F4U- Am I missing something here?

[Ripsnort]

I claim to be of no aviation expert, my only expertise comes from many, many books and many hours of reading in my spare time, but, it seems as though the F4U should dive alittle better than it does...with all that weight, I should be able to B and Z alittle better, eh?  Whats your take on it?  I haven't tested anything officially, but, how about the climb rate?

[Yeager]

I dunno Rip......I myself am trying hard to remain objective about the FMs.  Gawd, its still BETA and many things still need to be implimented but the whole sim feels so damn good that it is tempting to figure it as done
already..hehe...

The F4U seems a little out of sorts from what I have learned over the years but hey, I think all the FMs are still out of sorts.  Give it another three months I figure and we should have a basically stable bunch of FMs to examine.

Yeager

[AKDejaVu]

I'm curious as to how a plane's weight affects its diving ability.  A wingman and I were talking about it the other day.  In an ideal situation, weight shouldn't matter... but I don't know how this is affected when drag comes into play.  It seems a heavier plane would be less incumbered by the drag coeficient... though it should also have a greater ammount of lift (since it is a heavier aircraft) and thus create other issues in a dive.

Could someone with a little knowledge/experience in this area explain what are the most significant factors for increasing dive performance?

AKDejaVu

[wells]

You're right, weight shouldn't matter, and it doesn't at the planes maximum level speed, where thrust and drag are equal.  Having a higher top speed helps!  Below that speed, thrust helps the acceleration and above that speed, there is a net drag force.  What those forces are in proportion to the weight of the plane will determine how much it helps or hinders dive performance.  Some examples...

I am going to equate engine power with thrust and drag for this example.  They are not equal, but are related, and should be adequate for 'ranking' diving ability and not figuring exact dive accelerations.  A plane that needs more power to hit a certain speed has more drag.

P-51 - 365 mph, 1700 hp, 9600 lbs
SpitfireIX - 336 mph, 1700 hp, 7300 lbs
109G10 - 340 mph, 1800 hp, 7300 lbs
Fw-190A8 - 355 mph, 1700 hp, 9100 lbs
F4u-1 - 360 mph, 2250 hp, 11700 lbs

First, adjust the hp(drag) figures for set speeds of 200, 300 and 400 mph (V/Vmax)^2 * hp).  The acceleration is the (surplus power + the weight) divided by the weight.  

200 mph numbers:

P-51D - 510 hp, 9600 lbs, a = 1.12
Spit IX - 600 hp, 7300 lbs, a = 1.15
109G10 - 620 hp, 7300 lbs, a = 1.16
Fw-190A8 - 540 hp, 9100 lbs, a = 1.13
F4u-1 - 690 hp, 11700 lbs, a = 1.13

The 109 and Spit have the best dive acceleration at this speed.

300 mph numbers:

P-51D - 1150 hp, 9600 lbs, a = 1.06
Spit IX - 1355 hp, 7300 lbs, a = 1.05
109G10 - 1400 hp, 7300 lbs, a = 1.05
109a8 - 1215 hp, 9100 lbs, a = 1.05
F4u-1 - 1560 hp, 11700 lbs, a = 1.06

Pretty even field, F4u, 190 and P-51 starting to catch up.

400 mph numbers:

P-51D - 2040 hp, 9600 lbs, a = 0.96
Spit IX - 2410 hp, 7300 lbs, a = 0.90
109G10 - 2490 hp, 7300 lbs, a = 0.91
109a8 - 2160 hp, 9100 lbs, a = 0.95
F4u-1 - 2780 hp, 11700 lbs, a = 0.95

F4u, 190 and P-51 are pretty close and increasingly superior to the Spit and 109.

[Ripsnort]

"You're right, weight shouldn't matter"...Hmmm, I was talking about diving speed, not level flight.  Fact: P47 was the fastest diver in the theatre at its time I believe.  F4U doesn't quite have the weight of the P47, but it has the same engine, and is heavier than all the fighters currently in AH, right?  So shouldn't it dive the fastest?

[CombatWombat]

Actually weight shouldn't matter, even in a dive (assuming equal drag at all airspeeds and AOA).
Didnt newton do some thing where he rolled different weighted balls down a plank or something?  And they all finished at the same time?
The only thing weight would affect would be the momentum of the aircraft.  Perhaps having larger momentum would help "Burn through" the drag in a dive?

[hitech]

I realy should stay out of this discusion. And let someone else explain it to you but.

Weight DOES matter. Simple experment would be take 2 feathers. Tape a coin to one of them. which do you think will hit the ground first in an atmospheric envoirment.

Anyway basicly 3 factors effect unloaded (0 g) dive acceleration and speed.

Thrust,weight and drag.

HiTech

[dolomite]

 

Originally posted by hitech:
I realy should stay out of this discusion. And let someone else explain it to you but.

Weight DOES matter. Simple experment would be take 2 feathers. Tape a coin to one of them. which do you think will hit the ground first in an atmospheric envoirment.

Anyway basicly 3 factors effect unloaded (0 g) dive acceleration and speed.

Thrust,weight and drag.

HiTech

Only in a vacuum is weight unimportant. The analogy you mention is making reference to weight vs. surface area. The more surface area, the more drag. Distributing weight over a sufficient area can create enough drag to slow a plane.

There is a balance to the whole mess, but it certainly isn't black-and-white (at least for me it's not).

[Duckwing6]

use the VECTORS Luke  

Point the airplane straight down .. Thrust = straight down... Weight = straight down... Drag = straight up -> Weight adds to the initial thrust lift points somewhere else and doesn't really help here  

[wells]

Acceleration due to gravity is constant, neglecting drag.  The reason the feather with the coin on it will drop faster is because the drag (about equal for both feathers) is less in comparison to the weight.  The deceleration due to drag being

drag/weight

At a planes top speed, thrust and drag cancel each other, so neither is a factor there, which is why a plane with a higher top speed has an advantage in a dive.  

Power loading is a good indication of both acceleration (below top speed) and deceleration (above top speed).  A low power loading favours good initial dive acceleration at slower speeds, whereas a higher power loading favours better acceleration at higher speeds.

[CptTrips]

never mind



[This message has been edited by AKWabbit (edited 01-05-2000).]

[xmab]

Imagine 2 identical planes in vertical dives in a vacuum (no air resistance and thus no friction or drag), except one weighs twice as much as the other. They both would accelerate exactly the same (32.2 ft/s^2) and would have unlimited terminal velocities (disregarding structural limits of course). Now add air to the equation. Air acts as vicous friction and thus adds a retarding force to the direction of motion that is proportional to velocity. Now, the heavier plane will both accelerate faster and have a higher terminal velocity because the net force acting on the plane is larger. The net force is equal to weight - drag or mass*gravity-velocity*drag (simplified of course and neglecting lift). Thus for 2 planes with equal drag the heavier (i.e. the one with more mass) one will accelerate faster.

[Sundog]

Actaully WEIGHT does matter. Just as a HEAVIER aircraft climbs slower....a Heavier aircraft will ACCELERATE in a dive faster until it reaches terminal dive speed. What people are confusing here is the difference between acceleration and terminal velocity in a dive. Force equals mass times acceleration. G, the Earths Pull, is relatively constant (Assuming flat earth for calcs etc.). The mass is not constant due to fuel burn. However, a heavier aircraft should ACCELERATE faster in a dive then a lighter aircraft. That is why the P-47 was able to dive away from most attackers so QUICKLY. e.g.- It could extend quickly. However, in terms of terminal dive speed, e.g. the fastest diver, my understanding is that the P-51 had the HIGHEST dive speed, due to the laminar airfoil delaying the onset of compressibility.

Now, aerodynamics do play a part, but going from level flight into a dive allowed much greater speeds to be attained faster if the aircraft was heavier. The Weight was directly related to the aircraft's ability to accelerate in a dive, more so then aerodynamics (Assuming from steady level flight). The aerodynamics (and possibly stability/control and structural limits depending on the aircraft) are what determined the terminal dive speed though.

Of course, now we can argue about the terminal dive speed or the maximum `controllable' dive speed ;-)

[Sundog]

Note:
When I said Heavier aircraft, I was refering to 2 aircraft of same type. Obviously wing loading and power loading play a part as well, as can be shown by the best rate of climb (ROC) for a prop aircraft as follows:

V(Best ROC) = V(AC)*sin(gamma)
=((550*(eta))/weight)-(drag*V(AC))/Weight))
where
V(ac)=Velocity (Aircraft)
gamma=climb angle
eta=the propellor efficiency
eta=Prop thrust/engine horsepower

However, in a dive, the initial acceleration will mostly be governed by weight, as all WW2 Propellor aircraft had a T/W ratio substantially below unity (as do most propellor aircraft).

[AKDejaVu]

   

((550*(eta))/weight)-(drag*V(AC))/Weight))

Doing a little factoring:

((550*(eta))-(Drag*V(ac)))/weight

This would indicate to me that increasing prop efficiency and/or decreasing drag would increase V(Best ROC).  Increasing the weight would actually decrease V(Best ROC)... all else equal.

It seems there is a balance between drag and weight in conjunction with thrust.  I'm not exactly sure what the formula is, but it does seem that the best way to calculate the results is to hop in an airplane and point it downward with a stopwatch on.  Either that or the formulas we are seeing are way too simplistic.

AKDejaVu

[This message has been edited by AKDejaVu (edited 01-06-2000).]