Author Topic: Climb and acceleration(engineers please) (Read 1266 times)

F4UDOA · « **on:** July 10, 2002, 01:16:58 PM »

Gents,

I have always questioned the relation of climb versus accleration and I have always been told that they are directly proportionate. When I have argued this relation I have been beaten about the head and shoulders. However what if a test where performed between 4 A/C in both climb and acceleration then the results should be identical right? WRONG!!

In 1989 a group of modern military test pilots known as the "Socioty of Experamental Test Pilots" SETP Web Page Perfomed a test called the "End of the Arguement" between the P-51D, F4U-1D, P-47D-40 and F6F-5 to determine which was the best fighter of WW2 using modern evaluation techniques.

The findings are very interesting indeed but the point I will make if this. None of the A/C were even close to being the same finish in climb and acceleration.

Here are the listed weights of each A/C in the test.

P-47D
Weight= 11,535LBS <=3,000lbs light of full combat weight

FG-1D
Weight= 11,000LBS <=1,000LBS light of full combat weight

F6F-5
Weight= 10,681 <==2000LBS light of full combat weight

P-51D= 8,900 <= 1,000LBS light of full combat weight

Climb test to 10,000FT Mil power

1. F6F-5 = 4:15

2. FG-1D = 4:45

3. P-51D = 4:55 <=hard to see exactly on chart

4. P-47D = 5:00

Acceleration Test Mil power at 10,000FT to top speed.

Note: the P-51D started at 120KNOTS. P-47D started at 110KNOTS. The F6F-5 and FG-1D started at 100KNOTS.

1. P-51D<== By a fair margin Top speed attained approx 240knots in 130seconds

2. F6F-5= Top speed attained 210Knots in 110seconds

3. P-47D= Top speed attained 215Knots in 130seconds

4. FG-1D= Top speed attained approx 230Knots in 160 seconds

So none of these a/c finished in the same order in both test.

What gives?

funkedup · « **Reply #1 on:** July 10, 2002, 01:21:57 PM »

The climb = accel thing only applies to instantaneous measurements, not average measurements.

F4UDOA · « **Reply #2 on:** July 10, 2002, 01:46:13 PM »

Funked,

All the climb and accleration numbers in AH are sustained not instentanious as are these test. I have always said that they are not directly related. I have test data with 4 dissimiler A/C and according to theory they should finish with the same result.

It does not matter what aircraft type was tested. It could have been 4 cessna's. The point being that they should finish in the same order if climb and accleration are tied together.

funkedup · « **Reply #3 on:** July 10, 2002, 02:22:52 PM »

The climb charts on the HTC website are instantaneous rate of climb measured at the airspeed which gives the best sustained rate. AFAIK HTC never published any acceleration figures.

hitech · « **Reply #4 on:** July 10, 2002, 02:23:02 PM »

F4UDOA, Your taking the word instantious wrong, meens at a single speed for acceleration. I.E. The acceleration from 169 to 170 is different than acceleration from 200 - 201.

Accelerations would be in a unit like Mph change in 1 sec.

Also climb rate at 200 is different than climb rate at 169.

The chart your looking at gives a climb rate at 1 speed, and an average acceleration over different speeds.

When we say Climb rate is always perportional to acceleration we meen that if you have an accelerations of 10 mph per sec and a climb rate of 5000 fps, at 160 mph.

Then if acceleration at 200 mph = 5 mph per sec the climb rate at 200 mph must be 2500.

Note you must hold a constant 200 during climb by either raising or lowering the nose.

And they will always be directly perportional.

HiTech

Regurge · « **Reply #5 on:** July 10, 2002, 03:52:46 PM »

Those climb times look to be from a standing start (only 2000fpm average for a 3000lb underweight P47). So a good portion of that test is not even at the sustained climb speed. I'd expect the F6F to do well from a standing start just because it can get the gear up faster.

As was said, the accelerations were averaged over a range of speeds, rather than at the climb speed. The F6F acceleration might have been the best for the 1st 50mph or so but then dropped off drasticly. Likewise the P51 might have been mediocre at low speeds, but kept pulling strong all the way to 300mph.

GRUNHERZ · « **Reply #6 on:** July 10, 2002, 05:11:38 PM »

Perfomed a test called the "End of the Arguement" between the P-51D, F4U-1D, P-47D-40 and F6F-5 to determine which was the best fighter of WW2 using modern evaluation techniques.

Easy, Bf109G6!

udet · « **Reply #7 on:** July 10, 2002, 05:17:46 PM »

rate of climb is proportional to the excess power of your engine.(i don't remember the exact formula)
That is, the power besides that used to overcome drag at a certain speed.
If you think of power as force*velocity, then, at a given speed your excess power would be (thrust-drag)*velocity.
the difference between thrust and drag is what would cause your plane to accelerate, as, according to good old boy Newton, mass*acceleration equals the algebraic sum of the forces.
Therefore, indeed, climb and acceleration are proportional.

F4UDOA · « **Reply #8 on:** July 10, 2002, 05:33:56 PM »

Gents(and Hitech;) ),

I am a little confused here still. I understand that when you calculate accleration or climb it is always at one speed.

However my arguement has always been that I felt the F4U acclerated to slowly because of it relative power to weight and low parasite drag. What I was told is that becuase of it's high induced drag it slows it terribly. I always felt this hard to believe that induced drag could slow an A/C that had such good power to weight and low Cdo. But the answer is always that if it climbs bad it has to acclerate bad.

However now I have real world test data that shows the A/C that acclerates the best doesn't really climb the best. According to the test data the P-51 should out climb the others by a large margin. But instead it was 3rd behind a F6F and a F4U. And I am willing to bet if that F6F were 500LBS heavier than the F4U instead of 500LBS lighter(1000LBS weight gain) that the F4U would have been #1 in the climb test.

Also a side note on the 2,000FPM cllimb is that the fuel used was not 100/130 as used in wartime on the Radials. It is said to have lost about 4 inches of map on those A/C. Also it mentions that this change did not affect the MAP of the P-51D water cooled engine.

hitech · « **Reply #9 on:** July 10, 2002, 06:04:37 PM »

F4UDOA: Try think of it this way lets say we have 2 planes plane A with a max speed of 300 PLANE B with a max speed of 350.

The Plane A best climb rate is 4000. Plane B is 3000. Boths best climb speed is 160.

Whos has the best acceleration at 160.

Who has the best acceleration at 300 mph

Who has the best climb rate at 300 mph.

"I understand that when you calculate accleration or climb it is always at one speed."

If you realy understood this you wouldn't be confused by the chart because it is not messuring acceleration at 1 speed. Its messuring acceleration over the entire speed range.

I.E. It's average acceleration. That would be like messuring climb rates at different speeds then taking the average of those rates.

We are realy talking about 2 completly different messurements, your viewing acceleration as time to reach top speed. Im viewing it as a change in speed per sec at any given speed.

If in my example Plane A & B both started at 160 and went full throttle Plane A would first pull head of plane B, i.e. better acceleration, as there speeds increased plane B would catch back up and pass plane A. So early on plane A has better acceleration, later plane B does.

As far as the F4 goes, it accelerates badly at slower speeds as compared to other planes , for the same resones it climbs poorly.

Acceleration and climb are 100% equivilent functions. And it realy comes down to Acceleration = Gravity, gravity is the oposing force in climbing, mass is the oposing force in acceleration.

(I know mass is not a force but for this description it works)

wells · « **Reply #10 on:** July 10, 2002, 06:20:31 PM »

DOA,

You need to know the climb speed, which was probably different for each plane? I can guarantee you that the P-51 will outclimb all other planes at 230 knts. The average acceleration to top speed is not indicative of how a plane will accelerate at it's climb speed.

a = Force / mass
RoC = Force * TAS / Weight

Note, that Weight = mass * g

g = gravitational constant (32.2 feet/s^2)

All you have to do to convert a climb rate into an acceleration (at the climb speed), is to mutiply by g and divide by TAS.

A plane that will climb at 50 feet/s, with climb speed of say 220 feet/s, will have an instantaneous acceleration of

50 * 32.2 / 220 = 7.3 feet/s^2

...in level flight. Note, that as soon as speed increases to 221 fps, climb rate and acceleration will both change.

F4UDOA · « **Reply #11 on:** July 10, 2002, 06:25:41 PM »

Just so I am being clear the acceleration chart is a graph. It shows the acceleration through the speed range.

The P-51D accelerates at a higher rate through out the speed range. Based on this the P-51D should climb quite well through out the speed range. But it finishes third behind the F6F and F4U(which accelerated the poorest).

Having done the testing in AH myself I can say that climb and acceleration are completely linear. This is not so IRL.

So if this relationship is not linear IRL why is it in AH??

F4UDOA · « **Reply #12 on:** July 10, 2002, 06:54:04 PM »

Wells,

I would expect the P-51D to accelerate faster at 230MPH but it is clearly superior throughout the speed range in the chart. However the climb of the P-51 is clearly inferior to the F6F and F4U.

This is the opposite of what I have been told.

1. I would like Hitech and Wells to get a copy of the 1989 Symposium. It was sent to me free within a week.

2. Is it possibe to calculate a range of acceleration from Vmin to Vmax? It must be because AH's FM's are working examples of this.

My conclusion:

I don't know the math and I probably won't anytime soon. But I want to see someone express a logical reason why a

A. F6F should outclimb or accelerate an F4U?

B. How a P-51D can accelerate better than a F6F but climb worse?

C. Draw a graph of these three things taking place

I have Zigrats performance calculator on my web page here

My homepage(F4UDOA)

F4UDOA · « **Reply #13 on:** July 10, 2002, 07:02:55 PM »

Hitech,

We are crossing post here.

Anyway I'm not reading acceleration as time to read max speed. I know thats how it sounds but I am looking at acceleration at a given speed.

Forgeting about the F4U for a minute and just look at the F6F and P-51. The P-51 has better acceleration througout the range and the F6F has better climb albeit the F6F is climbing at a constant speed. Both A/C are climbed at there best speeds.

The P-51 accelerates faster than the F6F at the F6F's best climb speed. This should not happen, correct?

wells · « **Reply #14 on:** July 10, 2002, 08:15:39 PM »

DOA,

I don't have the report to see this chart, but given the info you've provided, the acceleration test is at 10000 ft. The climb performance is measured from the ground up to 10000 ft, right?

These are laws of physics. Those planes are not defying those laws. The only info I can give you, is that at 10000 ft, the climb rate at a given speed will correspond to the acceleration, provided power is the same, etc. Perhaps at lower altitudes, the results are different. Surely, the power will be different at lower heights.