Thrust

[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.  

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  

[Jimdandy]

Didn't anyone look at my web site posts? It's pretty good info.

[Sundog]

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   . 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

[Widewing]

 

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

[juzz]

V is on the bottom of the equation. Making it smaller obviously makes T bigger. At 0 mph you get infinite thrust!  

[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.

[funked]

Sundog, yes they could measure drag and estimate thrust.  But then your thrust estimate is only as good as your drag measurement.  

[funked]

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).]

[juzz]

IOW, without an accurate chart of propellor efficiency vs speed for each a/c, you are better off comparing powerloadings.  

[F4UDOA]

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.

[J_A_B]

Maybe the P-51 has a much better propeller than the G-10?  

J_A_B

[Widewing]

 

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

[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

[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?

[av8or]

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.