Physics of a plane dropping bombs

Factual Questions
1

What happens to the aircraft the instant it drops it’s payload?

Assume a typical B-17 WWII bomber. We DO NOT need exact, nit-picking calculations on weight. All we need to know is that its total takeoff weight, fully loaded for a bombing run, is X + Y pounds: ‘Y’ being the bomb payload, ‘X’ being everything else (plane, crew, fuel, etc).

Also, assume that in this mission the bomber is dropping all it’s bombs in the same place at the same time (like a WWII “carpet bombing” mission), which I realize is not how it’s usually done today.

Searching the web I found lots of information on the B-17. The technical specs differ between various production models, but I did notice that bomb payload (Y) was about 20% of the total takeoff weight (X + Y). In one example, a pre-production test model for the Army was said to have a bomb payload of 8000 lbs. and a total takeoff weight (fully loaded and armed) of 40,000 lbs. Therefore, I get the rough estimate that, for this plane anyway, the bomb payload represented about 20% of the total weight that could be lifted off the ground. (Please correct me if I’m way way way off base, but no need to nit-pick, these are rough guesses anyway.)

OK, so they’re on their way, locate the target, and “bombs away!” What happens to the flight characteristics of the plane when it (suddenly) loses 20% of its weight? In what appears to be only seconds (albeit 15 or more, if the History Channel has served me correctly), the weight of the plane changes from X + Y to simply X.

What happens to the plane? What does the pilot do to compensate? I assume the plane is going along generating a certain amount of lift. What happens when the “lift to weight” ratio suddenly increases (dramatically)? Does the plane gain altitude, in fact “jump” up in the air? Does the pilot cut airspeed at the moment of the “drop”, effectively reducing the amount of lift being generated? Do they slow to near stall speed just before the drop?

Thanks.

2

I ANA pilot, but I read about them in books.

The plane actually does tend to lurch upward, but it’s not that big a deal. The pilot compensates by nosing the plane down a bit and adjusting the power settings.

Also the mass change is somewhat “disguised” by the aerodynamics of the plane while on it’s bomb run. The bomb bay doors are hanging open and creating significant drag and turbulence. The pilot had to adjust more for the doors than the bombs, and the doors would remain open for some time after the bombs fell. And when they closed, they closed SLOWLY.

3

The plane loses mass and thus gains speed and lift, both to a certain degree. The pilot takes all that into account when planning and executing his attack and flight path.

4

Well, what happens when an aircraft encounters a major updraft? Isn’t it the same problem? If pilots don’t want to gain altitude, they could change the ailerons which reduces the lifting force, or they could just reorient the entire plane so it moves more towards being in a shallow dive. Lifting force is proportional to angle of attack, so either tilt the trailing edge of the wing or tilt the entire wing by tilting the aircraft itself.

5

I’m not the most experienced pilot, but if I hit a major updraft, I’m not going to change the ailerons. I’m going to gently lower the nose.

Bomb dropping is a slightly different beast, because the changes are permanent and not temporary like an updraft. My guess is that when the bombs start dropping, the pilot will slowly put in some nose-down trim, and maybe even nudge the power back a little. This should counteract the increase in lift. And I think Monty is right–an experienced bomb-dropper will probably know, generally speaking, how much trim and power adjustment will be required.

6

One of the big advantages the US had during WW II was the Norden bombsight. That was considered a big secret that they did not want the Germans to get hold of. It was a big advance in dropping bombs more accurately.

From what I have heard the bombadier would actually end up steering the plane himself in order to get the bombs to drop right on the target. He did this only for a short time during the flight - right when they got near the target.

I bet a google search on Norden will bring up a lot of good info.

7

I don’t know about the B-17. With the Martin B-26 the upward lurch wasn’t noticeable. The formation was rather tight and wasn’t disturbed by the bomb release.

The bombay doors on this airplane opened and closed quickly. I would guess no more that 3-4 sec per operation.

8

I had this really cool guy once explain to me exactly how the Norden worked by pointing out the various controls on one in the Smithsonian. Wish I could remember his name.

The controls are exactly the same as one would find on a metal lathe, rings with knobs. Obviously, the plane can’t be steered that much from the Norden; it is really an automatic pilot which can be fine-tuned.

Here’s the most annoying part: I actually asked this guy, who had piloted B-17s in Europe, what happened when the bombs are released. Now I can’t remember what his answer was. My guess would be that the automatic pilot would compensate for the sudden shift in weight, and only afterward was the control of the plane turned back over to the pilot.

Also, I think that only the lead plane in the sub-formation was actually controlled by a Norden. In the other planes, the pilots lined up on the leader, and the bombardiers dropped when the leader did.

Hopefully, our resident bomber pilot and all around nice chap will be able to confirm all this, or expose me as an utter fool.

9

…And there he is!

10

I’ve never dropped a bomb, not jettisoned anything from an airplane but…

Let’s assume you’re making your bomb run. Your bomb bay doors are open and you’re flying straight and level. In this configuration your wings are developing just enough lift to keep you from descending, and not enough lift to make you climb. If you suddenly reduce the weight of your aircraft by 8,000 pounds, you are still making x pounds of lift. Therefore, the airplane will try to climb. If the pilot wishes to maintain the original altitude, he will have to adjust for it. (I dont think the bombardier is flying at this point, but I don’t know. In any case, “the pilot” here refers to whoever is controlling the aircraft.)

There are two ways to adjust lift: with the throttle and with the elevators. By reducing power, the aircraft slows down and lift is maintained at its original “setting”. In this way, the throttle controls lift. By pushing the elevators forward, the angle of attack is reduced and so is lift. Since the nose is down, speed will increase.

You could also bank the aircraft, which will move the lift vector out of vertical and introduce a lateral component. Take as an example the coordinated turn. You bank the aircraft with the ailerons and yaw the airplane with the rudder. But since the lift vector is no longer perpendicular to the ground, you need to add up-elevator to maintain altitude. In a 45° bank (or 60° – I forget which) you are pulling 2 gees, so if your airplane normally weighs 32,000 pounds without bombs it now “weighs” 64,000 pounds. If you’re trying to “make up for” 8,000 pounds of bombs, you can bank the aircraft at such an angle that it “weighs” 8,000 pounds heavier because of g-forces.

Of course, that’s all theoretical. I have no idea how bombers actually maneuvered after releasing its bombs.

11

Gee, Johnny, it has been awhile since you flew fixed wing, hasn’t it?

60 degrees of bank equals 2 g’s in level flight

45 degrees is something like 1.15 g

For an exact figure, it’s the inverse of the co-sign of the angle of bank. (I think - mostly I just go by how heavy my eyebrows get)

12

Heh. The last time I was in control of a fixed-wing was in a Cessna 172 on April 3, 1992. It’s all been rotary-wing since then. (And not nearly enough, dammit!)

Of course the gees are the same in a heli as they are in a Cessna. But it’s not one of those things that comes up in coversation, and I’ve never banked a heli to 60°. And I’ve never done negative gees in a heli:

:eek:

13

Again, I don’t know anything about the 8th Air Force operational methods. But in the medium bomber division of the 9th Air Force you are right.

[a primer]
The basic element was a flight of 6 aircraft in two vees one slightly behind and below the lead vee. These flights were then formed into boxes of three flights each, again flying in a vee with one flight being above, to the right and slightly behind the lead flight and another to the left, below and slightly behind. The typical group mission was two such boxes making a total of 36 aircraft in all. Occasionally a 3rd box was added for a total of 54 aircraft. This strained the availability of flyable planes for the typical group. With three boxes, the box grouping was like the flight grouping of a single box.

Bombing was done by three methods. By flights, by boxes and by the whole group.

Bombing by flights. Before the initial point the group broke into a line of flights in trail. The lead plane in each flight had a bombsight and a bombardier and the rest dropped on the leader. The lead plane of the second vee of the flight usually also carried sight and bombardier in case of trouble by the leader.

Bombing by boxes. The technique was like bombing by flights. Everyone dropped on the leader. Several planes in the box carried bombsights and bombardiers so they could take over the lead in case of trouble with the original leader.

Bombing by groups was for the times when the weather was so bad that a pathfinder aicraft was used. The pathfinder flew a radio “beam” to the intersection with check points identified by crossing radio beams. The pathfinder dropped bombs when the drop point was crossed. Everyone else dropped on the pathfinder.
[/primer]

I had a flight lead bombardier tell me that he always set his aiming point slightly short of the target. This allowed for the reaction time of the other aircraft and, he hoped, spead the drop over the target.

It would not be possible to maintain a formation with all the planes in it flying an independent bomb run. So a compromise was needed and the tacticians who knew how to figure these things out came up with the scenarios I have described.

A word about the Norden sight. It was a marvelous device. However, it was part of a complex system that involved the bombardier, the pilot or the autopilot in some cases, the wind, the aerodynamics of the bomb, the attitude of the plane at release and maybe a few other things that I have forgotten.

Given a day with zero wind, a pilot and bombardier who were not under stress and who identified the correct target, a bomb with nominal aerodynamics (i.e. no bent fins or anything like that), straight and level flight at release etc., the sight could drop bombs in a pretty tight pattern. I have no idea what the Circular Error Probable was, but I suspect that it was pretty good under ideal conditions. Ideal conditions never existed.

14

This is interesting and certainly makes sense. I do remember reading, though, that the lead plane would sometimes intentionally drop its bombs slightly past the target since subsequent planes, and waves of planes, would invariably release their loads earlier and earlier (I have no idea where to find the cite). I believe this had to do with some of the Allied bombing of Hamburg in particular. IIRC, the justification was in part that it was human nature to drop bombs just ahead of ground targets already aflame, and in part due to the fact that later waves were more likely to be spotted and attacked by the defenders and hence more likely to just want to drop their bombs and get the hell out of there ASAP.

15

IANA Bomber pilot, but a friend of mine used to fly water bombers, and we discussed this a bit. The aircraft definitely lurches upwards when a load is dumped, for the reason Johnny mentioned. The pilots get in the habit of pulling the release lever and pushing the nose down at the same time to maintain altitude.

The lurch may not be noticeable in a bomber if the weight of the load is a smaller fraction of the weight of the entire aircraft.

16

A friend of mine was a Lead Bombadier in WWII. He flew in the B-17 and should be able to aswer all this stuff first hand. I will ask if there was a noticeable RISE in the plane after dropping or did the pilot compensate or what. And see if he tried to aim short to compensate or anything like that.
I’ll give him a call Saturday and post back. Anyone have anything else I should ask?

17

Tell him happy birthday for me and I hope he has manny more. Also how old was he on his first combat flight?

18

OK… I got some answers for you guys:

Yes the plane rose the instant the bombs were dropped. It was not enough to be of much concern, but enough for the crew to feel the change. The pilot did nothing to compensate. All the planes rose after dropping the bombs so the formation was not effected.

When the bomb doors were opened, “you could tell”. But it did not create a lot of drag like the landing gear would.

The bomb doors opened “pretty quick”. It was comparable to the landing gear. But, he adds, if there was a problem, you’d have to crank them open manually and that took a while.

Aiming the bomb site. He did not try to “lead” the targer or line of the cross hairs ahead or behind the intended target. Before setting the sites, all the calculations were figured such as velocity, altitude and ballistic coefficient of the particular bombs being dropped. When it came time to aim, the cross hairs were placed directly on the target - not behind or in front.
He adds that you had to have all the information to accurately dropped. If something happened to prevent the planes from maintaining the predetermined altitude or speed, the bomb site was useless. He brought up something about the Memphis Belle movie where apparantly they had to back through the bombing run (which is crap he says. it would never happen) and their altitude had changed or something, and the pilot was like, “Let’s make these count”. He says they could not have been aimed properly if the variables had changed.
Note: I dont remember the movie, so I am not sure what happend in the example he gave… but either way I think we get the idea.

Also, he was 20 years old on his first bombing mission with the 91st Heavy Bomber Group.

Hope some of this helped.

19

Bear_Nenno, the implication seems to be that once the target was programmed into the bombsight, it couldn’t be changed, and if the planes couldn’t fly their programmed course, the whole exercise was for naught.

I KNOW that most missions had secondary target, if the primary was obscured by clouds or something. So the bombsight had to be reprogrammable in flight.

Am I misinterpreting something?

20

The Nordon sight was a type of analog computer. No programming was involved. The formation was brought to the initial point at which time the bombardier lined his cross hairs on the target. If the horizontal crosshair drifted, say, ahead of the target he adjusted the RATE knob slightly. If the vertical crosshair drifted right or left he turned the sight so as to bring it back on target. After a few iterations he had the RATE and the AZIMUTH adjusted so that the crosshairs stayed on target.

The AZIMUTH bombsight output was connected either to the autopilot in which case the sight directed the plane, or to what was called a PDI needle. This was a right-left indicator needle (a dial with a pointer in it) that the pilot followed and this got the plane onto the proper path.

The whole process took about a minute and a half, which seemed like forever.

If the bamardier couldn’t identify the target because of weather the run was aborted and if a secondary target had been picked that was attacked. If that was also obscured or there wasn’t one, when the raid was in occupied Europe, like Belgium or France, our group salvoed the bombs in the English Channel. If the target was in Germany a “target of opportunity” was picked and attacked or else the bombs were just dropped willy-nilly.

By the way, the OP mentioned the physics of bomb dropping. So I made some computations on the amount of rise and the velocity of same. In an earlier post I mentioned an upward “lurch.” I semi retract that. For the Martin B-26 the plane with bombs weighed about 32,000 poinds. The bomb load was 4000 pounds. The up and down forces are in balance before the drop. After the drop the upward force exceeds the down by 4000 pounds. The mass to be accelerated is the plane without the bombs or 28000/32.2 slugs (don’t blame me, I didn’t name the units in the English system). This gives an upward acceleration of 4.6 ft/sec[sup]2[/sup]. So at the end of 1 second the plane will have an upward velocity of 4.6 ft/sec which is about 300 ft/min which was the usual climb rate of a B-26 formation. Immediately upon bomb release the standard escape was to turn either right or left and lose 1000 ft of altitude so any upward motion disappeared the the post-drop maneuver as the throttle was cut back, the nose was lowered and the turn was made…