Improve That Design: How Aircraft Could Have Been Made Better

[GrauGeist]

Now we see "center bearing support for flexible splines coupling", and a whole new arena of potential disaster scenarios opens up. I hope that center bearing support is bolted to some mighty robust structure, or I see a potential wrecking ball.

A follow-on to Thomas' diagram (in answer to Swampyankee's query), there was a center joint supported by a bearing.

Shown here, from a P-39Q, is the driveline between the V-1710-85 and the propellor's gearbox. The first segment of shaft (60 inches) runs beneath the cockpit. The last segment (60 inches) runs beneath the weapon bay.

 

[XBe02Drvr]

Shown here, from a P-39Q, is the driveline between the V-1710-85 and the propellor's gearbox. The first segment of shaft (60 inches) runs beneath the cockpit. The last segment (60 inches) runs beneath the weapon bay.

Very interesting. And apparently devoid of U-joints.

 

[special ed]

Now we need a description how the flexible splines coupling works.

 

[P-39 Expert]

Suggest you take a P39. Put the engine in front of the pilot, saving the weight and complexity of the drive shaft, its supporting structure and making it less twitchy on COG issues. Move the guns to the wings with uniform 0.50.
It would weigh a lot less than the P40 with similar power equalling a better performance.

The drive shaft only weighed about 10lbs. If you put the engine in the front you increase the weight a couple hundred pounds for a new engine mount. Basically no engine mount on the P-39 since it was mounted on the wing.

 

[P-39 Expert]

The gearbox and reduction box at either end were armored for a reason.

The driveshaft transferred a great deal of energy and if either gearbox was damaged, then all that energy stored in the shaft under load has to go somewhere.

A bullet through the prop was not enough to stop the aircraft - a P-47 took a 20mm round through a prop and flew back to England. Matyer of fact, a P-47 flew through an Olive grove, bending back all four of the props (amongst other things) and still flew over 100 miles back to base.

So back to the core of the issue:
Is the armor at the prop's gearbox nessecary? Yes.
Is the armor at the engine's gearbox nessecary? Yes.
Reason for the armor? To prevent catastrophic failure if struck by projectiles while in combat.

Reduction gearbox armor on the P-39 is unnecessary since no other planes had armored reduction gearboxes. Now the gearbox itself may be sturdy enough to withstand a direct hit, but I doubt it.

There were no structural issues with the driveshaft. No need for the nose gearbox armor plate, no issues with the symmetrical airfoil, and no tumbling problems. Only problem with the early P-39 was it was too heavy to climb above 20000ft with a drop tank. Dump the wing .30s and the nose armor and even that is fixed. Move on.

 

[GrauGeist]

lmao...no other aircraft had nose armor because they didn't have 10 feet of driveshaft between it and the engine.

It's EXTREMELY obvious that a few people here don't understand why the gearbox was armored, but removing the armor would have been sheer folly and is why it was never done, so, move on.

 

[GrauGeist]

Just an example of what a driveshaft can do when it fails under load:

 

[XBe02Drvr]

Just an example of what a driveshaft can do when it fails under load:

Can you imagine that right underneath a full load of 37MM and .50 cal ammo?

 

[XBe02Drvr]

Reduction gearbox armor on the P-39 is unnecessary since no other planes had armored reduction gearboxes.
There were no structural issues with the driveshaft. No need for the nose gearbox armor plate, no issues with the symmetrical airfoil, and no tumbling problems. Only problem was it was too heavy to climb above 20000ft with a drop tank. Dump the wing .30s and the nose armor and even that is fixed. Move on.

"To argue with a person who has renounced the use of reason is like administering medicine to the dead."
Thomas Paine

 

[GrauGeist]

Can you imagine that right underneath a full load of 37MM and .50 cal ammo?

Notice that the shaft breached the safety devices and ripped through the Camero's steel floorpan and steel tunnel?

Typically, a stock driveshaft on a '69 Camaro weighs about 25 pounds, I'm guessing his was reinforced and rebalance for pro-street.

But a 10 foot long shaft, which had considerably more weight, coming loose in a predominately aluminum structure would be a terrible mess.

 

[wuzak]

Can you imagine that right underneath a full load of 37MM and .50 cal ammo?

Or your balls!

 

[wuzak]

The drive shaft only weighed about 10lbs. If you put the engine in the front you increase the weight a couple hundred pounds for a new engine mount. Basically no engine mount on the P-39 since it was mounted on the wing.

The fuselage structure was stronger behind the pilot (the engine bay) than a typical aircraft with a nose mounted engine.

And the engine was not mounted on the wing.

Also, how much did the gearbox weigh?

 

[BiffF15]

Or your balls!

Testicular fortitude required.

 

[XBe02Drvr]

Testicular fortitude required.

Armor plated codpiece, anyone?

 

[GrauGeist]

Also, how much did the gearbox weigh?

Not sure about the gearbox's weight, but the shaft (each one) would have weighed at least 75 pounds. They weren't hollow like an automobile, but rather turned from solid stock (much like a hydraulic ram) in order to handle the torque between the engine and the prop's loading as well as being "tuned" (balanced rotationally).

I would also add that the center carrier bearing assembly that joined the two shafts weighed in the neighborhood of 10 pounds.

 

[wuzak]

Not sure about the gearbox's weight, but the shaft (each one) would have weighed at least 75 pounds. They weren't hollow like an automobile, but rather turned from solid stock (much like a hydraulic ram) in order to handle the torque between the engine and the prop's loading as well as being "tuned" (balanced rotationally).

75lbs each? I was assured that it only weighed 10lb!

 

[wuzak]

Now we need a description how the flexible splines coupling works.

I would think that the male spline would be spherical and the female spline straight.

This would allow for small misalignments in the shafts and the forward shaft to translate fore/aft as the airframe flexes.

 

[GrauGeist]

75lbs each? I was assured that it only weighed 10lb!

I saw that original comment and figured it was supposed to read "100 pounds", which would be realistic.

 

[Shortround6]

There may have been 3 different shafts used between the XP-39 and production P-39s.

The XP-39 was fitted with shafts of 2.500 diameter and 0.156 wall thickness to begin with but new shafts were fabricated while the XP-39 was at Langley.
The New shafts (weight increase 11-15lbs but not specified as to each shaft or both shafts) were 2.550 in diameter and wall thickness 0.200.

Production P-39s got 3.000 diameter shafts that were only 5 lbs heavier (book doesn't give wall thickness.) all in the larger couplings and flanges.
The 3 in shafts were supposed to have the same torsional characteristics as the 2.550 in shafts.


a 60 in solid shaft 3 in in diameter would weigh about 118lbs (?) depending on exact alloy.

BTW the Shaft is one of the reasons there is considerable doubt about the XP-39 ever coming close to 390mph in it's original form (pre Langley) as the engine was restricted to either 2600rpm or 2700rpm due to potential vibration problems should the engine misfire at higher RPM.

The 2nd shaft was supposed to be about 30% stiffer and raised the vibration problem area by 400rpm which was above the normal operating range of the Allison at that time.

The entire drive shaft may have weighed 100lbs? (or 50lbs plus the fuselage weighing another 50lbs more than a front engine plane due to extra stiffening?)
I can't remember were I read that or the exact numbers assigned to each part (shaft/s and fuselage)

The drive shaft (the the flexible coupling in the middle) was designed to accommodate +/- 1 degree of misalignment between the engine drive flange and the gearbox input flange.
This was supposed to be a total of 3.64in of possible travel over 10 ft.

 

[Admiral Beez]

OTOH - engine was firmly behind the curve by 1940.
All in all, perhaps going with Fulmar or Henley as a dive bomber nets you a better solution for 1939-42?

That's probably where the Skua needs help the most, where its 905 hp Perseus (55" dia, 1,025 lb) engine needs a replacement. What pre-Hercules options do we have? The Bristol Taurus is rated at 1,050 hp and at 46" wide and 1,301 lb. it could allow the Skua's engine bay to be narrowed and more streamlined, but it's 30% heavier.

If we're keeping the Perseus, reject the fighter role and remove two of the forward .303 guns to allow for a heavier bombload.