Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Ex-jazz on September 14, 2009, 02:32:21 PM
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Hi
I try to understand the airplane total drag issue. Yes, I know, it is a very very complex area.
Is there a real life data available about the P51D minimum frontal area in 'clean condition' and controls in neutral position?
According to my own measurements, the minimum frontal area is a 4.43m2 (47.68ft2), between the ~ -2.5 – +3.5 degrees AoA range.
I would like to know, if my measurements are in same ballpark with real life data at all.
Thank you
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Some data:
P-51B: f:4.61ft^2, Wetted area:874.0 ft^2, CDswet .0053
P-51D: f:4.65ft^2, Wetted area:882.2 ft^2, CDswet .0053
Source: EAA january 1999, David Lednicer
-C+
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Pretty good numbers there Ex-Jazz...
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Hi
I try to understand the airplane total drag issue. Yes, I know, it is a very very complex area.
Regarding that, frontal area has very little direct effect on the drag.
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Regarding that, frontal area has very little direct effect on the drag.
Your joking right?
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Thank you for your feedback.
I found an rounding error from my code and after the fix the rerun give a 4.77m2(minimum) @ 0.25 degree AoA.
Charge,
Your figures must be in m2, not in ft2, or those figures must have then some kind of adjust parameter in it.
bozon,
If I understood right, the form drag is very must up to the frontal area. The frontal area is easy to measure, but define the whole plane Cd would be a major PITA.
(edit)
My measurement figures are from necessarily not so accurate P51 3D model... So, be gently :)
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Thank you for your feedback.
I found an rounding error from my code and after the fix the rerun give a 4.77m2(minimum) @ 0.25 degree AoA.
Whoops! Just caught that your original numbers were in meters instead of feet. May be a problem with that 4.77m^2 figure.
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Ex-jazz is probably calculating the actual projected frontal area. I bet the 4.6 sq ft number is the equivalent flat plate drag area. Obviously a P51D viewed from the front blocks more than a 2' x 2.3' rectangle but, because it's streamlined, it probably has the equivalent drag of a flat rectangle that size???
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Hi
I try to understand the airplane total drag issue. Yes, I know,
When you get it figured out, fill me in will you? ;)
HiTech
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Your joking right?
Not really.
Drag is not caused by pushing of the air around the object in a non-rotational way (flow lines not forming circles). Drag is caused by depositing some of the flow kinetic energy into rotational flow. If no circulations are created, the object will leave the airflow behind it undisturbed from how it was before it - that means no energy was lost or gained, or in other words, no drag no matter how large your frontal area is.
Large frontal area means one needs to push more air around the plane, but once you are in a steady state, no more energy is required. Accelerating will require more kinetic energy in the flow around the plane, so there is energy input here, but in air (as opposed to lets say, water) this accelerating resistance is small - of the order of the acceleration times the mass of air in the volume of the object. That would be very small in air since the air density is so much smaller than the average plane mass density. But even this is more of a volume thing than just frontal area.
The SHAPE as opposed to the area of the front has a lot to do with drag. This is what causes the flow to break into part laminar, part turbulence and part ordered rotational flow. What happens at the trailing edge often has a much larger impact on the drag than what happens at the front. In the front, there will always be built a pressure-front that will divert most of the air around the body and into large scale flow, where dissipation is minimal. On the trailing edge, there is no more the barrier of the solid object. Large scale disconnected flow from above and below (or sides) of the object suddenly meet and breaks into rotational flow in all scales - turbulent wake. This is the largest source of drag, assuming the object surface itself is not full of irregularities to produce small scale turbulence while the air is flowing across.
People tend to think that radial planes are less aerodynamically efficient than inlines, because they have a larger frontal cross-section. The real reason is that radials have an aerodynamic inefficient SHAPE to their front because they need to get the air into the engine to cool it, instead of diverting the flow around the body. Inline engines do not need to do that, but instead have to stick a radiator into the airflow that produce a lot of drag. It is easier to design an aerodynamic efficient radiator that will give some of the lost energy back by heating and accelerating the air out of the radiator than it is to control the flow of air around an engine and out through the fuselage to get the same effect. Good design of the engine cowling can dramatically improve the situation for radials. The XP-47J is such an example and is claimed to break the 500 mph. It was almost 40 mph faster than a regular jug at the same engine power (military). The biggest difference is a redesigned cowling. The total frontal area could not have changed much as it is limited by the size of the radial engine.
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Large scale disconnected flow from above and below (or sides) of the object suddenly meet and breaks into rotational flow in all scales - turbulent wake. This is the largest source of drag, assuming the object surface itself is not full of irregularities to produce small scale turbulence while the air is flowing across.
So the biggest cause of turbulence is the jump discontinuity in the velocity vectors of the airflows which meet at the trailing edge?
Sorry, my brain is going nuts with words like "Green's function" and "delta function". :lol
Actually, from what I remember, isn't high Reynolds number the primary indicator of turbulence? And roughly Googling wikipedia, Reynolds number is not proportional to area - ok props to you bozon. I assume Reynolds number and the jump discontinuity are related somehow, and I've forgotten how.
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The Reynolds number is L*v/nu:
L = scale length. Typical size on which the flow varies
v = flow velocity
nu = viscosity (per unit density)
The origin of this dimensionless parameter is the ratio between the convective term and the dissipative term in the flow equation. Simply put, it tells us what is more important on the scales L we are interested in - changes in the local flow because it moves, or because it dissipates. High Ry number indicate that the convection dominates, so a vortex created in the flow will survive past the length scale of interest - you will not see it decay (dissipate) while it passes. This is why high Ry numbers are usually associated with turbulence - disturbances to the flow (vortices) will not decay. Try to stir your tea - it will continue rotating for a short while. Now try stirring maple syrup, it will stop immediately. On the other hand, high Ry numbers have no source term for turbulance and will not generated any either.
To generate vorticity you need small Reynolds numbers where the viscosity can come into play. This means places where the flow changes sharply on small scales. This is where turbulence is both generated and dissipates quickly. It could be because the flow is forced to change - like around a corner of a solid boundary surface or a sheer in the flow, like where the flow from around the object meet at the other side. It is easy to imagine all the corners and small turns the air has to do as it goes through a radial engine and this is why it is wasteful. The whole cowling area is a "corner" for the flow, some goes in some need to go around it.
The black magic of flight is the ability to generate and maintain ordered, large scale vorticities and minimize the turbulence. The difference between the two is that turbulence has a spectrum of length scales for the vortices down to very small sizes. The energy is dissipated fast in the small scales and the large ones break into new small vortices etc etc. While the flow around the wing for example has a big component that circles the entire wing and another, counter rotating big one at the trailing edge. However, this structure, even though it dissipates energy relatively slow, must still be maintained by injecting more vorticity. Therefore one has to pay for the lift by some drag. No drag - no lift. It always amazed me how gliders can stay up with so little energy required vs. what a similar mass would need in order to stay up by rockets blasting downward.
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When you get it figured out, fill me in will you? ;)
HiTech
Haha! :D
Of course I laugh because I know exactly what you mean!
Tango
412th FS Braunco Mustangs
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Ex-jazz:
To summarize what bozon is talking about, don't think frontal area with regard to drag. Infact in most cases the frontal area concept is a conversion method to try and provide a means of drag comparison between unlike things by comparing them all to a theoretical flat plate, not actual "frontal area".
Instead think of adverse pressure gradients across the objects in question. Resulting boundary layer separations due to adverse pressure gradients means bad juju and drag. It gets a gazillion times more hairy because all this actually happens in 3D involving formations of 3D vortices like bozon mentions and the like.
Tango
412th FS Braunco Mustangs
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An then you do simple little things like put 1/2 a wing tip in your cowl out let to remove the turbulence that has been vibrating your feet for the last 8 years, and pick up 3 Knots.
HiTech
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You guys make me wish I had been playing this game about 8 months ago when I was actually taking a fluids class. Would have made it a lot easier to understand. :salute
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The frustration... Oh mine.
I just assumed, I could guesstimate total drag at least in some level with projected frontal area data, but this aerodynamics... It's a bottomless swamp of mind pending equations & formulas... :rolleyes:
I put that darn drag thang now aside...
Anyhow, I still would like to know, if my calculator is working so, any idea about that P51D projected frontal area?
Thank you for your informative comments.
Hitech, sure I will :D
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I assume your 3D modelling program will compute surface area of the model? If so, the first thing you could check is the total area of the model versus the wetted area numbers that Charge posted?
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An then you do simple little things like put 1/2 a wing tip in your cowl out let to remove the turbulence that has been vibrating your feet for the last 8 years, and pick up 3 Knots.
HiTech
:lol
Your version of removing radiator duct rumble eh?? Very cool.
Tango
412th FS Braunco Mustangs
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I assume your 3D modelling program will compute surface area of the model? If so, the first thing you could check is the total area of the model versus the wetted area numbers that Charge posted?
The given P51D 3D model wetted area is 82.6m2 (889.1ft2). I guess, it's ok then :)
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Good! That means the model is configured pretty well. Knowing the wetted area of the aircraft allows you to do the most accurate drag approximations. Just remember that they're approximations only. Even CFD software is only "predicting" drag.
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Ahh physics and applied physics. Everything's an approximation. Taylor series anyone?
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Ahh physics and applied physics. Everything's an approximation. Taylor series anyone?
Everything is an approximation. Roughly speaking.
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Everything is an approximation. Roughly speaking.
Monitor spew.
In theory there is no difference between theory and practice. But, in practice, there is.
HiTech
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This is why I laugh at X-Plane's claims about the realism/accuracy of its "blade-element" flight modeling.
Even with the most powerful computers in the world using the best modeling techniques available, the computed results can be radically different from the predicted results.
It is a science where a 20% error between a predicted coefficient and the actual value is considered pretty close despite the fact that a 5% error can mean the difference between flyable and fatally unflyable.
Using simplified approximations is an absolute necessity and X-Plane's editor that estimates flight performance based on the geometry of the 3-d model is pretty cool, but I prefer empirically derived data from the actual aircraft any day, with scaled down wind tunnel results taking a close second.
None of the calculated models available to the public on PCs can model the XB-70 to any useful level, but NASA has tons of charts available on the web depicting nearly all of the parameters (stability co-efficients as well as basic lift, drag, etc.) that permit detailed models based on tables of emprical data to capture a reasonable approximation of the XB-70 from stall speed to Mach 3.
Of course, when insufficent empirical data is unavailable, what choice do you have besides mathematical approximations?
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Of course, when insufficent empirical data is unavailable, what choice do you have besides mathematical approximations?
Tell this to the guy in the flap deployment thread... :)