PB4Y Privateer / B-24 Liberator

[LT Ducky]

Title says it all; Basically the same airframe but why the different tails?

 

A Navy requirement?

Edited December 5, 2008 by LT Ducky

[Bob B]

It was supposed to improve handling, and the Army was looking at a Ford designed singled tail version of the B-24 at the end of the war, the B-24N. From what I understand Consolidated didn't want to be upstaged, with their own design, by Ford, who had done the original conversion. They pulled strings to get the ordered canceled. Likewise the Navy was worried that they might once again have to use an off the shelf Army plane, instead of their own PB4Y version, and they also pressed to get it canceled.

 

 

Lt Ducky, some designers advocated twin rudders for multi engine designs, while others advocated big single rudders. Then there was the triple rudder Lockheed Constellation, which I always thought was one of the most beautiful aircraft ever built. When I was a kid I can remember taking several trips on them. IIRC, you have flown a B17, do you think it makes a difference.

 

 

I always thought the twin tail version of the Lib looked better.

Edited December 5, 2008 by Bob B

[sunday]

Title says it all; Basically the same airframe but why the different tails?

 

A Navy requirement?

 

One of my firsts model built was a Airfix PB4Y-2. Pretty plane.

 

The -2 has also a stretched fuselage (and no turbos).

[LT Ducky]

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QUOTE(Bob B @ Fri 5 Dec 2008 1137) 624488[/snapback]

Lt Ducky, some designers advocated twin rudders for multi engine designs, while others advocated big single rudders. Then there was the triple rudder Lockheed Constellation, which I always thought was one of the most beautiful aircraft ever built. When I was a kid I can remember taking several trips on them. IIRC, you have flown a B17, do you think it makes a difference.

I always thought the twin tail version of the Lib looked better.

 

I flew IN a B-17, not at the controls (I wish!)

 

The design philosophy for twin tails was to put the rudders in the 'prop-wash' of an engine, giving more lateral authority with a smaller rudder

[Ken Estes]

It was slightly ironic that, although as early as 1942 the AAF decided that the Liberator would have greater stability with a single fin-and-rudder assembly, it went through its entire operational career with AAF in its original twin-tailed configuration.

1943 Ford converts a B-24D to XB-24K, single tail also more powerful R-1830-75 [1350hp at takeoff], 11 mph faster than previous Liberators, greatly improved climb rate.

 

Apr44 AAF decision that all future Liberators will have single tail fin, Ford redesigns XB-24K to XB-24N with tail gun like B-17 and nose ball turret. Ford builds seven YB-24N before production of Liberators ceases on May 31, 1945, cancelling 5168 B-24N on order!

 

USN order for over 720 PB4Y-2 had been made May43, first one flew Sept 20, 1943.

 

The PB4Y-2 was a rather more radical development of the Liberator than the AAF single-fin variants, and although it retained the same Davis wing and std undercarriage, ...forward fuselage lengthened 7' .. to normal Consolidated nose, tail and Martin dorsal turrets, , added two Erco blister waist gun positions and an additional Martin mid-upper turret = 12 MGs.

 

The vertical tail was of much higher aspect ratio than that of the B-24N, the tailplane being given ome dihedral to augment directional stability.... A few PB4Y-2s reached operational squadrons of the USN during the closing months of the war, but most of the extensive global career of the Privavteer was to be achieved after V-J Day.

 

William Green, Famous Bombers of the Second World War, (NY: Hanover Hse, 1959) - one of the best investments I ever made as a kid. The USNR sqadrons at Sand Point NAS, Seattle showed off their P2V and PB4Y-4 all through the 195os at openhouse, Navy Day, etc.

[hojutsuka]

The design philosophy for twin tails was to put the rudders in the 'prop-wash' of an engine, giving more lateral authority with a smaller rudder

I have always felt a bit dubious about this argument, since the loss of an engine, especially with a twin engined plane, would reduce your rudder authority just when you needed it the most to counter the asymmetric thrust. Anyone with knowledge of aircraft design care to comment? Ivanhoe?

 

Hojutsuka

[Ivanhoe]

Not my area of expertise, but whenever I see a single vertical stabilizer seemingly overproportioned to the rest of the beast, on a contentional airplane layout, first thing that comes to my mind is "loss of control authority or yaw stability at high alpha". H tails and V tails have the advantage of generally having at least some of their working surfaces in fairly clean air at almost all positive alphas. Looking at some measure such as "area of vertical stabilizer per weight of airplane" I would expect a conventional tail to have the highest ratio, V-tail next, H-tail lowest.

 

Another trick that can be done with H-tails is to angle the verticals such that the prop wash hitting each vertical has a net side force inwards (i.e. the net aerodynamic force on the fuselage by the fin) is towards the a/c centerline. Thus, if your starboard engine quits, the net side force generated by the portside vertical creates a net yaw moment opposing the yaw due to asymmetric thrust. Of course you can also try to work the swirl component of the prop wash into the mix. Not 100%, but back in the days of iron men and steel control rods, reducing pedal forces by even 25% could be a big deal. What I don't know is how often this trick was used; it does have its cost, since in normal symmetric flight with 2 happy engines, those pre-yawed verticals are generating some induced drag along with profile drag. On a big 4-motor I doubt they used this. On the flip side, if you assume you'll always have prop wash, depending on the relative geometry of the props and H-tail verticals, you could downsize the verticals a bit. I'm skeptical of this, simply due to safety of flight issues, but in the 1918-1945 period the design philophies were not as conservative as the post-war period IMHO.

 

The main thing about having your tailfeathers in prop wash is control authority at very low speeds (including speeds below stall). In these days of concrete, ILS, and 10kft runways, we forget how much of the time those old prop jobs spent maneuvering on the ground, and flying low & slow (often scud running). Back then, "rudder" was a very close second fiddle in the "stick and rudder" duet.

 

When looking at empennages, another thing that comes to mind, more on modern jets than on classics, is "what kind of vortices from the nose/canopy/wing roots are hitting the tail, and what did the design team have to do to alleviate the problem?" Back in the day, when men were men and large aircraft were beautifully hand-crafted smooth sculptures of Alclad, the "wake" of the front end tended to be broad and disorganized AIUI. These days, with sharp noses, forebody strakes, root extensions etc there can be pretty strong and coherent vortices crisscrossing the verticals, causing vibration and in some cases appreciable fatigue. In my callow youth, I recall a period when we mere mortals were locked out of our computer accounts at MCAUTO while the F-18 product team worked night and day (and then some!) getting a handle on the early Hornet's tail woes. IIRC it was vortices coming off the forebody strakes and impinging on the verticals, causing a fatigue problem.

[Ivanhoe]

I have always felt a bit dubious about this argument, since the loss of an engine, especially with a twin engined plane, would reduce your rudder authority just when you needed it the most to counter the asymmetric thrust. Anyone with knowledge of aircraft design care to comment? Ivanhoe?

 

I guess another way to look at it is to prioritize into "likely fatal" and "unlikely fatal". Most common failure, and the one involving UFIT, is engine failure during takeoff or climbout. In this case, due to the low airspeeds and lack of altitude to gain airspeed, generating enough yaw moment with the rudders to balance the yaw moment from thrust is the whole ballgame. If you can achieve a rough yaw equilibrium, you can fly out and live to tell; if you don't, spiral city.

 

The second most probable failure, AIUI, is in level steady. As long as airspeed is up around cruise numbers, in the short run as long as you have enough rudder authority to maintain control, you can trade altitude for airspeed and crank the snot out of the yaw trim wheel so that your leg doesn't collapse from the strain. Once you've got a new stable airspeed and altitude and have trimmed most of the force out of the pedals, everything gets easier. Recovery from that condition should be straightforward; head for your secondary, trade altitude for airspeed so that your remaining engine is only part-throttle, that sort of thing.

[hojutsuka]

I guess another way to look at it is to prioritize into "likely fatal" and "unlikely fatal". Most common failure, and the one involving UFIT, is engine failure during takeoff or climbout. In this case, due to the low airspeeds and lack of altitude to gain airspeed, generating enough yaw moment with the rudders to balance the yaw moment from thrust is the whole ballgame. If you can achieve a rough yaw equilibrium, you can fly out and live to tell; if you don't, spiral city.

 

The second most probable failure, AIUI, is in level steady. As long as airspeed is up around cruise numbers, in the short run as long as you have enough rudder authority to maintain control, you can trade altitude for airspeed and crank the snot out of the yaw trim wheel so that your leg doesn't collapse from the strain. Once you've got a new stable airspeed and altitude and have trimmed most of the force out of the pedals, everything gets easier. Recovery from that condition should be straightforward; head for your secondary, trade altitude for airspeed so that your remaining engine is only part-throttle, that sort of thing.

Thank you, Ivanhoe. I had heard that engine failure on takeoff/climbout was the critical case driving the size of the vertical fin and rudder on aircraft like the C-130 Hercules.

 

A very rough and ready calculation seems to show that if k is the improvement in rudder effectiveness due to the rudder being in the prop wash, then the total area for H-configuration compared to the total area for a single central fin and rudder for adequate rudder authority in case of engine failure on one side is 2/(2+k). So some improvement, but not as much as if we could assume the engines would never fail, in which case the ratio would be 1/(1+k).

 

A tall central fin and rudder is located above the fuselage and wings, so must generate a rolling force when the rudder is used. With an H-configuration, the fins and rudders are typically much closer in vertical distance to the wing and fuselage, so I would expect the rolling force to be much less. Do you know if this might be a factor in choosing the H-configuration for some aircraft?

 

Regards,

Hojutsuka

[DKTanker]

I guess another way to look at it is to prioritize into "likely fatal" and "unlikely fatal". Most common failure, and the one involving UFIT, is engine failure during takeoff or climbout. In this case, due to the low airspeeds and lack of altitude to gain airspeed, generating enough yaw moment with the rudders to balance the yaw moment from thrust is the whole ballgame. If you can achieve a rough yaw equilibrium, you can fly out and live to tell; if you don't, spiral city.

Hence the huge and esthetically unpleasing vertical stabilzer of the C-17?

[shep854]

Low-speed control is a large consideration in the vertical stabilizer size. Besides having enough surface area to be effective at low airspeeds (necessary for short/soft-field operations), the fin needs to be tall enough to clear fuselage turbulence at the high angles of attack required for short/soft ops. Also, maneuvering at low altitudes and relatively low speeds make for high AoA, where high control authority is vital. The B-24 was designed as a straight-and-level bomber, whereas the Navy needed a plane capable of tight-turning, on-the-deck combat (a four-engine fighter, in a manner of speaking).

 

Given the low/slow nature of WWII maritime patrol, did the Navy find the single large fin more controllable when compared to the original mission of high/fast bombing? If the majority of the flying is low/slow, then the additional drag should not be a concern. The classical patrol mission was pretty much straight-and-level, but the realities of combat necessitated a plane that could mix it up without suddenly turning on its crew.

Edited December 6, 2008 by shep854

[beans4]

Interesting pics from a Privateer restoration on the Warbird Info Exchange forum. The twin gun, fully-enclosed side blister turrets were new to me. Allegedly all turrets will be fully functional when the restoration is finished.

 

http://warbirdinformationexchange.org/phpB...pic.php?t=26185

[shep854]

Beans4, THAT IS AWESOME! Thank you! I have always wondered about those waist positions; I could not figure how a powered turret went in there, and couldn't find any details or photos. I always thought the guns were handheld with some sort of power assist. Those -2's were certainly gun-heavy.

[sunday]

Beans4, THAT IS AWESOME! Thank you! I have always wondered about those waist positions; I could not figure how a powered turret went in there, and couldn't find any details or photos. I always thought the guns were handheld with some sort of power assist. Those -2's were certainly gun-heavy.

 

Ditto. I know that those blister were powered defensive positions, but not idea of their layout.

[DanielStarseer]

OK, I'll bite.

How much more development went into these later B-24 variants before we saw the definitive layout of the B-32 come forth?

 

...or was the B-32 a clean design from the ground up, so to speak?

[beans4]

You're welcome. IIRC, there is a Privateer that is being restored to flying status somewhere; not sure if it's this one, though.

 

WIX is a neat forum, lots of fun stuff going on there. If you have time, there is a very long thread (with pics) about the Commemorative Air Force returning their LB-30 "Diamond Lil" back to it's origins as a rare B-24A.

[shep854]

You're welcome. IIRC, there is a Privateer that is being restored to flying status somewhere; not sure if it's this one, though.

 

WIX is a neat forum, lots of fun stuff going on there. If you have time, there is a very long thread (with pics) about the Commemorative Air Force returning their LB-30 "Diamond Lil" back to it's origins as a rare B-24A.

 

I bookmarked the page. One of the warbird mags had what seemed to be a fairly detailed article on the CAF's reconversion of "Diamond Lil".

 

Back to the Privateer; it suddenly occurred to me a little while ago just how much the Millenium Falcon's turrets resembled the -2's waist turrets.

[BansheeOne]

That was my first thought upon seeing the pics, actually.

[shep854]

That was my first thought upon seeing the pics, actually.

 

My first though upon seeing those picks was, "Oh WOW! So that's what they look like on the inside!"

 

But then, considering how much WWII ordnance Spielberg and Lucas adapted for Star Wars, it should not be too surprising.

[DKTanker]

My first though upon seeing those picks was, "Oh WOW! So that's what they look like on the inside!"

 

But then, considering how much WWII ordnance Spielberg and Lucas adapted for Star Wars, it should not be too surprising.

Spielberg...Star Wars?

[shep854]

Spielberg...Star Wars?

 

Got me.

[nitflegal]

Spielberg...Star Wars?

 

The rumor has long been that Lucas used Spielberg as a sounding board during the SW process for many things, including where the aerial influences came from. Never confirmed to my knowledge.

 

Matt

[Bob B]

Beans4, THAT IS AWESOME! Thank you! I have always wondered about those waist positions; I could not figure how a powered turret went in there, and couldn't find any details or photos. I always thought the guns were handheld with some sort of power assist. Those -2's were certainly gun-heavy.

 

 

I also got curious about the turret and finally found a photo of one that shows some of the inside details:

 

http://www.air-and-space.com/20070106%20Ch...inboard%20l.jpg

[shep854]

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QUOTE(Bob B @ Mon 8 Dec 2008 1537) 625039[/snapback]

I also got curious about the turret and finally found a photo of one that shows some of the inside details:

 

http://www.air-and-space.com/20070106%20Ch...inboard%20l.jpg

 

Cool! Thanks! Now...if a (training) video of the thing would turn up...

 

As an aside; any guesses as to what those models hanging in the background represent? They resemble Frogfoots (Frogfeet? ) or P-59 Airacomets.

[Mike Steele]

Spielberg...Star Wars?

Speilberg had his folks review WWII footage of various engagements so his folks could get the 'feel' right.