Bombers and G-Loads

[KJ_Lesnick]

I'm wondering here what the maximum g-load of light-bombers like the A-20, A-26, and medium bombers like the B-25.  Does anybody have anything?

[Old Wombat]

That depends on how you're looking at "g's" because it is more complex than most people realise. My physics days are long behind me & I can't remember the equations but eventually "g" comes down the being a function of mass, distance, time & direction, & changes in these. When people speak of "g-forces" they generally mean inertia acting against the force of changing momentum acting on a human body during a change in direction. However, it can just as easily be carried over into aircraft but you have to greatly increase the mass involved when calculating the inertia & momentum, plus you have to take into account the fact that the force is not being exerted down the spine of the aircraft but, rather, along the spine.

For a example, let's take bungee jumping (far, far away, as far as I'm concerned ); if a normal bungee jump, was to be carried out with the jumper strapped into a seat, the force is taken down the axis of your spine (& you're now 2" shorter). So, instead of tying the rope to a chair & sitting in it, let's have you spread eagled with ropes tied to your wrists & ankles.... now, imagine what will happen to you when you hit the end of the bungee .

Same "g-force", different effect.

However, bombers are built to carry all that extra weight, so their frames are stronger & in normal operations they are experiencing forces greater than those of fighters - it's just that the crew aren't. Now, if all excess mass was removed from the aircraft (especially the fuselage) many light/medium bombers could pull as many or more g's than most fighters. However, with the usual extra weight that nasty pair of little monsters, Inertia & Angular Momentum, make pulling tight turns an experiment in instant design modification as they try to rip the fuselage out from beneath the wings.



Note: that does not (necessarily) mean they could turn inside a smaller, single-engine fighter but they could certainly give it a run for it's money!


PS: for specific g-loads I would suggest you try to find anything you can from the original testing but I doubt you'll find actual figures because until late in the war, even into the 50's, much of that stuff was worked out (guesstimated) empirically - by test pilots.

[Hobbes]

IMO, G is the second derivative (i.e. accelleration) of the change in direction of an object. The G-value has nothing to do with the mass of the object.

The shape of the structure, its mass and its strength is important for calculating how much G it can withstand.

Now, if all excess mass was removed from the aircraft (especially the fuselage) many light/medium bombers could pull as many or more g's than most fighters.

No they couldn't, because you've just removed the mass that makes up the structure of the aircraft. An empty bomber can do tighter turns (ie generate more G) than a fully loaded bomber, true.

[raafif]

not really relevant to the Q but a guy in the States was building an F4U Corsair from new -- apart from the engine, 100% totally new metal using modern design materials, techniques, fixings (rivets/bolts etc).  He said he has added more gusset-plates, better joints etc.  Think he said it would end up slightly lighter than the WW2 version.

Claims he has upped the G-rating of the aircraft structure from a max of 4g to 7g -- now he only has to finish the bird & then do the same to his own structure so's he doesn't black out

[KJ_Lesnick]

Old Wombat,

However, bombers are built to carry all that extra weight, so their frames are stronger & in normal operations they are experiencing forces greater than those of fighters - it's just that the crew aren't. Now, if all excess mass was removed from the aircraft (especially the fuselage) many light/medium bombers could pull as many or more g's than most fighters.

I never thought of it that way.  Just to be clear, when you say extra mass you're talking about bombs and most of the fuel?  With that said does this mean if the plane could pull 4g's at 65,000 pounds, and a little more than half that weight was fuel and bombs, at 32,000 pounds flying on fumes it could pull 8g? 


Hobbes,

IMO, G is the second derivative (i.e. accelleration) of the change in direction of an object. The G-value has nothing to do with the mass of the object.

Yeah, but as I understand it the acceleration forces are an example of increasing the structural load on the plane (in this case by acceleration).  Adding extra mass would also increase the load on the plane structurally too.

No they couldn't, because you've just removed the mass that makes up the structure of the aircraft. An empty bomber can do tighter turns (ie generate more G) than a fully loaded bomber, true.

If however the bombs and most of the fuel were tossed off however...

[Hobbes]

I never thought of it that way.  Just to be clear, when you say extra mass you're talking about bombs and most of the fuel?  With that said does this mean if the plane could pull 4g's at 65,000 pounds, and a little more than half that weight was fuel and bombs, at 32,000 pounds flying on fumes it could pull 8g?

Not necessarily. In order to withstand 8G, the entire structure needs to be strong enough. The tailplanes and cockpit section don't need to carry much weight, so they aren't going to be built strongly. Jet engines warp under stress, so the fans are going to hit the stators at 8G unless the engine has been designed to tolerate this load. 

[rickshaw]

Does that mean that piston engines also distort under stress?  Do cam shafts distort and pistons fail to reach full compression or hit the cylinder heads?

[PR19_Kit]

Does that mean that piston engines also distort under stress?  Do cam shafts distort and pistons fail to reach full compression or hit the cylinder heads?

No. Because much of a piston engine is operating under INCREDIBLE internal G loads anyway, just because of the way they work. Plus they have to be built pretty darn strongly because of those same forces. It amazes me that Junkers ever managed to make an airborne diesel light enough to fly yet strong enough to work!

[Old Wombat]

Hobbes, dude, I did say "excess" mass.

That means bombs, bomb-bays, turrets, any now unnecessary crew (& their equipment, seats, life-jackets, parachutes, etc.) &, perhaps, some of the fuel tanks (except many of them were in &/or part of the box-frame of the wing-beams). Cockpit areas were stronger than necessary just for the pilots because they, also, had to carry the weight of nose turrets & equipment further forward, as the rest of the aircraft was stronger to carry all the mass we've just removed, too.

KJ, it's not quite that simple, as there are two major forces at work (inertia & angular momentum) & neither of them are precisely direct ratios to the mass, but that plane might get up to 6-to-7g or as little as 5-to-6g, if you're adding extra mass (like guns & ammunition).

[famvburg]

      I bet if you ask over at WIX  www.warbirdinformationexchange.org, somebody over there will know. I don't think the answers will be as complicated as some are making it sound over here either.

I'm wondering here what the maximum g-load of light-bombers like the A-20, A-26, and medium bombers like the B-25.  Does anybody have anything?

[Old Wombat]

I suppose I have made it sound a bit complex and, in the realm of physics, it is an interestingly complex interaction of forces.

However, in the realm of WW2 modifications, they would probably have just ripped out all the excess mass, stuck some lucky bugger in the cockpit and said "Just take this for a quick spin, will you, old chap, & throw in a few tight turn for good measure, what!", and see what happened.

[KJ_Lesnick]

No answer at all

[Hobbes]

Google unearthed this:
http://www.warbirdinformationexchange.org/phpBB3/viewtopic.php?p=398961

[sagallacci]

I would WAG that G-load data would actually be readily available in the flight ops manuals for the various types and would likely include data for full and empty weights. For light and medium bomber/attack types, they were expected to be maneuverable and/or have some level of dive bombing capability but did have limits, least something important break.

[famvburg]

     Hmm. looks like where I suggested she ask........ Also, if you're that desperate, you could look around eBay for flight manuals/pilot handbooks for the a/c. There are several sellers who offer them on CD for usually less than $15 - $20. Even less if you do a download.

Google unearthed this:
http://www.warbirdinformationexchange.org/phpBB3/viewtopic.php?p=398961