the area necessary at the proper moment from the CG to keep the plane pitch stable.  Therefore, I need to figure out the pitching moments.  I see them listed on the airfoil data charts, but don't know how to interpret them.

Second, while most aircraft are designed with range in mind, a race plane is more concerned about speed, so that's why I'm interested in design lift coefficient with respect to Vmax instead of Vcruise.  Mostly I'm looking at the 6-series airfoils.  So with respect to design lift coefficient, I'm thinking I need to find an airfoil that gives me Alpha(zerolift) at Vmax or close to it.

Thanks again

[joeblogs]

I think the distinction between Vcruise and Vmax is not going to be important for you. I say that because the coefficients being used are dimensionless. The airfoil that maximizes lift relative to drag at the cruise, to a first approximation, is going to give you the maximum speed.

The qualification here is that we are looking at the same angle of attack at the two amounts of thrust. The pitching moment is being cause by the lift vector not being pointed straight up. That is caused by (1) the angle of attack of the wing (the lift vector will be perpendicular to the chord of the wing) and (I believe) the difference between the center of gravity and the center of lift, which depends on where the wing is located. There is possibly a third factor, which is any force outside the horizontal plane induced by the source of thrust (on some planes, the propeller is not perpendicular to the direction of flight), and the magnitude of that vector will depend on the amount of thrust. The analogy for a car is how the nose rises relative to the trunk when you slam on the gas.

So if I am right, the pitching moment is going to depend on these three variables and you are not going to be able to derive them from a table of lift coefficients for different airfoil sections.

At a minimum you need an introductory book on aerodynamics, but you really need a trained engineer to help with this.

Two books you might look at are Mair & Birdsall's Aircraft Performance or the older book Airplane Performance, Stability, & Control by Perkins & Hage.

-Blogs

Originally posted by Stoney74
First, I appreciate the responses so far...

Second, I should have preceded my post with a small bit of context.  I'm in the initial stages of coming up with a Formula 1 design for Reno.  I've taken a look at all of the most competitive planes--Nemesis (the original DR-9, not the Sport class NXT), Endeavour, Mariah, etc. and their sizes and shapes.  The craft that went into the original Nemesis is probably beyond my abilities due to the fact that the guy that designed it worked for Lockheed's skunk works and that it was basically assembled by parts made at Scaled Composites.  That being said...

Pitching moment of the airfoil has an effect on horizontal tail size and moment, so therefore, after selecting an airfoil, you make the tail have the area necessary at the proper moment from the CG to keep the plane pitch stable.  Therefore, I need to figure out the pitching moments.  I see them listed on the airfoil data charts, but don't know how to interpret them.

Second, while most aircraft are designed with range in mind, a race plane is more concerned about speed, so that's why I'm interested in design lift coefficient with respect to Vmax instead of Vcruise.  Mostly I'm looking at the 6-series airfoils.  So with respect to design lift coefficient, I'm thinking I need to find an airfoil that gives me Alpha(zerolift) at Vmax or close to it.

Thanks again

[Stoney74]

Thanks for the tips Joe.  Obviously right now I'm just making drawings and dusting off some rusty math skills.  Before I start putting anything together I'll have some professional advice.  IF1 has a technical director that you can send designs to and they'll help keep you safe and within the realm of reality.  There are a lot of other resources here locally--a benefit of living in Reno.  Regardless, I'll take a look at those books and bounce it off what I already know.  I've been pouring through a copy of Aerodynamics for Naval Aviators and have a couple more that I'm going to order from Amazon.  I'll add these two titles to my list.

[Stoney74]

Finally found what I was looking for.  Thanks for the input...

[Knegel]

Originally posted by joeblogs
While it is certainly true the shape of the wing (looking down from the top) influences the ratio of lift to drag (higher aspect ratios are generally better) I don't think you can argue this is more important than the airfoil shape (the view of the wing from the side).

 

Hi,

as long as there get very different airfoild used, Airfoil and Wingshape are  same important, but in WWII most fighters had very similar airfoils.
Unlike to the WWI time, where the scientists just started to gain knowledge, resulting is very big differents.

If you look to the extremes, you will see that the Wingshape and Airfoil are same important.
A squary wing with a Aspectratio of 3 or smaler, but a highly effective asymertic Airfoil, dont will create more lift than a trapezium Wing with a aspectratio of 11, but a flat symetrical airfoil.  

I took the Spitfire and P51´s as exception, cause they have rather seldom airfoils in WWII, both with a rather bad influence to the CLmax, but with a good incluende to the drag.

Originally posted by F4UDOA

Funny thing about airfoils. Many WW2 fighters shared the same one. F4U, F6F, P-38, F7F and I believe the FW190 as well all used the 2300 series.

IMHO it is much easier to reverse engineer based on airframe than one section at a time. I like to read the range charts of various aricraft to determine which was most efficient. For instance look at the amount of fuel required to fly a given distance and compare several different aircraft types for instance P-51D, P-38L, F4U and F6F. You can look at fuel required, distance and HP and see which aircraft had the least drag at cruise speed and most efficient engine. The P-51D is truely a marvel in this regard because of the laminar wing.

I dont think you can see much due to this, cause you never know if the engine, the propeller or the airframe is responsible for the low fuel consum or high cruise speed.
Imho, Stall speed, Vmax, climb ratio and critical Mach are values to work with, in combination with wing load, span load, wingarea, aspectratio,  powerload etc.  

btw, i doubt the P51 Airfoil was a the main factor for its long range. Rather the very clean surface condition, the very streamlined fuselage, the thrust producing radiator system and a very good fitting aragement between engine, reduction gear, propeller and speed of smalest drag was the cause for the high speed and low fuel consumption.
The rather low critical mach number of the P51 dont give a hint that the airfoil was a main factor. Tests did show that the smalest pollution did disturb the laminar effect anyway, and the P51 anyway only had a semi laminar wing.
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.
The Vmax different between 109G10 and K4 show what covered wheels do.


Greetings,

Knegel

[joeblogs]

I should be more careful. If you are taking the plane into the transsonic regime, wing planform (what you call wingshape) is going to matter a whole lot.

Below the transsonic region, however, the properties of a wing's planform can be determined on paper and so engineers are not going to choose wildly inefficient shapes. They will choose different shapes, but the effects tend to be second order compared to things like dihedral and twist - hence the huge debate over the Spitfire wing shape.

With airfoils however, much less was known until NACA's pathbreaking experimental research. That is one reason why engineers were so conservative about selecting airfoils.

In any case, none of this helps Stoney74. Fortunately he has already found what he was looking for.

-Blogs

Originally posted by Knegel
Hi,

as long as there get very different airfoild used, Airfoil and Wingshape are  same important, but in WWII most fighters had very similar airfoils.
Unlike to the WWI time, where the scientists just started to gain knowledge, resulting is very big differents.

If you look to the extremes, you will see that the Wingshape and Airfoil are same important.
A squary wing with a Aspectratio of 3 or smaler, but a highly effective asymertic Airfoil, dont will create more lift than a trapezium Wing with a aspectratio of 11, but a flat symetrical airfoil.  

...


Greetings,

Knegel

[Stoney74]

Originally posted by joeblogs
So if I am right, the pitching moment is going to depend on these three variables and you are not going to be able to derive them from a table of lift coefficients for different airfoil sections.

Well, the pitching moment of the airfoil section has an influence on the sizes and moments of the horizontal stabilizers.  Some airfoils have higher pitching moments, which either require a larger horiz. stab at the same moment, or a smaller horiz stab at a larger moment...or so I've come to understand it.  Pitching moments at different lift coefficients are listed on the airfoil charts.  That was the catalyst for the original question.  Design lift coefficient is also listed as a part of the airfoil number.  (From what I've read) The design lift coefficient is the lift coefficient at which the airfoil section produces the least drag.  In the airfoil nomenclature, i.e. a 64415 airfoil, the third digit (4 in this case) is the design lift coefficient in tenths.  So, for this airfoil section, the design lift coefficient is (.4).  It relates to cruise speeds in a manner that you should try to match the airfoil that is best suited to the cruise speed desired.  So, in order to determine the proper tail sizing, it seems you have to know the pitching moment of the airfoil at its design lift coefficient to ensure pitch stability at Vcruise (or Vmax in my case).  Other characteristics of the airfoil will effect stall speeds, Clmax, pressure distributions, etc.

Again, this is initial information, and not something I'm hanging my hat on.  Merely a starting point on what, if carried to fruition, will be a long, detailed process.

[Knegel]

Originally posted by joeblogs
I should be more careful. If you are taking the plane into the transsonic regime, wing planform (what you call wingshape) is going to matter a whole lot.

Below the transsonic region, however, the properties of a wing's planform can be determined on paper and so engineers are not going to choose wildly inefficient shapes. They will choose different shapes, but the effects tend to be second order compared to things like dihedral and twist - hence the huge debate over the Spitfire wing shape.

With airfoils however, much less was known until NACA's pathbreaking experimental research. That is one reason why engineers were so conservative about selecting airfoils.

In any case, none of this helps Stoney74. Fortunately he has already found what he was looking for.

-Blogs

Hi,

also at slow speed, specialy while flying with high AoA´s the planform make a huge different. To understand this influence and to create a effective planform is at least as difficult as to create a good airfoil. Actually one dont work perfect without the other.

Typical fighters in WWII had a wing aspectratio between 4,6 and 8,8, this make a huge different, at least as big as the flat Spit airfoil to the wide spreaded 2300 airfoil.

The 1st good airfoils, based on exact researches got used already in the FokkerDr.1 and D.VII and D.VIII. The Götingern University made this researches, resulting in the "Göttingen Airfoil", which did provide a much superior handling(stall character).

I guess the constructors was so conservative while selecting airfoils, cause mach related problems was new and the used airfoils was already close to perfect(good mix between good lift and smal drag) for speeds below critical mach.

Since most planes in WWII had rather smilar airfoils, the influence to the max CL is rather smal.
As i wrote in my 1st post, aspect ratio, trapezium or squary, the form of the wingtips, swept wings etc are the more important factors in WWII( "etc" also include dihedral and twist, simply all not airfoil related influences).
 
Greetings,

Knegel

[joeblogs]

What is intersting about the evolution of airfoils is that we started with very thin wings when speeds were quite slow. Fokker and some of the German aerodynamicists figured out a thick wing was much more efficient at those speeds.

We then went through a 20 year period where wings were pretty thick, but speeds had nearly tripled. By then, a thinner wing was more efficient, but it took a lot of wind tunnel tests to convince people it was worth engineering thin, but strong wings. Then came laminar flow...

Originally posted by Knegel
Hi,

The 1st good airfoils, based on exact researches got used already in the FokkerDr.1 and D.VII and D.VIII. The Götingern University made this researches, resulting in the "Göttingen Airfoil", which did provide a much superior handling(stall character).

Knegel