P-51 drag tests. Lots of good stuff here. Note that this also used a model in a wind tunnel but run at the same velocities (or more specifically Mach numbers) as the full size test aircraft that it was compared with.
Check out the photo's - the full-size one was the world's fastest air-launched glider (until the Shuttle, I guess)
Also, a Cd 0f 0.022 is a lot higher than Mustang fans tend to quote. Pretty much what Creaking Door was saying. Of course, this was engine-off - so no Meredith effect!
Of course, Cd is dimensionless - so the size of the model surely doesn't affect it? It just affects the force that the drag exerts. I guess in drag modelling the problems CD highlighted with the velocity squared rule don't apply if all you are after is the drag co-efficient.
The squaring adjustment happens in the squared relationship change in reference area between model and full size - and that is what this apparently arbitrary element is actually for!
By: Graham Boak
- 26th June 2015 at 14:49Permalink- Edited 1st January 1970 at 01:00
Despite your doubts, the performance can be and was calculated to a useful accuracy from drag and lift coefficients - or equivalent methods such as D100 as mentioned above. The basic equations of flight go back to WW1 or earlier. No, they weren't as good at it then as they are now, but it still wouldn't do to underestimate their capabilities. Most of the problems, then and now, come from allowing the correct values for engine power/thrust, intake efficiencies, and unexpected interactions. (And investigating the last is perhaps where this started?)
You are right about the scale speed problems in wind tunnels: look under the term "Reynolds Number". However, this is only useful below high subsonic Mach numbers. The German supersonic wind tunnels were indeed run to investigate supersonic flight - the drag increase due to compressibility, and the movement of shock waves, made them totally useless for subsonic work. How many of them were capable of anything more significant than research work into configurations is another matter - we are talking about very small tunnels.
I suspect the P-51 results you are referring to are those done in the Langley full scale tunnel - but certainly not based on Mach numbers as the Langley tunnel was unable to reach anything significant (AFAIK). The Meredith effect is basically just flow through ducts - it would be present with or without propwash. The magnitude might be different, but it would not disappear. The Spitfire and Bf109, and indeed many other aircraft, also used the Meredith effect in their radiator systems.
"The squaring adjustment happens in the squared relationship change in reference area between model and full size - and that is what this apparently arbitrary element is actually for!" Sorry, I've no idea what you mean by this.
By: Beermat
- 26th June 2015 at 15:38Permalink- Edited 1st January 1970 at 01:00
Graham, I worry that something on the general discussion board has made you oppositional to me
I suspect the P-51 results you are referring to are those done in the Langley full scale tunnel - but certainly not based on Mach numbers as the Langley tunnel was unable to reach anything significant (AFAIK)..
No.. please read the report that I referred to and linked to before telling me what it contained and that I am wrong about it. It discusses Mach when describing matching the conditions of the test tunnel - at Ames, not Langley - and the full-size tests. A bit of reading around shows that it was known then that matching Mach was important, and not airspeed.
The Meredith effect is basically just flow through ducts - it would be present with or without propwash. The magnitude might be different, but it would not disappear. .
It would if the air was not heated by the radiator (the effect doesn't tap into a magic source of energy). As in this case, because the engine is off.
"The squaring adjustment happens in the squared relationship change in reference area between model and full size - and that is what this apparently arbitrary element is actually for!" Sorry, I've no idea what you mean by this.
Look at the equations a bit longer (I had to stare until my forehead bled), think about how the units cancel out to make the result a dimensionless co-efficient and you'll see it. Basically I was going full circle and realising the dimensionless nature of the coefficient meant that the dimensions of the body it applied to were irrelevant in deriving it, but doing it the hard way.
Here is it put another way, in "The Peenemünde Wind Tunnels: A Memoir" by Peter P. Wegener (which talks about the Mach4 tunnels):
[ATTACH=CONFIG]238675[/ATTACH]
The force varies with size - but the Cd doesn't. It doesn't because the reference area is scaled, cancelling out the change in force in the calculation.
By: Beermat
- 26th June 2015 at 16:55Permalink- Edited 1st January 1970 at 01:00
Or to put it yet another way, the way to scale up from the model forces to derive the full size ones is to scale up the reference area from the model's wing area to the full size aircrafts wing area, keeping everything else in the equation (including Cd) the same. I have arrived at what I was trying to say at last - sorry it was a bit 'round the houses' if only to get to something you already knew!
I am aware that there are slight qualifications to this.
Posts: 3,447
By: Beermat - 26th June 2015 at 12:49 Permalink - Edited 1st January 1970 at 01:00
Me too.. but this is FASCINATING. My best Google find yet - http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CCoQFjAB&url=http%3A%2F%2Fwww.dtic.mil%2Fcgi-bin%2FGetTRDoc%3FAD%3DADB804999&ei=vzmNVefzOebT7Qb8voOoBw&usg=AFQjCNHg09CjCoP4Yy5fgLoCDpwqdMXchw&sig2=q8GcyAzIZMTudrrOwEsT9Q&bvm=bv.96782255,d.ZGU
..you have to download from here, I can't find a better way of doing this.
P-51 drag tests. Lots of good stuff here. Note that this also used a model in a wind tunnel but run at the same velocities (or more specifically Mach numbers) as the full size test aircraft that it was compared with.
Check out the photo's - the full-size one was the world's fastest air-launched glider (until the Shuttle, I guess)
Also, a Cd 0f 0.022 is a lot higher than Mustang fans tend to quote. Pretty much what Creaking Door was saying. Of course, this was engine-off - so no Meredith effect!
Of course, Cd is dimensionless - so the size of the model surely doesn't affect it? It just affects the force that the drag exerts. I guess in drag modelling the problems CD highlighted with the velocity squared rule don't apply if all you are after is the drag co-efficient.
The squaring adjustment happens in the squared relationship change in reference area between model and full size - and that is what this apparently arbitrary element is actually for!
Lightbulb moment.
Posts: 957
By: Graham Boak - 26th June 2015 at 14:49 Permalink - Edited 1st January 1970 at 01:00
Despite your doubts, the performance can be and was calculated to a useful accuracy from drag and lift coefficients - or equivalent methods such as D100 as mentioned above. The basic equations of flight go back to WW1 or earlier. No, they weren't as good at it then as they are now, but it still wouldn't do to underestimate their capabilities. Most of the problems, then and now, come from allowing the correct values for engine power/thrust, intake efficiencies, and unexpected interactions. (And investigating the last is perhaps where this started?)
You are right about the scale speed problems in wind tunnels: look under the term "Reynolds Number". However, this is only useful below high subsonic Mach numbers. The German supersonic wind tunnels were indeed run to investigate supersonic flight - the drag increase due to compressibility, and the movement of shock waves, made them totally useless for subsonic work. How many of them were capable of anything more significant than research work into configurations is another matter - we are talking about very small tunnels.
I suspect the P-51 results you are referring to are those done in the Langley full scale tunnel - but certainly not based on Mach numbers as the Langley tunnel was unable to reach anything significant (AFAIK). The Meredith effect is basically just flow through ducts - it would be present with or without propwash. The magnitude might be different, but it would not disappear. The Spitfire and Bf109, and indeed many other aircraft, also used the Meredith effect in their radiator systems.
"The squaring adjustment happens in the squared relationship change in reference area between model and full size - and that is what this apparently arbitrary element is actually for!" Sorry, I've no idea what you mean by this.
Posts: 8,505
By: mike currill - 26th June 2015 at 15:04 Permalink - Edited 1st January 1970 at 01:00
My brain hurts:)
Posts: 3,447
By: Beermat - 26th June 2015 at 15:38 Permalink - Edited 1st January 1970 at 01:00
Graham, I worry that something on the general discussion board has made you oppositional to me
No.. please read the report that I referred to and linked to before telling me what it contained and that I am wrong about it. It discusses Mach when describing matching the conditions of the test tunnel - at Ames, not Langley - and the full-size tests. A bit of reading around shows that it was known then that matching Mach was important, and not airspeed. It would if the air was not heated by the radiator (the effect doesn't tap into a magic source of energy). As in this case, because the engine is off.
Look at the equations a bit longer (I had to stare until my forehead bled), think about how the units cancel out to make the result a dimensionless co-efficient and you'll see it. Basically I was going full circle and realising the dimensionless nature of the coefficient meant that the dimensions of the body it applied to were irrelevant in deriving it, but doing it the hard way.Here is it put another way, in "The Peenemünde Wind Tunnels: A Memoir" by Peter P. Wegener (which talks about the Mach4 tunnels):
[ATTACH=CONFIG]238675[/ATTACH]
The force varies with size - but the Cd doesn't. It doesn't because the reference area is scaled, cancelling out the change in force in the calculation.
Posts: 957
By: Graham Boak - 26th June 2015 at 16:12 Permalink - Edited 1st January 1970 at 01:00
I beg your pardon, I was thinking of the full-scale tunnel where there'd be no means of differentiating between velocity and mach.
Posts: 3,447
By: Beermat - 26th June 2015 at 16:21 Permalink - Edited 1st January 1970 at 01:00
Ah yes, I see - thanks Graham. I'm new to this.
Posts: 3,447
By: Beermat - 26th June 2015 at 16:55 Permalink - Edited 1st January 1970 at 01:00
Or to put it yet another way, the way to scale up from the model forces to derive the full size ones is to scale up the reference area from the model's wing area to the full size aircrafts wing area, keeping everything else in the equation (including Cd) the same. I have arrived at what I was trying to say at last - sorry it was a bit 'round the houses' if only to get to something you already knew!
I am aware that there are slight qualifications to this.