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Thanks for that Rocket... I don't think I need to explain what it means if the wing is below the zero lift line in stable straight and level flight.

JB

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Buzz313th
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Now I know I said I wouldn't compare with photos of real P-51's, since the camera could be slanted, the plane might not be level and we don't know any details of flight data...

But I found a really good document that includes a picture shot that is the best representation I could find.

The Doc is here.... http://www.spitfireperformance.com/must ... gtest.html

CapFlyer, If this guy is flying level and if he is at an indicated airspeed close to cruise and if his load is more typical than not, then as you can see the prop is around perpendicular to the horizon... I know this doesn't prove anything, but the included document is good to have.

JB

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Here's one more document that references some data I refered to in an earlier post....

Here is a Graph on page 87 that looks like theoretical data generated in a wind tunnel, or on a computer for a redesigned wing of a P-51 for air racing. It mentions an Alpha of Negative -0.963 on a P-51 wing traveling at Mach 0.78. True Airspeed at that Mach number at Sea Level = 514 knots and at 40k its 447. The highest IAS I was able to achieve in level flight in my screenshot examples was 380 knots. Now given, we don't have a clue what altitude this graph represents, but mach 0.78 in the P-51 is far exceeding VMO. So whats my point? If the theoretical alpha of the P-51 wing at .78 mach in level flight is -0.93 and I showed a screenshot of the P-51 at 380 knots indicated, in level flight with an alpha around -2.0 degrees, then I can say with some certainty that the model is flying with too low of an AoA.

http://www.google.com/url?sa=t&rct=j&q= ... kUEel5L9-g

And here is a link to a mach to TAS converter with correction for altitude if you wanna plug some numbers in...... http://www.globalaircraft.org/converter.htm
JB

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Buzz313th
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Correct me if I am wrong, but the wings on the P-51 air racer are shortened right? It would have to have different lifting characteristics which may possibly imply a higher Alpha for a given co-efficient of lift

Just to clarify though, when you mentioned the zero lift line and the P-51 having -2 degrees angle of attack, that was based on 0 CL right? I just wanted to note that you can travel pretty close to 0 simply by increasing the speed of an aircraft. According to this ATPL question (http://www.thedailyatpl.com/atpl/pof/an ... of-flight/), formula included, if you double an aircraft's velocity in straight and level flight, the coefficient of lift becomes a quarter of it's original value. That chart you provided couple pages back (the 6th page of the slide), seemed to assign a value of 0.2 CL for cruise flight. Assuming we can double the speed of cruise, the CL can be as low as 0.05, which could bring a value pretty close to -2 degrees angle of attack in straight and level flight.

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Not sure about the shortened wings. But the first graph on the left states that it is an unmodified wing. The right one is modified. Before or after shortening the wings, I don't know. Edit.... If the P-51Racer Teams are shortening the wings, then I can only assume that they include modifications to counter any loss of lift, since any modification that may cause a need for a higher AoA at given airspeed will create an increase in induced drag and it's my understanding that the reason they modify the wings in the first place would be to decrease drag both in subsonic and transonic flight.

The zero lift line is the point at where the wing makes no lift. So to get an aircraft wing at the zero lift line would require the pilot to push the plane over until load is zero G's.

To double the speed would bring the IAS of my example to 760 KIAS. Which is 145 knots faster than Mach .78 at sea level. Considering your passing through the transonic barrier and going supersonic these assumptions get thrown out the window and new formulas and calculations would need to come into play that deal with Supersonic Aerodynamics.. I see your point, but I will let you decide whether they are relevant for the P-51 in this situation.

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LOL,

I gotta tell ya all, you guys are a really hard "Nut to Crack".

I feel like I should have taken a debate class before posting in this thread.



JB

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What I meant was the 0.2 was assigned to CLCruise on that particular chart, which came close to 0 angle of attack. Assuming CLCruise is straight and level flight, to push the lift coefficient lower would put alpha into the negative range. Now, it didn't state the conditions (in particular, the speed) at which CLCruise was obtained, but 380 knots is pretty fast and while I don't have any performance tables in front of me, I would say it is quite a bit above cruise, which would then equal lower CL, and hence, a negative Angle of Attack.

It's not so much about flying 760 KTAS, but how much 380 KTAS is in relation to the speed at which CLCruise is obtained... which we don't know exactly

Not trying to be disagreeable or anything. I want to see the model as accurate as it can too . Just saying what is making sense to me right now lol

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Your right and to be honest, I have learned a tremendous amount while researching info to backup my opinion. My original asumption was that no plane should not be able to fly with a negative AoA, this is obviously wrong and I'm fine admitting that I was wrong. But I still believe that the model is flying with too low of an AoA.

JB

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Lemme see if I can find a good range of cruise speed... Fastest cruise on the charts for 2k feet (Since we are comparing to the 380KIAS Screenshot) is 285KIAS, best endurance at 2k is 200KIAS.

I see your point and it's valid IMHO.. But I changed my stance in this small debate and decided to try and compare only calculations that we know. We know that theoretical data shows level flight at -0.93 alpha at M0.78 and we know that the Zero Lift line is -2.0 degrees. This is the reason why my new screenshots are based on flight data at the highest sustained IAS, instead of Cruise speed, since we don't know what CLcruise was derived from.

JB

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Buzz, can you clarify something? I thought you said the -0.93 alpha was only valid for a modified wing on a P-51 racer. Are you assuming this is valid for the almost completely symetrical wing on the production aircraft? That AoA still seems hokey to me, just from looking at the shape of the wing.

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The only info I have is from the charts I posted. Have a look at em. They are both showing before and after a camber adjustment made to the upper portion of the wing to reduce drag and further decrease AoA at .78 mach. There is nothing that states that the first graph is a stock wing, it only states that the graph represents the values from before this specific modification and after this specific modefication. So I don't know if the first graph represents a wing that is bone stock or has been modified in some way.

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Ok Rocket Bird,

Lets plug in the highest and lowest cruise speeds into CLcruise and compare..

CLcruise=0.2

If CLcruise was derived from 200kias then 400 kias CL would be 0.05 which on the graph would be roughly -0.5 to -0.75 AoA, right? I know the graph is hard to interpolate at those decimals.

Now if CLcruise was derived by using 285kias then double the IAS would be 570kias which would make CL at 570KIAS the same at 0.05 which would again give us -0.5 to -0.75AoA

Our most plausable cruise speed for CLcruise would be right in the middle of the two... at 250kias right?

Please back up my math would ya... My head is starting to spin...

So at 380kias, then based off of the 250kias CLcruise our AoA should be..... around -0.2 to -0.4 AoA?

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There is one more thing we have to take into consideration.. As we move into transonic speeds, we will start experiencing Mach tuck. This will create a foreward pitching moment and force the pilot to increase back elevator to maintain level flight. This will force the wing to run at a higher AoA as we push farther into transonic speed. I believe, and correct me if I am wrong, that this is the exact situation that the racing teams are trying avoid by making modifications to the wing that push this further up the Mach scale.

JB

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Buzz313th
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One sec... need to find the math side of my brain *searches through cobwebs*

Okay, here goes:

-For every time we double airspeed by 2, we have to divide the lift coefficient by 4.
-380 knots is 1.52 times more than 250 knots (250 x 1.52 = 380)
-Doing a little cross multiplication, if we take (1.52 x 4Cl)/ 2, we get 3.04.
-Thus we would have to divide the lift coefficient of 0.2 by 3.04
-0.2 / 3.04 = a Cl of 0.067.
-Reading 0.067 on the graph is difficult lol. It's about -1 degree to -1.5 from the looks of it, but I dunno!

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Seems quite a bit excessive at -1 to -1.5 aoa, especialy considering that our example of the P-51 wing at Mach.78 is only going to -0.93 AoA. (And thats a wing that could be modified, we don't really know) And depending on altitude, 0.78Mach is 46 knots faster than 380 at FL400 and 134 knots faster than 380 at sea level. And I'm sure the graph from the racing team is biased towards a lower altitude, since almost all of their races are done from 100 ft MSL to 5000 MSL through the pylons. And furthermore, we aren't even figuring in what the impact Mach Tuck is gonna have on the AoA as we get into the higher airspeeds...

Anyway, I'm gonna run and take the kids down to In N Out Burger.. Then stop by Van Nuys and see if I can get in contact with a P-51 Owner. Hey Dudley.. Whats your wingmans plane look like when you guys are flying in level flight while in formation?

Seeya all a bit later..

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Buzz313th
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