Internal weight and max speed

[Krusty]

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?

[BaldEagl]

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.

[dtango]

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

[Noir]

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.

[Krusty]

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.

[hammer]

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

[detch01]

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

[dtango]

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

[Krusty]

[straffo]

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 ...

[Viking]

Originally posted by Krusty

What's that matter? Dtango's short, but to the point explanation is correct. Elementary aerodynamics isn't that hard to understand.

[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.

[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.

[straffo]

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.

[dtango]

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