Question For B-17 Pilots, Past or Present RE: Vmc

[Randy Haskin]

Perfect! Thanks for all the answers, gents. Much appreciated!

[JDK]

I'm certainly no B-17 expert, but...

The original statement has all the hallmarks of the exaggerated myth to wow the young punks. File with 'the gun was so big it stopped the airplane in flight' etc...

There are numerous types from the era which would be significantly more vulnerable to the gear affecting low speed or take off performance; the de Havilland Mosquito's 30 second retraction period came up in conversation yesterday. But high power twins like the Mozzie have always been more vulnerable to engine P factors or torque in this regimen, I understand.

The B-17 would be (I'd guess from the visuals) one of the lowest wing-loading heavies of W.W.II, and has a relatively small area of exposure of the gear compared to most other types. The higher wing loading and take-off speed (again I assume) as well as greater gear exposure to airflow of the B-24 comes to mind; never mind the Short Stirling's massive gear ironmongery. And then what goes for the B-17 would presumably go double for the B-25?

But if you are wowing neophytes with scary tales of 'way back when', it needs to be the only heavy bomber they've heard of...

Regards,

[Chris]

the Mains retract forward into the gear well so the CG would move forward a little bit but not much. As was stated in some previous responses, we call out 92 kts as our point of decision whether to fly or not to fly if adequate runway would even permit an abort. Also it's very true the gear takes a long time to fully retract. We are usually at about 135 or so by the time the gear is in the well. I suppose if you lost the outboard on takeoff and the gear on the same side happened to fail to come up, you could have some directional control issues. I've experienced one of each but never both on the same takeoff. There is a ton of rudder authority on the B-17 to counter an outboard engine failure. I've heard in the past of rumors which I can neither confirm or deny that B-17s have been ferried on three engines with the prop in feathered position and roped stuffed into the cylinder through the sparkplug hole to prevent windmilling should the prop unfeather. Like I said, just a rumor

[B25PBYGUY]

As a Liberty Belle B-17 captain/instructor, with a thousand hours in type, and some expertise, I'll make a couple of comments.
B-17 gear is electric and takes 45 sec. to retract/extend at 28.5V (as per maint. manual) depending on whether F or G motors are installed which differ in ratio.
The concept of a 'published' Vmc exists as a certification value (as used in more present day certification). It is generally accepted to be 106 mph in the 17. The critical engine being #1 aerodynamically, which if it were flight tested for certification, would be used. 'Actual' (as opposed to published) Vmc depends on several factors, (I'm not taking the time to explain it here) the first of which is determined by which powerplant has failed. For certification, gear extended reduces Vmc since the form drag would provide some resistance to yaw. That said, gear extended would rob performance. Potentially, the performance to accelerate to Vyse, or Vxse. Gear extended raises the stalling speed since a higher alpha is required to support the drag.....and vice versa.

The comment about pattern work and leaving the gear down, is to preclude the gear motor clutches from overheating, and failing on retraction.

The dampning moment of a B-17 wing planform somewhat retards the rolling moment, induced by the resultant yaw, of an outboard engine malfunction. The yaw would (hopefully) be arrested by the pilot applying opposite rudder pedal. Rudder authority being a function of forward speed and design. The loss of vertical lift of the rearward moving wing is a function of the Cosine of the angle (to the relative wind) times the TAS of the aircraft.

The 25 has hydraulically actuated gear and after liftoff at 120 mph, the pilot accellerate's to a Vy of 160, so as to pass thru the utilized Vmc of 145. Between liftoff at 120 and the accepted Vmc of 145, the rudder lacks the authority to preserve directional control and preclude the rolling moment toward the inoperative engine. Hence, a rapid acceleration to 160, since a catastrophic failure will disipate forward speed toward Vmc compromising controllability.

As was suggested in a previous post, the original comment is decidedly hearsay/war story type of proliferated BS.

Lastly, as mentioned in another post, the B-17 being ferried on 3 engines is not uncommon.

Toodles....

[bdk]

the Mains retract forward into the gear well so the CG would move forward a little bit but not much.

I've heard in the past of rumors which I can neither confirm or deny that B-17s have been ferried on three engines with the prop in feathered position and roped stuffed into the cylinder through the sparkplug hole to prevent windmilling should the prop unfeather. Like I said, just a rumor

So much for that theory!

C-17's did 3-engined takeoffs in flight test and think thee is something in the book about a 3-engined ferry.

[B25PBYGUY]

Many 4 engine airplanes have a procedure for a 3 engine ferry. In the DC-4, below 51,000 lbs., prop feathered.... etc.
I dont quite understand how rope in a spark plug hole could possibly preclude the mass of a rotating blade (that must not be fully feathered - aerodynamically) and overcome its inertia.

Interestingly, for an engine, that say, has a failed starter with no immediate replacement. It is possible to depart with that propeller feathered, and then unfeather it in flight to take advantage of the centrifugal twisting moment of the propeller (towards low pitch) to affect rotation for starting.
That's been done......

[Larry Kraus]

I think that the rope in the cylinder business was,as Chris mentioned,used with the prop feathered and was supposed to prevent the engine from rotating in the event that the prop began to unfeather in flight.I remember hearing about this trick in the 1960's from Kenny Stubbs,who was the Director of Maintenance at TBM.He spoke of setting a cylinder at bottom dead center and then filling the cylinder completely with clothesline rope via the spark plug hole.As the clothesline rope would not compress to any degree,the engine would not be able to rotate,but there should be enough give to prevent bending the connecting rod.I've never tried it,but Kenny spoke of the technique as being legitimate.

By the way,we 3 engine ferry DC-7's routinely without problems and have a supplemental section in our approved flight manual manual at Butler dealing with 3 engine ferry procedures and limitations.I think that my longest 3 engine ferry was just under 4 hours.I don't recall ever needing to 3 engine ferry a B-17,but we had few engine problems with the B-17 that couldn't be mended with a cylinder change.

[b29driver]

We also had procedures for three engine takeoffs and windmill engine starts in the KC-97. Multiengine airplanes with nosewheel steering lend themselves to these variations much better than a tailwheel airplane. The B-17 would need and extremely long runway to get off on three engines because of directional control issues. I can't think of any situation where I would consider doing it today.