Internal weight and max speed

[SgtPappy]

anyone know exactly how much an empty fuel tank will bite performance?

after looking up info for the spit8 and 9, the 8's TARE weight was only about 180 lb.s more (5,931 vs 5,749 lbs.). if i drained the fuel in the wing tanks on the Mk.VIII, would it in real life be as maneuverable as the IX?

Furthermore, would an empty 29 imp gal ferryfuel tank (self-sealing) stuck in the rear of a Mk.IX make it maneuver as sloppily as the VIII devoid of wing fuel (but still in possession of those 14 imp gal wing tanks themselves) in-game?

*note -  in-game tanks

Mk.VIII fuel tanks

Main (top) - 48 IG
Main (bottom) - 48 IG
LW - 14 IG
RW 14 IG

Mk.IX fuel tanks

Main - 85 IG

[Knegel]

Hi,

the influence of internal weight differ with relation between the current speed(in IAS) and "Speed of smalest drag(IAS related)".

In low alt the planes have the tencency to fly much faster than the "Speed of smalest drag"(but this depends to the contruction), so at a speed where the induced drag normaly dont count that much.
With increasing altitude and decreasing power the the IAS decrease and so the planes get closer to the "speed of smalest drag", or even below it, and so the induced drag(depending to the load) have a much more big influence.

The "speed gain discrepancy" between fighters and bombers should occur only when the Bomber fly at or below the "speed of smalest drag", while the fighter still is faster and when the bomber lose much more relative weight than the fighter.

The B17G fuel load + bomb load are around 30% of the take off weight, while even the P47D fuelload(around 1000kg) are only 15% of the plane weight.  This different also will have a more big influence to the Vmax.

Greetings,

Knegel

[Knegel]

Originally posted by Benny Moore
It's very little; trim tabs are small, and do not move very far into the airflow.  Many simulator users believe that trimming a real airplane will improve the speed because a human cannot hold the stick steady if improperly trimmed, but this is doubly wrong.  First, it is quite possible to hold a ship as steady without trimming as with; in a real airplane, it is a matter of strength, not precision (quite unlike the case in simulators).  Second, proper trimming actually causes the trim tabs to deflect into the airflow, theoretically reducing speed.  Of course, as I said at the beginning of the paragraph, the difference is negligible.

Hi,

with the for most WWII common trim system trimming or not trimming dont matter. Though, in planes like the Me109 and FW190(maybe others?), where the whole vert stab get moved, a good trimmed plane also cause less drag. With this system the pilot need to pull or push the stick to fly strait, if the plane isnt trimmed, as result the vert stab and elevator section cause a not to smal drag(specialy at highspeed of course). If the plane is trimmed, the vert stab and elevator are in one line, so the drag is as smal as it can be.

Greetings,

Knegel

[bozon]

Originally posted by SgtPappy
anyone know exactly how much an empty fuel tank will bite performance?  

Try the DT on the F6F. When the speed gets a little high it feels like you are dragging a circus tent. OK, I'm exaggerating, but it is noticeable (very big tank).

[SgtPappy]

good idea...

i tried it in the F6F-5 and i emptied the ext. tank. the handling felt exactly the same. so i tried it in the Spitfre IX as well. same results.

does this game actually cause an empty external tank to hurt performance at all?

[Krusty]

The tank adds drag if it's onboard, regardless of being full or not. It adds drag the same way bombs and rockets do -- it juts out into the airstream and slams into the airflow.

[Charge]

"Weight shouldn't make any appreciable difference on the top end level speed of airplane. Parasite drag greatly overshadows any induced drag and therefore is the dominant factor in determining max level speed. Weight affects induced drag therefore it's impact is completely overshadowed by parasite drag in effecting max level speed."

That is how I have understood it too. That is why I have wondered why 190A8 is  so much slower than, say, A5. The same thing between Spit IX, XIV and XVI if you consider weight and power.

Of course the official speed charts are an undisputable source but I still wonder what are the factors that cause such huge drops in Vmax. There has to be somekind of visible drag increase causing this e.g. in A8.

AFAIK there were different cooling blades for FW which increased cooling but provided also more drag and it does not make much sense to install a more powerful engine requiring more cooling if casuse it to be slower than its predecessor.

-C+

[Knegel]

Hi,

there is no real different in the low level speed of the 190A8, A5 and A6 if they all carry the same loadout.

The A8(with 1.42 ata) often get displayed around 5-7km/h(3-4,3mph) slower, but the A8 thats not that much.  

Another reason for different test results might be different engine settings.
There was a BMW801D and a BMW801Dg, the Dg was downrated to 1750PS, while the D had 1800PS at sea level. Afaik later they mainly used the Dg together with the "Sondernotleistung". While combat the different wasnt big, but the Dg was more reliable, so this version often got used.

As we know the surface condition is also important same like the radiator flap setting.

The bumped cowling due to the bigger MG´s also might be a reason.

The main reason for this differents probably is a rather high "speed of smalest drag", what is typical for high wing loaded planes.
With a "speed of smalest drag" rather close to the Vmax, the influence of the induced drag while a level flight is more big.  

It also might be that the smal AoA changing due to more weight cause more parasite drag around the fuselage.

Testresults also might vary with the test technique. Is the Vmax measured out of a acceleration or out of a deceleration? How long was the course how good the pilot did keep the levelflight(a very smal climb or dive made a very big different)?? etc etc.

I saw datas regarding the 190A8 with 1.42ata from 540 to 556km/h, while the early tests (without Sondernot) was in general 10km/h faster at sea level, so i guess it was the 1800PS (1.42ata) engine, while the later one did use the 1750PS(1.42ata) engine.

Of course all this is rather speculative.

Greetings,

Knegel

[SgtPappy]

well for a spitfire pilot, speed wasn't ever really something to rely on completely. im wondering about the maneuverability of the aircraft.

the spitfire VIII is about 180 lbs heavier than the spitfire IX when both a/c are completely void of ammo, fuel and oil (guns, fuel tanks, ballasts etc are all intact). In-game, this seems to make the spit8 REALLY sloppy when compared to the spit9. Sometimes it feels like i'm flying a mustang while the spit9 feels floaty like a Zeke. keep in mind that i 'tested' their maneuverability while at full up trim with both planes.

The spit8 at 100 mph under all conditions will be shuddering while the spit9 will only do so (and very little) when pulling a high AoA immleman. so i tested both planes and i drained the spit8's wing tanks and made sure that i had about 25% of the main tanks' fuel left (thats 24 Imp Gal) and i had made sure i had 30% of fuel left in the spit9's main tank (25 Imp Gal). The spit9 felt much more 'floaty' under all conditions.

[Stoney74]

CG can have an affect on Vmax as well.  Planes loaded near to the aft limit fly faster than the same plane loaded near the forward limit, assuming gross weight is equal.  Some of you guys may be able to correct me or expound on this, but it might be interesting to determine the CG of a B-17 with bombs versus right after they get dropped.  Certainly wouldn't be the whole difference, but in my plane, it can make the difference in 5mph TAS.  Also, my TAS at solo weight is higher than TAS at max gross, again by about 5 mph.  In a plane that trues at 140-150 mph, these are high percentage gains.

Krusty, at what altitude did you test the P-47N?  Its Vmax is at 34K or so.  Was this where you tested it?

[Krusty]

A bit under 15k. I took off on the DA map, 15k field, shallow dive for quick speed, then leveled out. Already had spent hours testing the B17s at 25k, didn't want to waste much time.

[Stoney74]

Ok,

Went on the TA map offline.  Launched out of one of the 30K bases.  Made a climbing 45 degree turn due east, hit alt-x and climbed to 34K.  Waited for speed to stabilize for over a few minutes before "calling it".  Fuel burn was set to 0.001.  Picked 34K as it is pretty close to critical altitude for the Jug-N.

P-47N, 100% internal fuel, 8X.50cal with 425 rpg=16,692lbs.  Top speed at 34K in this configuration was 432mph TAS at 54" (Mil Power)

P-47N, 4 gallons fuel, 6X.50cal with 0 rpg=12,233lbs.  Top speed at 34K in this configuration was 457mph TAS at 54" (Mil Power)

25mph TAS difference in top speed.  Acceleration was tremendously different (and although not entirely scientific) it took me almost 100 miles to climb, level and hit the heavy speed.  It took just over 50 miles to climb, level, and hit the light speed.  (used WEP up to about 400 TAS, then 54" of MP).  Almost a 4400 lb difference in weight.  No difference in drag (unless the gun barrels are modelled).

So, what's the difference?  Tango, in all honesty, I have a hard time believing that the heavy jug would have made it all the way to 457.  If it had, it would have taken hours.  I understand your graph, but I'm skeptical none-the-less.  What's the formula for determining the speed at which drag is lowest?  I guess we're going to need some drag coefficient numbers for the B-17, yeah?

[dtango]

Stoney, et al.

I need to clarify my original statement.  It's misleading because it's a generalization.  Things are a bit more complicated than that.

Here's the induced drag equation for level flight:

Di =                          2*Weight^2
      -----------------------------------------------------
      pi*air-density*span-eff*V^2*wingspan^2

For a given airframe there are 3 variables that can increase or decrease induced drag:
(1) Weight
(2) Altitude (air density)
(3) Airspeed

For the 1st graph I posted (I didn't create it BTW) it plots drag vs. airspeed where weight and altitude are fixed.

Weight does impact overall drag by increasing induced drag.  The question though is by how much does it effect the max level speed of an airplane.  The following is a graph of induced drag and total drag for a P-51D at different fuel load outs at sea level:



At sea level the max speed of the P-51D is ~366mph.  Notice that around that velocity the total drag curves are pretty much on top of each other which means the total drag is very similar to each other despite the differences in weight in fuel load.

Here's a closer look at what the differences in drag would mean in terms of max level speed:

Alt %F Weight rho Di CD0 Dp Dtotal Thrust V mph Mach
0 100 12100 0.0023 124 0.019 1427 1551 1552 361 0.474
0 75 10606 0.0023 93 0.019 1458 1551 1552 365 0.479
0 50 9486 0.0023 74 0.019 1477 1551 1552 367 0.482
0 25 8739 0.0023 62 0.019 1490 1552 1552 369 0.484

Note the lbs of induced drag (Di) for the given fuel weight and the max level airspeed associated for a given load out.  There's not very much variation because at these speeds and alt parsite drag is much larger than induced drag.

The reason the change in loaded fuel weight doesn't impact a fighter as much is because the % distribution of the total weight is lower compared to that of a bomber.

Plane F % F_lbs Wt_lbs %Wt
P-51D 100% 1494 12100 12%
P-51D 25% 374 8739 4%
B-17G 100% 16680 65500 25%
B-17G 25% 4170 52990 8%

Altitude plays a part on induced drag as well.  Here is a plot of the P-51D induced and total drag for different fuel loads at 24,500 ft:



The max level speed of the P-51D is ~437mph at 24,500.  At an higher altitude the induced drag difference due to different fuel loads is little more noticeable.  Around the 437mph mark the total drag curves are more spread out than how they look for the sea level graph.

Here's a closer look at the impact on max level airspeed for the P-51D at 24.5k alt for the different fuel load outs:

Alt %F Weight rho Di CD0 Dp Dtotal T V mph Mach
24500 100 12100 0.0010 199 0.022 1046 1245 1245 433 0.61777285
24500 75 10606 0.0010 148 0.0226 1096 1244 1245 440 0.629293089
24500 50 9486 0.0010 117 0.0229 1128 1245 1245 443 0.636493239
24500 25 8739 0.0010 99 0.0233 1145 1244 1245 443 0.640813329

The above estimate also factors in compressibility drag as well.  The spread in max level speed between 100% fuel vs. 25% at 24.5k is a little more than what is was for at Sea Level.

Without compressibility drag factored in the max level speeds would look something like this:

Alt %F Weight V mph
24500 100 12100 433
24500 75 10606 444
24500 50 9486 451
24500 25 8739 455

If I get a chance, I'll try and work up similar estimates for the B-17G as well.

Tango, XO
412th FS Braunco Mustangs

[Stoney74]

This generates two questions from me:

1) Is compressibility drag modelled in AH?  I noticed that without the compressibility correction, the P-51 is 25mph faster at 25% than at 100%.  Is that what happened with my Jug-N test?

2) What is the use of knowing the lowest total drag speeds at a given altitude and configuration, i.e. does it correlate to maximum range or best cruise speed?

I've been thumbing through my Aerodynamics for Naval Aviators for a few weeks now trying to figure some of this stuff out, so I appreciate the help and expertise Tango...

[Krusty]

Good question, Stoney. Maybe one of the folks at HTC will drop in and give us an answer.


(*waits expectantly*)