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I dont think it is really possible to measure just by looking at airplane profile. I could be wrong, but fsx could have a function to show actual angle of attack. I believe the visual representation could vary ( graphically)

By the way what wing dihedral P -51 has?

The more I look at that profile picture, something keeps popping out at me, but I don't have the knowledge to understand what I'm looking at. I am surprised to see that the wingtips in the profile screenshot appear to have a higher angle of incidence than the wing roots (where I measured the AoA). My prior understanding of 'washout' was that the tips always had a lower angle of incidence than the roots, in order that the root would stall first and provide a buffet. Since I apparently have this backwards, could someone explain how this works on the RW P-51? Be gentle, I probably don't know enough to understand your explanation.

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Sorry didn't have the right word for it, but I meant the idea that the chord does not change through the length of the wing... And it doesn't I can send you a graph that I found for the wing that shows it is constant.

Can you please explain your logic with the angle offset for the prop as I don't quite understand..


Regarding your comment about VNE.. I don't quite understand how engine, prop and other powerplant factors would change the VNE... Please explain...

Thanks

JB

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Buzz313th
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I have a graph in a test manual that shows that the AOI is consistant through the wing length...

Heres the link page 6...

http://www.google.com/url?sa=t&rct=j&q= ... bPMIQ1OdOQ

CL vs alpha graph.

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Okay, I see that graph. So what are we seeing in the profile picture between the root and the tip, the one annotated by me with the angles? Maybe the screenshot isn't a high enough resolution to make out the angle of attack at the wingtip, but it certainly looks to the eye that it is higher, and straighter with the horizon than the root. That baffles me a little bit. I'll say it again, I have no background in aero eng, so I'm in a perpetual state of learning, and this has been bugging me a bit.

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I suck at reading graphs like these, but I checked out that link for curiosity sake (since I value the accuracy of my P-51 ), went to page 6 of the slide which displays all the graphs, and since this whole topic is about angle of attack, the first thing that stood out for me in that graph is how the dots on that angle of attack chart is all in the negative. Now, I'm reading this graph without any context, and I have no idea exactly how the values on the graph are generated, but could this have any relevance to what is being discussed here about a negative AOA at cruise?

In my car so my reply will be brief.

Cruise CL is 0.2. Which reads on the graPh as ~ +0.5 aoa. And max CL before the wing stalls is 1.6 or 1.5 forget. Hump of the curve represents best lift for given aoa at ~ 15 degrees aoa. The curve descends representing the beginning of the stall and buffet.

X axis is aoa and y axis is coefficient of lift.

Hope that helps. By the way since the data points are all on the same x axis it means that the tip and the root has the same Aoi

And by the way. This graph represents modern day P-51s with fuselage tank removed. So we can't use it for our aoa discussion since a2a p-51 has the fuselage tank.

Lol one more note. I found CLcruise in other documents to say 0.3 - 0.5 and we have no idea what load config the CLcruise of 0.2 calculated at.

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Buzz313th just died because he was texting and driving...

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Oh right, thanks for clearing that up. I understand now



Oh wait, hold on a minute. Okay, maybe I don't fully understand, since I'm not quite getting that ~ +0.5 AoA you arrived at (though I did follow you up to the hump, which indicates buffet and stall). Looking at "CL vs Alpha" graph, if Cl Cruise is 0.2, then reading 0.2 in the Y axis puts AoA at pretty much close to 0. Then, referencing the third graph, the points there are in the 0 to -0.1 range (implying possible negative AoA)? Also, while the graph is based on a modern P-51 with the fuselage tank removed and is therefore irrelevant, I was still just thinking: A rear fuselage tank would put the P-51 tail heavy, no? (hence the nose down trim for take-off). Wouldn't this require a more pitch down attitude (an even lesser angle of attack) then to keep the thing level? Sorry if I'm all over the place with this. It's just that for some reason, those graphs make me feel that a negative AoA seems somewhat plausible, even if it's based on a modern P-51.

Don't read this while your driving by the way, lol.

LOL, ok at the comp and not driving...


Looked at the graph again and it does infact look like cruise at 0.2 CL is at zero aoa. Still better than the -1.67 degrees. I wish there was more info with the graph to establish what factors helped to calculate CLcruise. After many hours searching the web that one graph is the only place I was able to find any mention of the Coefficient of lift durring cruise (CLcruise) for the P-51. I was infact able to find numerous pages from flight tests that have CLmax, which makes sense, since they designed this plane to be a dogfighter, but any mention of CLCruise is limited to that one paper. I still find it hard to believe that the P-51 can maintain level flight, under a typical weight at a cruise power setting with 0 AoA. But it is still alot more believable than -1.67.

In regards to the third graph... This is the amount of pitching moment (torque) or pitching moment coefficient (CM) the airfoil (wing) exhibits at certain AoA. This would help define dynamic longitudinal stability and this force would need to be countered by the horizontal stabalizer and elevator. This would have a very small effect on cruise AoA, so you can ignore that graph.

In regards to with or without the fuselage tank....

If aircraft weight and airspeed remains a constant, then by moving CG foreward (within the CG limits), will make cruise AoA increase and by moving it rearward (within the CG limits), will make cruise AoA decrease. But also, by increasing aircraft weight without moving CG, will make the cruise AoA higher. So, depending on where the fuselage tank sits along the Longitudinal axis in relation to the aircrafts empty CG and the wings Aerodynamic Center Of Lift will determine how much difference the fuselage tank will make to the planes AoA at cruise.

Ouch that hurt...

Edit....

I still didn't answer your question about the fuselage tank... If we had the weight and ballance worksheets for the P-51 with the length that the fuselage tank sits along the longitudinal axis from the CG, then we would have the arm, with that we could determine the weight of the tank filled and empty and then we could come up with the moment of inertia for a full tank, an empty one and no tank and calculate the effect it would have on the AoA.

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Buzz313th
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Have you noticed on the screenshot, on which I've drawn the angles, that the wing root is at a lower angle of attack than the wingtips. I understand that to be "washout". This doesn't make sense to me, because 1) your chart indicates that there is no washout, and 2) The washout occurs opposite of what I expected. I thought that typically planes were designed so that the tips were at a lower angle of attack than the root, so that the root stalls first; but on the this P-51, it appears to be opposite. What's the right answer here?

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The chart indicates that there is no change in AoA from root to wingtip, but the chart does in fact show the wingtip to stall before the root. At about 9 degrees alpha the tips lose lift and continue to drop off while the root continues to increase lift until about 15 degrees alpha where the onset of the stall happens.

The chord length at the root is alot longer than the chord length at the tip. This is the primary reason why the tip will stall first.

The P-51 wasn't designed to train people how to fly. It wasn't designed to carry passengers. It was designed to fly long range and dogfight when it got there. There must be some advantage as to why the design of the wing allows the tips to stall first. Maybe it helps reduce induced drag durring high alpha?

There is a ton of info on the NACA 6 series airfoils and more specifically the NACA 45-100 as you mentioned. Have a second look and see if you can find any twist to the length of the wing.

JB

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Buzz313th
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Well, the internet is full of anecdotes. I've come across 1 or 2 unreliable "internet discussion" type resources that indicate the Mustang had a 2 degree washout, but I can't confirm that anywhere. Maybe you're right and it didn't have any at all. On the A2A model, though, like I said, that doesn't look like Washout to me, it almost looks like "washin"...(that isn't a term I don't think), because the wingtip appears to have a higher AoA than the root. Is that correct? What do you see when you look at the screenshot?

EDIT: Okay here is what I found. As far as I know it might be complete Bullsh88t

"The point of maximum thickness is at about 50% of chord. The P-51
wing is quite thick and because its point of max thickness
is at nearly 50% of chord if has quite a lot of volume. In particular
the wing tips are even thicker by WW2 standards. Although developed
by computation the P-51 wing has almost the exact profile of an
Albacore Tuna, including the compound reflex curve of the tail fin,
when the tuna is viewed from above. The wing has -2 degrees negative
washout at the wing tips to ensure they stall last."

-Some Guy on the internet
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So am I seeing the opposite on the A2A model? Looks like the twist from root to tip is upwards, not downwards like the above "some guy" quote indicates. Hmm..don't know what to believe anymore...

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1)i7 980x 4.35 ghz, gtx 470s SLI, Matrox th2go, Creative x-fi
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When I look at the Screens, it does appear there is a very small difference, but then again, it's a screenshot and not the best to be honest.

Hey Spyder, or anyone else for that matter.. Wanna fly the 51 online for an hour or so?

We can meet on the Digital Themepark server and do a quick flight.. My brain hurts and I need an outlet..


Edit... Your quote of the guy on the net is interesting. So his washout opinion directly conflicts with the Alpha vs CL graph.. I mean the Graph shows that the tips lose lift first.

2nd Edit.... I thought about it for a little and realized that the most important thing for a dogfighter is that it is able to create as much lift as possible with as little drag as possible within the highest range of alpha right? So then why would you design a dogfighter so that a portion of its wing looses lift prematurely? You wouldn't, you would design the wing to hold onto that lift as long as possible. We are trying to design a fighter that outperforms it's enemy first and foremost. If the the plane departs abruptly then we will train the pilots to recover.

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Buzz313th
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