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General Forums => Aircraft and Vehicles => Topic started by: Krusty on March 25, 2007, 11:26:15 PM

Title: Internal weight and max speed
Post by: Krusty on March 25, 2007, 11:26:15 PM
This is odd... I've read a comment here on these forums [edit: long ago] from somebody at HTC that said "internal weight won't reduce max speed" or something along those lines.

The issue was top speed of the 190A-8 with and without the 30mms outboard (which add a lot of weight). I think it was said they only reduce it by 1 mph.

Now, I'd always taken that to mean you'll get, at most, 1mph difference between fully loaded and "light" -- given no externals, I mean.

Then I thought about how you always speed up when you drop bombs from your internal bomb bay. You always have to recalibrate for the next drop. Then I began to wonder about fuel as well.

I was doing some bomber speed tests at 25k last week with 100% load.

B17 at 25k with 6x1000lb bombs will max out at 250 mph TAS.

Today I did the exact same test, only with 25% fuel. To my astonishment, the bomber reached 285 mph TAS at 25k with full bomb load.

After dropping the bombs, it jumped all the way up to 296 mph TAS.

This doesn't seem right... So, bombers gain +35mph simply because they never take more than 25% fuel? Then gain +11 more for losing a measely 6000lbs of bombs?

So why is it the P-47N (which I chose because it has the most gas of any fighter in this game) can gain only 2mph difference between 100% internal and 25% internal, but the B-17G gains +46mph (35 + 11) by losing internal weight?

Surely it can't simply be weight.

This makes me call into question all of the AH speed and climb charts, as well! They say the B-17G makes over 1000fpm climb past 20k... I know *I* sure as heck wasn't climbing 1000fpm at that altitude! Not even with half my fuel burned off! With about half my fuel burned off I was eeking by 700-800 fpm climb with 6x 1000lb bombs at 14k

So, what? Are the climb charts all done with stripped down, faster-than-they-were, no-ord, no-bomb, versions of all aircraft?


I find this extremely misleading.


Going back to the first question, how can this be possible, that internal weight can affect bombers so much, but fighters almost not at all?
Title: Internal weight and max speed
Post by: BaldEagl on March 26, 2007, 02:44:20 AM
Krusty,  I also suspect something is amiss.  Here's part of a post I made a few weeks ago:

I was flying an F4F-4 last night in the EW arena for quite a while trying to really get it figured out to where I could consistantly land kills in it against most of the other planes in that arena. For most of the night I was flying it with 100% fuel and the small (4 .50's) gun package. I was doing alright in it but nothing special.

Just as I was beginning to think my experiences in this plane in the past had been an aboration and that it was really a POS, I put in the bigger (6 .50's) gun package and reduced the fuel load to 75% and it almost felt as good as a FM2 (one of my favorite planes to furball in). I mean this thing came to life. I was able to toss it around with Zekes and Hurris where earlier it had been a wallowing pile of mush even after burning off most of the fuel load.

Similarily, I often fly the F6F-5 fully loaded in an attack role. After unloading my ord I often find myself engaged by enemy fighters. The F6F is a capable fighter without a doubt, but, one night a squaddie and I upped them in purly a fighter role and I gained a whole new appreciation for this plane.

Similar to my F4F experience of last night, even once the ord is dropped from the F6F it's not as lively as if you never loaded the ord in the first place.

Now I'm pretty sure the E6B calculator shows the loss of weight from both fuel burn and dropping ord but in reality it feels through performance that you are carrying a penalty with you the entire flight.


Since that post I've flown both the F4F and the F6F (the F6F both with and without ord) with reduced fuel and my success rate in both has seemed much better, even though, once engaged, I actually have the same or more fuel now (i.e.  weight) than I used to.  

Alternately, I still fly Spits a fair amount and adding a drop tank to a Spit seems to make no difference in performance from not carrying one at all.  In fact, most of the time I land with the sipper tank empty but still attached.
Title: Internal weight and max speed
Post by: dtango on March 26, 2007, 12:56:41 PM
Krusty:

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.

What you are probably seeing is a difference in level acceleration.  More weight = more mass which means the rate at which you can reach (accelerate) to the max end speed will be impacted.  It takes longer to get to max level speed for an airplane that has more mass vs. one that has less mass.

In other words I'm guessing the B-17 airspeed hadn't topped out yet and you hadn't reached the top level speed of the aircraft yet when you had more weight because the more weight you add the slower your increase in velocity is going to be.

W=m*g --> m=W/g
:  mass increases with weight

F=m*a --> a = F/m
: acceleration is inversely porportional to mass.  Lower mass, higher acceleration, higher mass, lower acceleration

For a B-17G
100% fuel = 2780 gallons = 17600 lbs
25% fuel = 695 gallons = 4400 lbs

There's a huge difference in mass.


===============
BaldEagl

Weight makes a big difference on maneuvering performance of an aircraft.

Tango, XO
412th FS Braunco Mustangs
Title: Internal weight and max speed
Post by: Noir on March 26, 2007, 01:36:44 PM
Lancasters gain a lot of speed after droping bombs too, I usually gain 5-10mph just by droping the cookies on my 18x500lb+4K setup.
Title: Internal weight and max speed
Post by: Krusty on March 26, 2007, 02:10:02 PM
It's not just acceleration, though. The way they are in here, the bombs and fuel in bombers greatly affect the top speed it can reach.

That 250 mph TAS speed, I was faster than that, and slowed down to it. I'd check E6B constantly, and if it dropped I'd throttle back a second then throttle up, if it dropped again, I'd repeat. This way I'd eventually drop to a point it no longer decellerated, and I'd speed up 1-3mph to top speed.
Title: Internal weight and max speed
Post by: hammer on March 26, 2007, 02:12:48 PM
It seems to me weight would make the most difference in level speed by forcing an increase in the Angle of Attack due to the increased lift required to counter the weight. This increased AoA would increase drag, which would reduce max speed. Without looking at numbers, it makes sense that the buffs would show a larger difference than a fighter.

Also remember to take into account the pylons for external ordnance are not carried on some fighters if you take off clean. To trully see the difference caused by weight, you need to start in the same configuration and drop the extra weight.

Regards,

Hammer
Title: Internal weight and max speed
Post by: detch01 on March 26, 2007, 02:20:57 PM
The heavier the wing loading on a given aircraft, the higher the angle of attack needs to be to create the necessary lift. More lift created = more induced drag created. If the ordnance load an airplane can carry is a significant percentage of the total aircraft (loaded) weight there is going to be a noticeable increase in maximum level speed, service ceiling and climb rate when the ordnance is dropped.



asw
Title: Internal weight and max speed
Post by: dtango on March 26, 2007, 03:51:11 PM
(http://brauncomustangs.org/upload/avd.gif)

This is a typical view of an aircraft's drag.  Line D is the total drag of the airplane as it varies with velocity.

Note induced drag (Line B) which is a function of lift that it decreases with increasing airspeed.  You have to remember that lift is a function of BOTH angle of attack and velocity.  Higher velocity means lower angle of attack needed for the equivalent amount of lift.

Note the difference between parasite drag vs. induced drag at higher airspeeds.  If you increase weight the the induced drag curve will basically shift up on the Y axis but the shape of the curve will remain pretty much the same.  That's why weight for the most part doesn't appreciably affect the max level speed of fighters.

It's conceivable that for a B-17G top level speed is less to the right of the graph (so to speak) therefore the impact of weight is felt more.  Someone can do the number crunching to estimate what the impact is.

Whatever the case when we talk about max level speeds of airplanes, specifically fighters you can see why increased weight doesn't appreciably change the top speed because far to right of the graph induced drag is pretty small compared to the overall drag of an airplane at higher speeds even if induced drag moves up on the drag (Y) axis when you increase weight.

Tango, XO
412th FS Braunco Mustangs
Title: Internal weight and max speed
Post by: Krusty on March 26, 2007, 04:09:43 PM
:huh
Title: Internal weight and max speed
Post by: straffo on March 26, 2007, 04:11:29 PM
What do you think of the effect of trimming on flight attitude and top speed ?

Btw if the weight is changed the trimming must change so ...
Title: Internal weight and max speed
Post by: Viking on March 26, 2007, 04:36:17 PM
Quote
Originally posted by Krusty
:huh


What's that matter? Dtango's short, but to the point explanation is correct. Elementary aerodynamics isn't that hard to understand.
Title: Internal weight and max speed
Post by: Benny Moore on March 26, 2007, 04:38:53 PM
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.

(http://users.adelphia.net/~j.r.engdahl/josh/Trim.gif)
Title: Internal weight and max speed
Post by: bozon on March 27, 2007, 07:57:34 AM
Don't forget that the speed in dtango's plot is IAS, not TAS.
At high altitudes IAS is much lower than TAS, so weight will have a bigger impact on top speed up high.
Krusty was measuring speeds at 25k.
Title: Internal weight and max speed
Post by: straffo on March 27, 2007, 10:01:09 AM
Quote
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.
 


I agree with that ,in fact I was more thinking of centering variation.
Title: Internal weight and max speed
Post by: dtango on March 27, 2007, 01:29:50 PM
Quote
Originally posted by bozon
Don't forget that the speed in dtango's plot is IAS, not TAS.
At high altitudes IAS is much lower than TAS, so weight will have a bigger impact on top speed up high.
Krusty was measuring speeds at 25k.


Very true.  250mph TAS 25K ~ 168mph IAS (assuming no position or compressibility error of course).

Tango, XO
412th FS Braunco Mustangs
Title: Internal weight and max speed
Post by: SgtPappy on March 27, 2007, 10:53:19 PM
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
Title: Internal weight and max speed
Post by: Knegel on March 28, 2007, 07:14:22 AM
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
Title: Internal weight and max speed
Post by: Knegel on March 28, 2007, 07:25:34 AM
Quote
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
Title: Internal weight and max speed
Post by: bozon on March 28, 2007, 09:27:30 AM
Quote
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).
Title: Internal weight and max speed
Post by: SgtPappy on March 28, 2007, 04:34:40 PM
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?
Title: Internal weight and max speed
Post by: Krusty on March 28, 2007, 06:50:28 PM
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.
Title: Internal weight and max speed
Post by: Charge on March 29, 2007, 03:56:44 AM
"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+
Title: Internal weight and max speed
Post by: Knegel on March 29, 2007, 09:41:53 AM
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
Title: Internal weight and max speed
Post by: SgtPappy on March 29, 2007, 05:44:16 PM
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.
Title: CG Affects Speed
Post by: Stoney74 on March 29, 2007, 10:21:44 PM
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?
Title: Internal weight and max speed
Post by: Krusty on March 29, 2007, 10:30:39 PM
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.
Title: Internal weight and max speed
Post by: Stoney74 on March 29, 2007, 11:11:55 PM
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?
Title: Internal weight and max speed
Post by: dtango on March 30, 2007, 05:14:52 PM
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:

(http://brauncomustangs.org/upload/drag1.gif)

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:

(http://brauncomustangs.org/upload/drag4.gif)

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
Title: Internal weight and max speed
Post by: Stoney74 on March 30, 2007, 07:45:48 PM
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...
Title: Internal weight and max speed
Post by: Krusty on March 30, 2007, 08:34:06 PM
Good question, Stoney. Maybe one of the folks at HTC will drop in and give us an answer.


(*waits expectantly*)
Title: Internal weight and max speed
Post by: dtango on March 30, 2007, 10:34:40 PM
Stoney:

Compressibility Drag:
I haven't verified it but I'd be surprised if they didn't model compressibility drag.

4400 lbs of difference in weight in the P-47N's you tested is a considerable amount.  The tests were done at 34k as well.  Lastly there are 2 more gun barrels sticking out in the wind at high speed (8 guns vs 6 guns).  Total drag increases with the square of velocity which means changes in overall drag coefficient get magnified.  The combination of all these could translate to the differences in max level speed you're seeing for the P-47N's you tested.

EDIT: Last but not least, the AH flight model is more sophisticated than my calcs.  My calcs were done to illustrate the principles.  Lift-dependent profile drag is not estimated which I know is in the AH flight model.  Also my compressibility drag polar I'm estimating may be different than how HTC is implementing their's.

Minimum Drag:
The point of lowest drag is where induced drag = parasite drag.  What use is this?

(1) For a jet, because the thrust doesn't vary with airspeed the maximum excess thrust is at the airspeed for lowest drag.  This is the speed for the best-rate-of-climb for a jet.  This is not true for a piston-prop because thrust varies with velocity.

(2) Knowing the velocity for lowest drag for a prop or jet plane tells you the velocity for the best glide ratio.

(3) Min drag (which occurs at max L/D ratio) also tells us the max endurance for a jet (how long it can remain airborne).  Note that endurance and range are not the same thing. [Tanget: Of course the tricky thing here is that weight changes with fuel burn therefore you have to either reduce your airspeed with reducing weight or keep changing AoA to maintain L/Dmax!].  This is not the case for a prop-plane.

(4) Velocity for Min drag (L/Dmax) will give us the maximum range for a prop aircraft.

Tango, XO
412th FS Braunco Mustangs
Title: Internal weight and max speed
Post by: Knegel on March 31, 2007, 12:52:44 AM
Hi,

the different "speeds of total drag" of different planes and at different loadout are one of the most important factors, if it comes to determining plane performence relations.

Best would be to have the curves for two extremes to compare them, unfortunately i dont know this curves for the A6M2 and FW190A8(probably upper and lower end of the scale).

But its safe to assume, the A6M2 had a very low "speed of smalest drag", while the FW190A8 had a rather high "speed of smalest drag".

It must be something like this(only a assumption of course):
 
(http://www.raf-roy.com/share/knegel/Screenes/sosdrag.PNG)

This explain the good climb and acceleration of the Zero at slow speed and the good acceleration of the FW190A in medium speed. And this explain why the FW190 didnt climb that good and dont had a good Vmax, while it had a very high cruize speed and why it could outclimb the F4U-1 and the P47 at medium speed, while it couldnt do this at slow speed.  This also explain why the inlunce of weight to the Vmax is rather big.

Would be nice to have real datas regarding this, at least we have a new FW190A8 to test it in a wind tunnel. :)


Greetings,

Knegel
Title: Re: CG Affects Speed
Post by: Shiva on April 01, 2007, 02:03:09 PM
Quote
Originally posted by 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.

If I remember the standard design parameters for aircraft correctly, the center of lift of the main wing is behind the center of gravity of the aircraft, resulting in a pitch-down moment; the horizontal stabilizer provides a downforce to balance the pitch-down moment. Because the amount of drag from the horizontal stabilizer goes up with the lift produced by the stab, as an aircraft is loaded to move its CG aft, reducing the pitch-down moment from the main wing and the compensatory lift required from the stab, decreasing its drag, resulting in a higher speed, whereas moving the CG forward increases the pitch-down moment, requiring more compensation from the stab, increasing the drag, and reducing the aircraft's speed.
Title: Internal weight and max speed
Post by: dtango on April 02, 2007, 09:59:35 AM
Here are the estimates for the B-17G:

B17G Sea Level: assuming 1200hp ea. eng.
Alt %F Weight rho Di CD0 Dp Dtotal Thrust V mph Mach
0 100 65500 0.0023 1333 0.032 5305 6639 6642 216.0 0.275
0 75 61330 0.0023 1143 0.032 5423 6567 6569 219.0 0.278
0 50 57160 0.0023 974 0.032 5528 6503 6507 221.0 0.281
0 25 52990 0.0023 823 0.032 5623 6447 6451 223.0 0.283

B17G 25k: assuming 1200hp ea. eng.
Alt %F Weight rho Di CD0 Dp Dtotal T V mph Mach
25000 100 65500 0.0010 2084 0.032 3394 5479 5475 263.0 0.33
25000 75 61330 0.0010 1725 0.032 3596 5321 5319 270.0 0.339
25000 50 57160 0.0010 1430 0.032 3768 5198 5196 277.0 0.347
25000 25 52990 0.0010 1182 0.032 3916 5099 5097 282.0 0.353

Impact of increased drag due to weight, the effects are definitely seen more at altitude than at sea level.

I've updated the P-51D estimates as well.  I had thrust set constant but modified it to vary with airspeed as it does in reality.  Here they are:


P-51D SL: 1650hp + 200lbs exh. Thrust
Alt %F Weight rho Di CD0 Dp Dtotal Thrust V mph Mach
0 100 12100 0.0023 122 0.019 1441 1564 1564 362 0.476
0 75 10606 0.0023 93 0.019 1462 1555 1554 365 0.479
0 50 9486 0.0023 73 0.019 1475 1548 1548 367 0.481
0 25 8739 0.0023 62 0.019 1483 1545 1545 368 0.483

P-51D 24K: 1250hp + 200lbs exh. Thrust
compressibility drag factored
Alt %F Weight rho Di CD0 Dp Dtotal T V mph Mach
24500 100 12100 0.0010 220 0.021 891 1111 1112 411 0.616
24500 75 10606 0.0010 163 0.0213 933 1097 1097 418 0.627
24500 50 9486 0.0010 129 0.0216 959 1088 1090 421 0.631
24500 25 8739 0.0010 108 0.0218 981 1089 1086 423 0.634

P-51D 24K: 1250hp + 200lbs exh. Thrust
no compressibility drag
Alt %F Weight rho Di CD0 Dp Dtotal T V mph Mach
24500 100 12100 0.0010 203 0.019 872 1075 1076 428 0.642
24500 75 10606 0.0010 149 0.019 909 1059 1058 437 0.655
24500 50 9486 0.0010 117 0.019 930 1047 1048 442 0.663
24500 25 8739 0.0010 98 0.019 942 1040 1042 445 0.667


On the issue of compressibility drag, I used the following curves to estimate the amount to add based the NACA XP-51 compressibility drag curves.

(http://brauncomustangs.org/upload/cdrag1.gif)

There are other sources that have more generic compressibility drag polar estimations that have drag rise starting at a higher mach number than the polar I was using as seen here:

(http://brauncomustangs.org/upload/cdrag2.gif)

My gaming PC is dead at the moment (ugh!!!) so I haven't been able to flight test anything.  For the P-47N tests that Stoney performed, if AH has compressibility drag modelled they may be using a model like the generic one above which means drag rise doesn't factor in until beyond Mach .7.

Tango, XO
412th FS Braunco Mustangs
Title: Internal weight and max speed
Post by: Stoney74 on April 02, 2007, 07:52:41 PM
Very nice information...
Title: Internal weight and max speed
Post by: gripen on April 04, 2007, 05:56:20 AM
Quote
Originally posted by dtango

On the issue of compressibility drag, I used the following curves to estimate the amount to add based the NACA XP-51 compressibility drag curves.


The effects of the compressibility tend to vary really a lot depending on plane. As an example in the RAE wind tunnel tests on the Spitfire and Typhoon (about 1:6 scale models), the Cd0 of the Spitfire raised from about 0,02 to about 0,045 when the mach number raised form about 0,5 to 0,78 while in the case of the Typhoon the Cd0 raised from about 0,02 to about 0,15 at same mach number increase. The AoA for the Cd0 stayed nearly constant -0,5deg for the Spitfire while in the case of the Typhoon the AoA for the Cd0 raised from about -2deg to +4deg. Most of this difference is caused by much thicker wing profile of the Typhoon.
Title: Internal weight and max speed
Post by: dtango on April 04, 2007, 03:02:42 PM
Good info as always gripen.

Tango, XO
412th FS Braunco Mustangs
Title: Internal weight and max speed
Post by: Stoney74 on April 04, 2007, 08:03:48 PM
What characteristics tend to affect compressibility effects?  I saw the mention of wing thickness.  Are their others?
Title: Internal weight and max speed
Post by: gripen on April 04, 2007, 11:16:44 PM
There is probably an endless list of things affecting compressibility; Reynolds number, temperatute, profile characters,fuselage shape, air intakes, tail surfaces etc. In the case of the P-38, the wing alone did fine up to above mach 0,7, but when connected with the fuselage and the engine nacelles, the airstream accelerated between the fuselage and nacelles (venturi effect) causing loss of lift in that section and so called "tuck under" starting around mach 0,68.
Title: Internal weight and max speed
Post by: Charge on April 05, 2007, 05:18:46 AM
I think the wing profile may be a rather significant factor of drag behaviour in high speeds and high loads.

If we look at P51 and 190 the the difference is that the P51 uses a symmetrical profile which does not "force" the change in AoA in speed change the same way as 190 as the 190 has an asymmetrical profile which forces the a/c to change its attitude more in flight when speed changes.

I think that especially when flown laden and high the P51 is in its own element, where as the 190 starts to suffer from its weight/wing-area ratio so that the wing is forced into AoA where the profile starts to create excess drag to keep up the lift (when compared to symmetrical profile). Maybe this explains the drop in top speeds between A5 and A8? Down at the deck there is almost no difference but at higher altitude the difference in weight starts to kick in.

BTW is the test flight data always corrected to standard atmosphere and temperature etc? I'd think that it has a significant effect on results.

-C+