Warbird Information Exchange

Hi everyone,

This may be a silly question, but I've never looked into it before.

I was hoping someone out there could tell me what caused the shift in B-17 design from the small tail of the B's and C's to the larger tail on later models?

I think I've got a few reasons why in my head, but want to hear the right answer.

Did it have anything to do with insufficient directional stability, thus requiring a larger vertical stabilizer....

Or was it more to do with the aircraft's defenses and the requirement to have a tailgunner?

I'd like to hear what you all think.

Cheers,

David

tail gunner, and more room for the waist gunners as well.

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From what I have read the tail redesign was for the tail gunner.

I don't know that the rudder is all that much larger on the E vs the D model. I do think the directional stability and airframe strength must be better with the larger tail.

I know the Boeing 307 was developed from the B-17 and that it started life with a small tail as well. I don't know which came first the big tail B-17 or 307.

The B-17E came out in 1940/41 and the 307 started flying in 1939...

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Hi Davey;

Can't say for sure about the B-17, but the whole design "community" was going through a re-assessment of directional stability needs in multi-engine aircraft at that time. The original B-17, and contemporaries like the Do17 and some of the French twins had very small vertical surfaces, sized for minimum drag in normal cruise flight. Little or no thought was given to low speed, engine out, requirements. It took designers (and their customers) a while to realize that the safety advantage of multiple engines was usually negated by stall/spin accidents while trying to land with an engine out. These were generally power-on stalls in a turn at low altitude - never much fun. Old aviation joke from that time: on a twin, the second engine is there to get you to the accident site after the first one fails.

WARNING: optional math bit. The side force generated by a fin is directly proportional to fin area, and proportional to the square of airspeed. The need for side force is greatest when you are trying to balance one engine out. Lower speed to normal landing speeds, and the available side force quickly drops, until the pilot can't keep the nose straight. Up goes the drag, down goes the speed and you are in a vicious spiral - literally.

After a series of accidents like this, people started to investigate the causes, and came up with concepts like Vmc (minimum control speed with one engine out) - usually well above stall speed or even normal landing speed if the aircraft had a small vertical fin like the early B-17s. There were 2 ways to reduce these landing accidents: keep speed well up after an engine failed, which wasn't easy with airfield sizes back then, or grow a bigger fin so you could land at similar speeds to all-engines-on.

I suspect that when the need for a tail gun came up on the B-17s the designers realized it would be a major re-design of the tail cone and vertical fin, and took advantage of this to incorporate the larger fin area. There wasn't much change in rudder area, but the fixed fin grew considerably. Also, the large ventral fin prevented tail stall at high sideslip angles. Look at the leading edge extensions on something like an F-18. Same idea, just in a different direction.

Hope I haven't bored anybody.

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I think it was a control issue: but as a side benefit, I am certain a small tail B-17 may never have taken some of the damage a large tail could. I am talking about the more serious flak and a/c strikes, where it made it home with 1/3rd or more of the tail surface gone...

Robbie

Thanks guys. More than I expected, but happy to receive it!

Cheers,

David

The prototype 307 flew with the small tail at the end of 1938. The second airplane first flew in August of '39 with the larger B-17E style tail, but I have a photo of it with the small vertical. Evidently the larger tail was retrofitted after the prototype crashed but before the first flight of the second airplane. The info I have also has the third production aircraft being delivered to Howard Hughes with the small tail, being refitted with the larger fin later.

From the information I can find, the lateral stability of both airplanes was in question and Boeing was working on a fix very early in the program, using the 307 as the "test mule" for the larger tail surfaces.

As to the large vertical fin being much beefier than the sharkfin, most of that metal is just a fairing. The vertical stabilizer on both airplanes was plenty stout, just like the primary structure all over the B-17.

Scott