Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: J_A_B on February 21, 2001, 08:19:00 PM
-
I see a lot of people comparing WW2 planes using "power to weight". But seldom do I see anyone use "thrust to weight" for a WW2 plane, yet TTW is a much more useful measurement.
Horsepower, in and of itself, means nothing. It is that combined with the propeller that moves the plane.
Take two version of the Spitfire 1--one has a fixed-pitch 2 blade prop, the other has a variable-pitch 3 blade prop. Do they perform the same? Obviously not--yet they have the same power-to-weight ratio.
Which brings me to my question: Exactly how much thrust do these planes produce? Knowing that figure would be much better for determing what kind of maneuvers they could perform.
J_A_B
-
thrust varies with speed, power does not
-
Well, the point here is still valid.
The airfoil design of the props used should have a big difference on the performance.
HP to WEIGHT ratios would be fine if all the planes used the same airfoil prop with the same number of blades.
------------------
"Wing up, Get kills, Be happy"
Midnight
13th TAS
midnight@13thtas.com
"I see you have made your decision. Now let's see you enforce it." -Brandon Lee (The Crow)
-
Thrust is a pain in the bellybutton to calculate for these planes. The best numbers anybody has are still going to include a lot of assumptions and estimates, i.e. guesses.
AFAIK there was not an in-flight test of any of these planes with strain gauge or other device on the prop shaft for a direct measurement. I've seen photos of ground tests of this sort but thrust at zero speed is not thrust at flying speed.
Even if you could actually figure out how much thrust the prop is exerting, there is still thrust from exhaust stacks which is significant on some of the planes.
-
A rough figure would be great.
If it is possible to test this using the planes in AH, thats fine--tell me how. As I said, I'm happy with a ballpark figure, and the AH planes are close enough to reality to satisfy me.
I'm just curious how these planes stack up in terms of total thrust.
In partucular, I am willing to bet that some planes with a high level of horsepower, actually have LESS total thrust than some less powerful planes--which explains some seeming anomolies of performance.
Knowing even a ballpark figure of TTW for these planes would give me a much better idea of what these planes could do relative to each other. It isn't what I think of as vital knowledge....just interesting to know.
J_A_B
-
A rough figure would be something like:
T=(0.8*hp*550)/(V*5280/3600)
T is thrust in lbs
hp is power in horsepower
V is true airspeed in mph
[This message has been edited by funked (edited 02-21-2001).]
-
Using that calc we get(at s/l top speeds)
P-51D: 1382lbs
Fw 190A-5: 1465lbs
Spitfire F.IX: 1486lbs
Spitfire VC: 1416lbs
Me 109G-10: 1622lbs
Me 109G-6: 1313lbs
Me 109G-2: 1301lbs
Me 109F-4: 1181lbs
Typhoon Ib: 1760lbs
F4U-1D: 1875lbs
La-5FN: 1542lbs
Hmmm, exactly how does the G-6 make more thrust with the exact same engine and prop as the G-2...
SO, some thrust:weight ratios then
P-51D: 0.137
Fw 190A-5: 0.170
Spitfire F.IX: 0.198
Spitfire VC: 0.218
Me 109G-10: 0.216
Me 109G-6: 0.188
Me 109F-4: 0.194
Typhoon Ib: 0.156
F4U-1D: 0.150
La-5FN: 0.213
So what does this mean - that the Spitfire VC should accelerate and climb better than the Me 109G-10? I knew it was porked! (http://bbs.hitechcreations.com/smf/Smileys/default/wink.gif)
[This message has been edited by juzz (edited 02-22-2001).]
-
Funked said:
>A rough figure would be something like:
T=(0.8*hp*550)/(V*5280/3600)
> T is thrust in lbs
> hp is power in horsepower
> V is true airspeed in mph
So:
.8*750=600(yes 750hp)*550=330,000
398*5280=2101440/3600=583.7333..=
330,000/583.7333= 565.32667 (rounded)
The numbers are for the S.A.I.207
This would give it almost no drag to hit 398mph??
??? Hell, I don't even do old math well :-)
Then again, it's after midnight :-)
------------------
M.C.202
Dino in Reno
-
565lbs is "almost no drag"?
BTW: There was a 1940 Russian fighter prototype with two 220hp engines that did 422mph at 19,000ft. (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
-
Maximum thrust does not occur at maximum speed Juzz. Look at the function I wrote that is good (assuming the 0.8 efficiency is valid) at any speed.
But you probably knew that, nerd.
And the efficiency is most definitely not constant. But J_A_B specifically requested a rough figure. (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
-
I'm glad to see someone else is talking about thrust to weight.
juzz, I know these are ruff calculations but you might be seeing an improvement of the prop between the G-2 and G-6.
This is from one of my other posts. It deals more with props in the transonic region but talks about prop efficiencies:
"Damn niklas even as rusty as I'm I can still pull one out of my a.. I was right about the efficiency of the prop near M1. Here is a NACA study on the subject. It is looking at prop tip speeds as we had talked about a while back. http://naca.larc.nasa.gov/reports/1950/naca-report-999/ (http://naca.larc.nasa.gov/reports/1950/naca-report-999/)
It even explains some of the things you were seeing with the prop data. It explains why they went to the wide bladed props. It appears they are able to play with the shape of the prop and get higher critcal Mach numbers.
Jimdandy,
You might interested in this story about supersonic (actually, transonic) props.
http://home.att.net/~historyzone/Fisher.html (http://home.att.net/~historyzone/Fisher.html)
My regards,
Widewing
[This message has been edited by Jimdandy (edited 02-22-2001).]
[This message has been edited by Jimdandy (edited 02-22-2001).]
-
Or to clarify what Zig said, the thrust varies greatly with speed (i.e.- it is much higher at low speeds than at high speeds for a propellor driven aircraft) and for a piston engined aircraft, power is a constant. So, from an aircraft analysis perspective, thrust loading offers a 'relative' indication of performance. As stated previously, because there are so many variables that come into play with regards to propellor design, P-Factor, Blockage, etc., power loading is an easier number to calculate. If you wanted thrust to weight ratio, you would typically need 'flight test' data, that most companies wouldn't want to share for propietary reasons and would be much harder to come by from foreign nations. Whereas horsepower and weight numbers were much easier to gain access to.
------------------
Sundog
VMF-111 Devildogs (http://www.devildogs.com)
'Criticism is always easier than craftmanship.'
-
Originally posted by Sundog:
...If you wanted thrust to weight ratio, you would typically need 'flight test' data, that most companies wouldn't want to share for propietary reasons and would be much harder to come by from foreign nations. Whereas horsepower and weight numbers were much easier to gain access to...
Your right but we are talking about obsolete planes so I don't think that's a factor here. The factor in getting the data is more the 50+ years since the data was produced. Also the fact that a lot of the non US plane data was destroyed. I wouldn't be surprised that a bunch of this data still exists but where. Thrust to weight might show more clearly why some of these planes performed as well as they did at lower speeds and altitudes. I sure wish we could find the data. The site that Widewing supplied me with above gives some hint of the differences in performance of the German and US planes and that it might just be the prop design that was a major factor.
-
Sundog as I said above, AFAIK they did not have a way to measure thrust in flight during the war.
-
Thansk for the replys, this is what I wanted to know. It gives me a much better idea of why these planes perform as they do.
J_A_B
-
Actually Gents,
Based on that chart the best numbers for performance are the lowest ratio's.
1. P51= .146
2. F4U= .156
What is being measured is the ability of the A/C to convert HP into staright ahead speed. It appears the US planes are more efficient at this.
Other American iron
1. P38L
Thrust = 2782
Thrust to weight = .159
2. P-47D-30
Thrust = 2261
Thrust to weight = .155
I know the F4U-1D and P-47D used paddle blade props. I also beleive the P51D did although I am not sure it was defined as a paddle blade. Blades on the Spit always looked like toothpicks to me were as FW190's look very wide indeed.
FW190A-5
Thrust= 1576
T/W=.181
-
Didn't anyone look at my web site posts? It's pretty good info.
-
Yes, for power loading, you want a small number (Its the inverse of thrust to weight ratio).
Funked, they couldn't measure the thrust, but they could measure the drag (http://bbs.hitechcreations.com/smf/Smileys/default/wink.gif) . Granted, the methods back then weren't as accurate as today and the full scale wind tunnels couldn't attain speeds near top speeds of the fighters. However, with a good approximation of the overall drag of the vehicle they could determine the thrust, at least for steady state flight.
SD
-
Originally posted by funked:
Maximum thrust does not occur at maximum speed Juzz. Look at the function I wrote that is good (assuming the 0.8 efficiency is valid) at any speed.
Are you sure about that? Does not T=D?
Maximum drag will be at maximum speed.
A P-40 generates 964 lbs of thrust at 280 mph. It generates 1,000 lbs of thrust at 360 mph. Since thrust is always equal to drag, it's pretty obvious that max speed produces max drag. Or, am I missing something?
My regards,
Widewing
-
V is on the bottom of the equation. Making it smaller obviously makes T bigger. At 0 mph you get infinite thrust! (http://bbs.hitechcreations.com/smf/Smileys/default/wink.gif)
-
If the powerplant is operating at maximum power, and the aircraft is in level flight, thrust equals drag only at the maximum level speed. For speeds below this, thrust exceeds drag, and the aircraft accelerates. For speeds above this, drag exceeds thrust, and the aircraft decclerates.
-
Sundog, yes they could measure drag and estimate thrust. But then your thrust estimate is only as good as your drag measurement. (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
-
Juzz if you want to do a useful thrust/weight comparison you need to pick a speed and evaluate all the planes at that speed. However if you do a little algebra you'll find it boils down to power/weight.
And yes that equation is no good at zero airspeed. The efficiency (0.8 in the equation) is a function of airspeed, pitch, rpm, etc., and obviously it has to get pretty small at low airspeeds to keep things bounded. Again, this is a ROUGH estimate.
[This message has been edited by funked (edited 02-22-2001).]
-
IOW, without an accurate chart of propellor efficiency vs speed for each a/c, you are better off comparing powerloadings. (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
-
Funked,
Are you sure about the power to weight being more inportant than power to drag? I think drag is by far the more limiting factor in max speed. Otherwise the Me109-g10 should be king by far. It has more HP than the P-51D and is considerable lighter but is at a virtual dead heat with the Mustang at sea level. Drag being the differance.
-
Maybe the P-51 has a much better propeller than the G-10?
J_A_B
-
Originally posted by F4UDOA:
Actually Gents,
Based on that chart the best numbers for performance are the lowest ratio's.
1. P51= .146
2. F4U= .156
What is being measured is the ability of the A/C to convert HP into staright ahead speed. It appears the US planes are more efficient at this.
This is an interesting exercise, but we need to look at power loading too. Of the major American fighters, the lowly FM-2 has the lowest power loading at sea level (5.94 lb/hp). The F6F-5 and the F4U-1D come next. However, once we get to 4,000 feet, the P-38L (6.21 lb/hp) has taken over the lead position, and maintains it through 30,000 feet. The P-47D comes in a solid second.
The P-51 lags well behind the P-38 in power loading at all altitudes, and suffers greatly in comparison above 25,000 feet.
The P-38L was the best accelerating fighter in U.S. service, even better than the P-47M.
Other American iron
1. P38L
Thrust = 2782
Thrust to weight = .159
2. P-47D-30
Thrust = 2261
Thrust to weight = .155
If you are considering thrust to weight ratios as important, you're correct. However, there is one factor that is missing. Drag coefficients. When these are factored in, you get something called the performance index. A performance index above 4.5 was considered very good. The performance index may be an excellent method for comparing aircraft in that it considers hp, thrust (and consequently, total drag), weight and the coefficient of drag. I don't know who came up with the Performance index, but it makes comparisons easy. In addition, if you know the weight, horsepower at best speed and that speed, you can estimate the coefficient of drag fairly accurately. This method was suggested to me by an engineer at Dryden, who claims that this is the great "de-liar" of performance numbers. I have tried it on several aircraft and it seems to bear out known performance numbers.
Perhaps, some of you kind folks will try it and see if works reliably on different aircraft than I used.
I am using the typical combat weight on max internal fuel. Based upon 3,200 hp, the P-38J should generate no more than 2,253 lbs of thrust. Factor in its weight of 17,699 lbs, and the P-38 should have a thrust to weight ratio of .127 lbs thrust per pound of weight.
Let's do the same for the P-51D, allowing for 1,280 hp @ 25,000 ft, where it makes its best speed (437 mph) This produces 879 lbs of thrust at its maximum speed. 879/10,176 = .086 or .086 lbs thrust per pound of weight.
How about the P-47D? 2,200 hp @ 32,000 ft produces a speed of 426 mph. This generates 1549 lbs of thrust or .107 lbs thrust per pound of weight.
Finally, let's plug in our old friend, the P-40. In this case, let's use the P-40F with its Merlin. With 1,065 hp @ 20,000 ft, we can calculate that it generates 878 lbs of thrust. This calculates to .101 lbs thrust per pound of weight. That's better than the Mustang. Odd isn't it? Well, let's figure in each plane's drag coefficient.
P-38J: .127/.0270 = 4.70 P.I.
P-51D: .086/.0176 = 4.88 P.I.
P-47D: .107/.0226 = 4.73 P.I.
P-40F: .101/.0242 = 4.17 P.I.
Let's do a few more.
P-63A: 1,072 hp @ 25K. 410 mph. 784 lbs thrust. .087 lbs thrust/lb. .087/.182 = 4.52 P.I.
F6F-3: 1,725 hp @ 20K. 376 mph. 1,376 lbs thrust. .112 lbs thrust/lb. .112/.0260 = 4.29 P.I.
F4U-1D: 1,795 hp @ 20K. 417 mph. 1,291 lbs thrust. .107 lbs thrust/lb. .107/.0239 = 4.47 P.I.
N1K2-J: 1,230 hp @ 18K. 369 mph. 1,000 lbs thrust. .112 lbs thrust/lb. .112/.0263 = 4.26 P.I.
F4F-3: 910 hp @ 21.1K. 330 mph. 827 lbs thrust. .103 lbsthrust/lb. .103/.253 = 407 P.I.
So, we have calculated the performance index for the above aircraft as follows:
P-38J: 4.70 (426 mph)
P-51D: 4.88 (437 mph)
P-47D: 4.73 (428 mph)
P-40F: 4.17 (364 mph)
P-63A: 4.52 (410 mph)
F6F-3: 4.29 (376 mph)*
F4U-1D: 4.47 (417 mph)*
N1K2-J: 4.26 (369 mph)*
F4F-3: 4.07 (330 mph)
*The Cdo for the F6F-3 and F4U-1D are an average of two different CDo numbers reported. The CDo used for the N1K2 is an estimate based upon known top speed and combat weights. It is probably very close.
My regards,
Widewing
-
Originally posted by funked:
If the powerplant is operating at maximum power, and the aircraft is in level flight, thrust equals drag only at the maximum level speed. For speeds below this, thrust exceeds drag, and the aircraft accelerates. For speeds above this, drag exceeds thrust, and the aircraft decclerates.
We can go further and say that thrust equals drag at ANY constant speed.
My regards,
Widewing
-
OK, so they both do the same speed at sea level, but the G-10 has about 300hp more.
What does this mean? The G-10 propellor is only 85% as efficient as the P-51D's? The G-10 airframe has 17% more drag? A combination of the two? How can we tell?
-
well guys there are several factors to take into account in what you guys are talking about.take for instance the spitfire and the mustang both have the same engines same horse power under the hood right?should go at the same speed right nope.you have to take in account wing design,prop,engine,drag(and there are two types of drag)there are so many factors that have that play a role in the aircraft performance that it gets too confusing and complicated that even me an airplane pilot and mechanic and my brother being an engineer leaves us spell bound.now back to the 51 and spit similar but very much different planes now the mustang is going to have a real low induced drag ratio but a high parisitic drag ratio this is what plagues the high speed jets of today the induced drag plagues the lower speed planes.
now you are wondering what is parasite drac and induced drag right? induced drag is drag produced by creating lift IE wing design heanse high lift airplanes that hual a ton of crap are generally slow.(no i know you guys are going to say jet airliners are not slow but if you throw enough HP at something it will fly)ok parasite drag is generally affect high speed AC this drag is the drag produced but the toejam hanging out in the airstream like antennas gear etc. and yes antennas do produce alot of drag several of my friends have modified there airplanes to mount the antennas internally and "cleaning up" the airplane and have picked up alot of airspeed in cruise.anyhow tire of typing i do hope this helps a bit. (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
-
Originally posted by juzz:
OK, so they both do the same speed at sea level, but the G-10 has about 300hp more.
What does this mean? The G-10 propellor is only 85% as efficient as the P-51D's? The G-10 airframe has 17% more drag? A combination of the two? How can we tell?
I doubt that you will find deviations between different props that exceed +/- 3%. I think the G-10 suffers from a great deal higher drag. I would expect to see a drag coefficient up around .0280 or more. Just look at the aircraft. Bulges, scoops, radio masts, non-retracting tail wheel and the main wheels are only partially covered. You also have aileron counter weights out in the wind, a nearly vertical windscreen and the radiator and cooler inlets are in the boundary layer. Despite switching to a true cantilever tailplane, allowing the deletion of the braces, the G-10 probably has a higher total drag than the Bf 109B-1 of 1937.
Comparing it to the ultra-clean P-51 with its lower drag laminar flow wing, the G-10 shows it age. The Mustang also had the advantage of its thrust generating Meredith
Effect, energy recovering radiator duct design. North American's Lee Atwood thought that the radiator duct design was the key to the Mustang's performance> A few years ago, he wrote the following:
Atwood explained, "Both the British and German engineers at the time thought you could test a scale model in a wind tunnel. But the wind tunnel models didn't generate the engine-heat factor, which we successfully controlled within the air scoop to create positive thrust. They were all looking at the Mustang's laminar flow wing, which was noted for reducing air friction over the surface of aircraft wings."
"The laminar flow wing is great for jet airplanes or in a high-speed dive but had relatively little effect on the P-51's overall performance envelope. You have to attribute the speed increase to the radiator energy recovery (positive thrust), not the characteristic of the wing itself. The wing did help in a dive -- not in level flight. I never mentioned this to anyone during the war."
Atwood credited F.W. Meredith of the RAE Farnborough, U.K., whose August 1935 report known as the Meredith Effect greatly influenced his work on the P-51 cooling radiator and duct design.
My regards,
Widewing
-
F4UDOA: I never said anything about power/weight or thrust/weight being related to top speed. I'm not sure what you are talking about. Also you mentioned something about a "chart" above and I don't know what you are referring to. Confused here. (http://bbs.hitechcreations.com/smf/Smileys/default/frown.gif)
Widewing: Yes if speed is not changing, then thrust equals drag. I just wanted to make it clear that the max thrust speed is not the same as the max drag speed if the engine is operating at full power.
Juzz: "What does this mean? The G-10 propellor is only 85% as efficient as the P-51D's? The G-10 airframe has 17% more drag? A combination of the two? How can we tell?"
You can't really tell.
The best you can do is get drag estimates for the two planes (either from wind tunnel or CFD or other analytical tools), then figure how much thrust would be needed to achieve the top speeds. If you have a good power figure, then you can work out the efficiency of the prop.
Obviously the drag estimate may be different from the actual in-flight drag. Also the engine may produce more or less power when installed in the airplane compared to on the test stand, due to a number of effects. Finally the exhaust stacks on some WW2 planes generated a significant amount of thrust. All these factors could show up in your "prop efficiency", even though they have nothing to do with the performance of the prop.
Another way to do it would be to deal with the prop directly. Either use analytical tools to figure out how much thrust is generated for a given rpm/airspeed/pitch combination, or some sort of testing. I've seen photos of prop/engine combos rigged up with strain gauges (which can measure thrust load in the prop shaft) which were presumably being used for this sort of testing. Again there are some problems though, because the performance of the prop maybe different in flight due to aerodynamic interaction with the airframe. And there is still the exhaust thrust to deal with.
But all of those techniques can definitely get you in the ballpark, a lot closer than the "0.8" fudge factor. (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)
-
Thanks Widewing. I think we're getting a handle on it now. I'm still trying to figure out (the original question by fscott a few days ago) what is the cause of the N1K's "exceptional" performance in the loop example is. I don't see in these figures where the N1K should out perform the P-51D. Is it because the thrust to weight is a bigger factor at low speeds (drag increasing with v^2) where the drag is less of a factor? I hope you can pop a comment on before tomorrow afternoon because I will be off line for a week or two. Thanks again for the info Widewing.
-
"Is it because the thrust to weight is a bigger factor at low speeds (drag increasing with v^2) where the drag is less of a factor? "
Bingo!!!
-
You can get a spreadsheet I made here. It will calculate thrust, induced drag, profile drag, climb rate, glide ratio, sustained turning speed, stall speed, air density, prop efficiency..etc..
http://www.iaw.com/~general6/flight_performance.htm (http://www.iaw.com/~general6/flight_performance.htm)
I've only got one plane (The f4u-1) as a template right now, but I will put others up for comparison soon!
-
Originally posted by wells:
You can get a spreadsheet I made here. It will calculate thrust, induced drag, profile drag, climb rate, glide ratio, sustained turning speed, stall speed, air density, prop efficiency..etc..
http://www.iaw.com/~general6/flight_performance.htm (http://www.iaw.com/~general6/flight_performance.htm)
I noticed that your link does not work. At least I can't access it via our MCI T1 line.
My regards,
Widewing
-
Originally posted by funked:
"Is it because the thrust to weight is a bigger factor at low speeds (drag increasing with v^2) where the drag is less of a factor? "
Bingo!!!
Well damn it! (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif) We are back to what I was harping about in fscott's post. At the speed he was talking about thrust to weight is the factor to look at. I admit that hp/wt may be a very good estimate at those speeds but maybe the Japanese had a nice prop to go with a great engine and the thrust figures my be more telling than it might first appear. Could this be a reason for some of the higher hp estimates out there? I also remember something about the Japanese paying attention to thrust augmentation from the exhaust. The wing may have been lacking the high speed performance but the prop, engine, wing combo made for a wicked low to medium alt fighter.
-
I noticed that your link does not work. At least I can't access it via our
MCI T1 line.
Hmm, anyone else have a problem? I've been having problems accessing certain websites myself, but maybe it's on my end? I can try e-mailing it if you want? Let me know!
-
I was able to get in wells. I've been having trouble with some web sites and links too. Several of the posts have had pictures that only show as the little red x the last couple weeks. I don't know if it's me or them or the web.
[This message has been edited by Jimdandy (edited 02-23-2001).]
-
Originally posted by wells:
Hmm, anyone else have a problem? I've been having problems accessing certain websites myself, but maybe it's on my end? I can try e-mailing it if you want? Let me know!
Still no luck from the office. I will try again later from home. Fridays are a half day for the Engineering Department. Dress down day too. I spent my morning designing a roll-over sensor for a jet ski. Get it upsidedown and it kills the ignition. Naturally, they won't let me do the field testing.. (http://bbs.hitechcreations.com/smf/Smileys/default/frown.gif)
My regards,
widewing
-
Originally posted by Widewing:
...Naturally, they won't let me do the field testing.. (http://bbs.hitechcreations.com/smf/Smileys/default/frown.gif)
My regards,
widewing
I live on the beautiful Columbia River. Jet Ski barge wake heaven. Bring it up here and I'll test it. (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif)