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I would fly on line with you, but I have never done that before and have no idea how to do that. I'll have to figure out how and give you a raincheck on that

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1)i7 980x 4.35 ghz, gtx 470s SLI, Matrox th2go, Creative x-fi
2)i7 2600k, Gigabyte z68x, gtx 285s sli

Well, I'm pretty sure that the Spitfire does have washout, and it allows you to retain aileron authority at low speed. Plus, if the root starts to stall before the tip, you have time to do something about it before falling off into a spin. Beyond that I don't know because I don't know too much about wings.

I'm sure somebody like Mitchell will be able to tell us what the story is on the P-51 wing regarding this. Right now though, we seem to have conflicting information, and I'm going to be very curious what the right answer is.

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1)i7 980x 4.35 ghz, gtx 470s SLI, Matrox th2go, Creative x-fi
2)i7 2600k, Gigabyte z68x, gtx 285s sli

Hey Buzz,

I was just playing around with GIMP and doing some test flights (mostly because I have no life ). I had a look again at that screenshot that read 1.62 degrees negative and discovered that the angle isn't in actuality that large. The reason why it was reading 1.62 degrees was because the measurement was taken at an arbitrary point from behind the tail and above the trailing edge of the root of the wing. I remeasured it in GIMP and it turns out between -0.44 to -0.87 degrees from your screenshot, depending on which pixel I set my measurement tool on (GIMP measures angles from a straight horizontal reference point, which, if the screenshot was taken in spot view, is parallel to the horizon).

I made a flight myself without the fuselage tank and came closer to 0 at 10,000 ft ASL with a power setting of 35" and 2300 RPM. Adding the fuselage tank in flight, it didn't seem to make much of a difference though for a moment, it did lower the nose with autopilot on. At 30,000 ft ASL, it was a hair higher but still for the most part, 0. I can post my own screenshots if required.

They are really small numbers, and I think it really depends on what camera angles we are looking at the airplane. If the real P-51 does maintain level flight at values so close to 0, then this may be a non-issue (being a fighter, this certainly seems possible. The thing behaves like a dart). If it doesn't, well, a fix may be in order, but it depends on how much the gap differs. A lot of this has to do with how much lift those wings actually generate at a reference angle to relative wind. Even if the angle is a hair negative, it might still be able to generate enough lift to keep the plane level. Just some thoughts. It might still be an issue.

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The thrust line of an engine (and thus the prop) on a piston powered aircraft is very rarely directly in line with the rest of the airplane. Most have an offset slightly to one side and down. The left/right offset is determined by which way the engine/prop spins and is opposite of the direction of spin. This offsets both P-Factor and some torque. The down pitch offsets that most of the time the engine is mounted above the exact centerline of the aircraft. Thus, the "down" component offsets the tendency to pull the nose up. The P-51's engine is mounted with such an offset. Thus, when flying in a "level" attitude, the prop disk will appear to be down and "cockeyed" when viewed edge-on.

However, the amount of offset is determined not by having neutral forces at any one given situation (or phase of flight) but rather to enhance/maintain controlability when at high power settings.



The way I understand it with the Convair 240 in particular is that the VNE is reduced at certain altitudes because of some sort of harmonic or other interference that is created that the Augmentors can be damaged in a certain altitude range at a speed lower than structural VNE. As such, they reduced the VNE through that range. Also, use of anti-icing equipment can have an effect on VNE, especially if there are components that inflate or have ports that open. This can also result in VNE for certain altitudes being reduced. I've noticed on several older British types over the years have different VNE's as altitude increases, although I'm not aware as to why they changed it as it's listed as VNE not VNO/MMO.

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I stand by my figure of -1.6 degrees for the picture in question, and everybody can see that the lines I drew on the picture are geometrically sound.

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1)i7 980x 4.35 ghz, gtx 470s SLI, Matrox th2go, Creative x-fi
2)i7 2600k, Gigabyte z68x, gtx 285s sli

BTW, I just got a note from a reliable source and it was confirmed that the Angle of Incidence of the wing on the P-51 is variable along its span and has an average of ~1*, not 2.5 as previously reported. This confirms that there is washout of some level in the wing as the AoI wouldn't be variable if there wasn't some twist to the chord relative to the Centerline of the aircraft.



I have to come back to this quote because I couldn't find it when I was putting up my last one, but I do need to address this.

1) The Vought Corsair (F4U) did not have an adjustable wing for Angle of Incidence.
2) The Vought Corsair II (A-7) also did not have such a capability.
3) The Vought Crusader (F-8) has a pivoting wing which effectively changes the AoI for the wing, but has only 2 positions (up or down) and was simply used to allow for the low-slung fuselage to remain nearly level during approach to give sufficient visibility to the pilot and reduce the chance of a tail strike due to it's very low ground clearance. It could not be used in any situation other than approach. One of the prototypes that did not have the interlock installed was lost on a demonstration flight when the wing began and uncommanded pivot during a high speed pass.

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I think A2A is finally going to Clobber me once and for all, but I'm on the verge of saying that they modelled their tip washout upside down in the 3D model. I kid you not. Look at the angle of Incidence at the tip versus the angle of Incidence at the root in the Buzz's screenshot. I'm -not- saying that, yet, but that is where I'm leaning towards with this discussion...Somebody explain to me why the tip appears to have a higher angle of incidence than the root when it should be lower, by definition of "washout". Am I just seeing things or misunderstanding the definitions?

<running away as fast as I can>

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1)i7 980x 4.35 ghz, gtx 470s SLI, Matrox th2go, Creative x-fi
2)i7 2600k, Gigabyte z68x, gtx 285s sli

I'm gonna make this brief.

RocketBird,
The screenshot that was measured is definitely more than -0.44 degrees and it looks more like -1.7 like Spyder calculated. I value your opinion and if you choose to post screens go ahead, but make sure you include all the info and reference Indicated Airspeed and weight.

CapFlyer,
With all due respect, I really don't see how your explanation of the prop angle makes any sense. Any prop angle other than perpendicular to the relative wind on a single engine aircraft will create a yawing moment at high power settings due to P factor. And I have never seen a prop engine tilted purposely to compensate for it being higher or lower than the CG of the aircraft. Now if your aware of a specific aircraft that has a variable VNE because of deice equip or has a specific flux generator that when turned on requires a lower VNE then fine. I have never heard of an aircraft that has a variable VNE. THE P51 is the subject here and it has two limiting speeds. One is VNE and the other is VMO. VNE is not variable on the airspeed indicator and VMO is.

Apparently none of us can definitively answer the simple question of what a P51 should look like in cruise and for good reason...... Because we have never flown one. So I am often at Van Nuys airport as I have a freind of mine whom gives me time in his S2B about once every two weeks since I gave him time years ago in my Cherokee 140 when he was putting together his private and instrument. There are two privately owned P51s that I know of down there, one of which was sitting in Clay Lacys hanger. I'm gonna print these pics and show em to someone who has lots of time in the 51 and see what he says.

I appreciate all the comments and participation and if anyone feels a little turned off by this post, then please blame me, I'm the one who started this thread.

PS, Dudley has tons of hours in the 51. Why hasn't he chimed in? We coulda had an answer yesterday.

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Buzz313th
FASA Founder and dedicated Staff Member
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CapFlyer,

When I meant the Corsair, I meant the fighter version of the A7 as I have seen it landing with the wing tilted out of the root. Sorry I wasn't specific enough, but it shoulda been obvious. Glad you guys found info on the twist of the wing of the 51.

I know A2A has the resources to correct the issue (if there even is an issue) of the attitude of which the model apears to fly in FSX.

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Buzz313th
FASA Founder and dedicated Staff Member
TeamSpeak 3 @ "ts3.digitalthemepark.com"
Facebook @ http://www.facebook.com/FsxAirSportsAssociation
Website @ http://www.fsxairsports.com

Nah, don't worry about that, I'm sure they're having a similar discussion behind the scenes, albeit one with much more documented information, and they're getting their facts straight. They'll respond with the various answers about AoA, washout, and all the rest soon, I would imagine. Afterall, even Mitchell responded, and when he responds, A2A means business

The funny thing about a discussion of this nature is that I often times ask myself "do I really care if the pitch angle turns out being off by a degree or two?" I think the answer to that is NO, but I really enjoy the discussion part of all this. By debating over what amounts to functional minutia, I'm learning so much more of the larger picture...

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2)i7 2600k, Gigabyte z68x, gtx 285s sli

Nothing against you spider, but the problem with your methodology in measuring that -1.62 angle is that you drew your diagonal line way out from tail. Geometrically, you CAN do this, but the accuracy depends strictly on how closely your line actually falls on top of the chord of the wing. In the case of GIMP, you can achieve much greater accuracy if you simply drew a line right from the trailing edge to leading edge, since it measures based on the horizontal.

This is the same screenshot from Buzz that you used to measure the line. I have remeasured it in GIMP. If I move the reference point by 1 pixel, and 1 pixel alone, the result is -.44 degrees. The angle is referenced at the bottom left corner of the capture:



Here is one I did at 30,000 ft, using 35" and and 2400 RPM, autopilot on.. I neglected to shift+z this, but hopefully by the height of the water appears believable. This was an attempt of copy, and I even used the same paint job.



Here is my result from 10,000 feet. Same power settings. I refilled the tanks at takeoff, with the fuselage tank removed. It carries a slight amount more fuel than the top, but everything else is equal.



One final thing, and this is a question for you Buzz: I noticed that you had two different altitudes in your screenshots over in page 3 of this thread. One was at 10,300, the other was at 10,100. 200 foot difference. Were you in a bit of a shallow descent on the 10,300? Did you use the FS autopilot to maintain altitude?

In any case, I wanted to point out something interesting too, which may or may not be relevant. Going back to the CL vs Alpha graph you posted, it appears that the airfoil maintains a positive lift coefficient with a negative angle of attack until about -2 degrees, even if it is not CLcruise. It may perhaps explain a few things (or it may not), but I thought I'd bring that to your attention. Personally, I'm much more interested in the plausibility of a P-51 maintaining lift at a slight angle below 0 than exact angles themselves.

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Then to be blunt, you've never seen a high powered piston-engined aircraft. The F4U has it, the P-51, the P-38, the SPITFIRE, The DC-3 even has a slight negative thrust line. That is why it is ALSO canted to one side and not just down. It offsets both the thrust line being higher than the centerline of the aircraft and the torque presented. P-Factor is offset by the fixed rudder tab and/or vertical stabilizer offset, which the P-51 has several degrees of, as does the Spitfire. The fact is that's what was done and why. I'm sorry if you don't understand it.



I guess I need to be clearer - there is NO FIGHTER VERSION OF THE A-7. The F8U Crusader came BEFORE the A-7 and is a totally different aircraft. There are similarities because Vought used the same basic fuselage, but everything else is different including the landing gear (taller and stronger), wing (different design without tilting mechanism, bigger flaps, stronger to carry hard points), fuselage (tail sloped differently to provide additional clearance, wider, and shorter by 8 feet overall), and engine (non-afterburning Allison TF-41 versus the afterburning Pratt J57 of the F8U). The difference may be obvious to you, but there are a lot of aviation history luddites (meant in the nicest way) on this forum and they don't know that there were 2 Corsairs or that there was even an F8U.

While washout typically means the wingtip has a lower angle of incidence than the root, it is not universal. However, I don't see this as the case with the screenshots shown elsewhere without the harsh shadowing that makes it difficult to determine the chord line of the tip in the screenshots here. I see a slight washout from root to tip. The problem is the wing/body fairing is huge and not proportional to the wing shape making it deceptive to the actual wing chord. However, the charts referenced before seem to indicate the tip will stall before the root, so it's also possible that indeed the washout is inverse to normal. Remember, the overriding desire of the North American team was creating the most maneuverable airplane possible, but also one with the longest range. This is why the laminar flow wing was so important and why so much was done to ensure that it was the overriding point of focus for maintenance in the field behind the engine. There were things done to the wing that were unique to the P-51 because it was also the only one using a laminar flow wing at the time.

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Personally, I really don't care if I mistaken the Vaught Corsair for the F8 Crusader. Lets just talk about the P-51 so we can stay on topic...

Now, lets leave out any twin engine aircraft and just talk about piston singles so we can stay relevent with the P-51 please.

I never said that there aren't aircraft with an offset thrust line, I have seen many. We got on this topic because I was pointing attention to the extreme down angle of the prop on the A2A P-51 in relation to the relative wind durring straight and level flight to hopefully point out that the model is indeed flying at the wrong pitch angle.

Secondly, I do understand and I understand fully, trust me.



Now, lets back to the task at hand. Correct me if I am wrong, but you are defending the the idea that the P-51 prop angle in level flight in the cruise regime should be pitched down against the relative wind? And you say this is true because the negative thrust line will help equalize aerodynamic forces caused by, A) the fact that the thrust line is above the CG and or Center of lift of the aircrfraft (Which would cause a negative pitching moment upon adding thrust and a positive pitching moment upon removing thrust) and B) because of torque along the longitudinal axis of the aircraft caused by the rotation of the engine at high Power?

Am I correct in the above assumptions?

If so, then can you please explain how the acute negative angle of the thrust line in relation to the relative wind will help the situation by referencing the picture below.

Edit.... And CapFlyer, to be 100% clear, I understand fully the idea of offset thrust lines, but what I am questioning here is that the A2A bird is flying with too excessive of a pitch angle in level flight. And to further defend that opinion, I am refering to the Very Excessive Negative Thrust Line we are seeing when measured against the relative wind durring visual evaluation of the A2A bird in level flight.Thanks

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Buzz313th
FASA Founder and dedicated Staff Member
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Facebook @ http://www.facebook.com/FsxAirSportsAssociation
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I just took a few more screens of the P-51 to better compare with the actual aircraft. Since so many people have seen videos of the 51 doing a high speed low pass at an airshow, then I figured I might as well include a representation of that with a light load which would represent a typical 51 flying a demonstration.

All screens were shot with barametric Altitude hold on and I tried to keep the tanks perfectly at 50%. I also tried to keep the speed up to represent what we might see in the one scenario that would show the least positive AoA hoping to compare it to the Zero Lift Line that the P-51 has of negative 2.0 degrees.

The Zero Lift Line can be found on the PDF link that I posted yesterday on page 6. Just follow the line from a zero Coefficient of lift.

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Buzz313th
FASA Founder and dedicated Staff Member
TeamSpeak 3 @ "ts3.digitalthemepark.com"
Facebook @ http://www.facebook.com/FsxAirSportsAssociation
Website @ http://www.fsxairsports.com

Just for fun, it's pretty close to negative -2 degrees. I need a life

(Assuming we are getting pretty close to 0 there, this is based on your quickest groundspeed)

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