Bubble canopies?

[GregP]

The P-47 was not the P-51, and the bubble canopy did cause a slight instability. Since the P-51 drosal ahd already been tried and it worked, it was natural to try it out on the P-47, where it ALSO worked. A general rule of aerodynamics is that if you remove wetted area behind the CG, you must add it somewhere else to restore the lost directional stability. They could have accomplished the same thing if they simply increased the vertical fin area by a similar amout to the dorsal extension ... but that would have caused a production interruption ... a real no-no in WWII's middle to late stages for the U.S.A.

We were winning with the tools in place at the time, so we didn't want to interrupt the flow of war materiel to the various fronts. It turned out to be the correct decision, didn't it?

 

[R Pope]

Grumman's method of manufacture could have had a bearing on why they never went to a bubble. The fuselage was built up of rings of simple-curved sheet metal with rivetting flanges bent inward so the rivets were inside. Simple and strong, but requiring much redesign to make any changes. Not just a question of swing off the formers and reskinning.

 

[CobberKane]

The P-47 was not the P-51, and the bubble canopy did cause a slight instability. Since the P-51 drosal ahd already been tried and it worked, it was natural to try it out on the P-47, where it ALSO worked. A general rule of aerodynamics is that if you remove wetted area behind the CG, you must add it somewhere else to restore the lost directional stability. They could have accomplished the same thing if they simply increased the vertical fin area by a similar amout to the dorsal extension ... but that would have caused a production interruption ... a real no-no in WWII's middle to late stages for the U.S.A.

We were winning with the tools in place at the time, so we didn't want to interrupt the flow of war materiel to the various fronts. It turned out to be the correct decision, didn't it?

Sounds good to me! I believe Hawker went through pretty much the same process with the Tempest, although they redesigned the whole fin rather than sticking a fillet on it. How about the
FM2 compared to the F4F ? No bubble canopy, obviously - a design reaction to compensate for the extra torque of the more powerful engine?

 

[bobbysocks]

as i understand it the dorsal fin was more to address an issue the 51 had from the beginning even with the razorback. the eppenage had a structural issue and would break away from the fuse either in a high G pull out ( or turn ) ...or a high speed roll...like a snap roll. i cant remember which it was or if it was both. but it was a result also of the higher HP and torque of the merlin over the allison. the airframe had been designed witht he allison rating and was not modiflied in the tail section once the engines were swapped. IIRC the mod was after D-15 and retrofitted to all war fit ac...Cs and Bs ( if they still had any ). to address the stability they increased the size of the rudder and the horizontal stabilizer but that was later.

 

[GregP]

The tail did not break away in a high speed roll ... it was simply not stressed for snap rolls, which are almost always lower speed rolls anyway. NOBODY snap rolls from cruise speed! A snap roll is basically a horizontal spin, and nobody enters a spin from crusie speed either.

If the P-51 did a very high speed aileron roll, there was no problem, nor were point rolls or slow rolls, just snap rolls, usually called a flick roll (1 snap roll) or a vertical reverse (1/2 snap roll) in WWII. The P-51 was also not to be aerobated if there were any fuel in the fuselage tank ... it was suppose to be empty or nearly so since the fuselage tank effectively moved the CG too far aft when it was significantly filled.

For more explanation of my earlier post about an increase in blade area forward, the Bell P-63 used a 4-bladed prop and was designed for it. Some people tried to retrofit a G-series Allison and a 4-bladed prop to the P-39 for racing. When they did, it became alost unflyable due to the increase in blade area forward of the center of lift. Think of the Mike Carol P-39 accident in California. I know I said CG eariler, but I believe it is area ahead and behind the CL, not the CG. They are close to one another, but not the same.

Drogondog, do I have that right, increasing the blade are ahead of the CL requires compensation behind the CL with an increase in fin or rudder area, or moving the existing fin and rudder area back to create proper directional stability balance, right? That would mean yaw and pitch stability, not roll stability, which is unaffected by blade area ahead increases of the CL.

 

[barney]

as i understand it the dorsal fin was more to address an issue the 51 had from the beginning even with the razorback. the eppenage had a structural issue and would break away from the fuse either in a high G pull out ( or turn ) ...or a high speed roll...like a snap roll. i cant remember which it was or if it was both. but it was a result also of the higher HP and torque of the merlin over the allison. the airframe had been designed witht he allison rating and was not modiflied in the tail section once the engines were swapped. IIRC the mod was after D-15 and retrofitted to all war fit ac...Cs and Bs ( if they still had any ). to address the stability they increased the size of the rudder and the horizontal stabilizer but that was later.

"Unless a dorsal fin is installed on the P-51B, P-51C and P-51D airplanes, a snap roll may result when attempting a slow roll. The horizontal stabilizer will not withstand the effects of a snap roll. To prevent recurrence, the stabilizer should be reinforced in accordance with T.O. 01-60J-18 dated 8 April 1944 and a dorsal fin should be installed. Dorsal fin kits are being made available to overseas activities"

A U-boat sank the ship carrying these first kits.

As I recall, the reinforcement of the horizontal stabilizer amounted to the installation of larger diameter rivets.

 

[drgondog]

The tail did not break away in a high speed roll ... it was simply not stressed for snap rolls, which are almost always lower speed rolls anyway. NOBODY snap rolls from cruise speed! A snap roll is basically a horizontal spin, and nobody enters a spin from crusie speed either.Greg, IIRC there was a cautionary put out regarding slow rolls also but I'll have to research that

If the P-51 did a very high speed aileron roll, there was no problem, nor were point rolls or slow rolls, just snap rolls, usually called a flick roll (1 snap roll) or a vertical reverse (1/2 snap roll) in WWII. The P-51 was also not to be aerobated if there were any fuel in the fuselage tank ... it was suppose to be empty or nearly so since the fuselage tank effectively moved the CG too far aft when it was significantly filled.

For more explanation of my earlier post about an increase in blade area forward, the Bell P-63 used a 4-bladed prop and was designed for it. Some people tried to retrofit a G-series Allison and a 4-bladed prop to the P-39 for racing. When they did, it became alost unflyable due to the increase in blade area forward of the center of lift. Think of the Mike Carol P-39 accident in California. I know I said CG eariler, but I believe it is area ahead and behind the CL, not the CG. They are close to one another, but not the same.

Drogondog, do I have that right, increasing the blade are ahead of the CL requires compensation behind the CL with an increase in fin or rudder area, or moving the existing fin and rudder area back to create proper directional stability balance, right? That would mean yaw and pitch stability, not roll stability, which is unaffected by blade area ahead increases of the CL.

Greg - I do not know. Offhand I can't see any reason why switching from three blade to four would alter stability in any way. It was not a factor for any of the P-51 series including experimenting with a five blade prop - Ditto Spitty. It shouldn't alter thrust axis, could affect some characteristics of the prop vortex, but can't see how it affects stability around X, Y or Z axis.

 

[wuzak]

Greg - I do not know. Offhand I can't see any reason why switching from three blade to four would alter stability in any way. It was not a factor for any of the P-51 series including experimenting with a five blade prop - Ditto Spitty. It shouldn't alter thrust axis, could affect some characteristics of the prop vortex, but can't see how it affects stability around X, Y or Z axis.

But the extra power would cause the change?

Rolls-Royce's conversion was found to have very poor stability, until they added fin area. NAA altered more than Rolls-Royce did in changing to the P-51B. IIRC teh wing was moved relative to the fuselage.

I'm not sure with the XII, but the thrust line for the Spitfire XIV did change - the engine was angled down to give a better view over the nose for the pilot, which in turn required a smaller diameter prop and the need for 5 blades to absorb the power.

The engine in the XII wasn't angled down, but I am unsure as to the relative position of the thrust line.

 

[drgondog]

What I understand is that it was the reduction in vertical area aft of the C of G that caused stability issues, hence the fillet on the fin. It wasn't the bubble canopy (or whatever you want to call it)per se, but the other changes in the airframe as a result.

It was the addition of the 85 gallon tank aft of the seat which caused the aft cg issue. The vertical stabilizer for P-51A-D was slightly inadequate in low speed when the Merlin engine replaced the Allison. The aft fuselage structure was marginal at design load limits in asymmetric flight conditions (High speed roll) as well as a snap roll at any speed.

While the notion that loss of the turtleback was the cause of yaw issues in a dive, the primary reason was that the rudder was neither sized nor stressed for the major jump in torque for the Merlin version. The P-51B/C had these same issue to a slightly less degree at high speed.

The P-51H specifically addressed the associated problems when they introduced the vertical stabilizer/rudder`increase in size and Lengthened the airframe aft of the CG by 13". The P-51H had Very few common parts with any previous P-51A-D.

The ventral fin did slightly improve yaw characteristics but that was it. I will have to scratch my head and see what if anything NAA did to improve the structural capabilty of the eppenage section - but I doubt that increasing rivet size in the elevator was performed. Rivets are there to introduce shear loadpaths for skins and loss of the eppenage should not have anything to do with shear panel buckling per se - the buckling, if any occurred, would have been due to the primary load path (Spar/bulkhead) failure. IIRC the observed failure mode was first the vertical stab/rudder failed along with the aft section of the fuse where the vert tail main spar connected to the fuselage.

Candidly I never heard of an elevator/horizontal stabilizer being the cause of any structural failure - although the fabric covered elevators were replaced by metal elevators beginning with the P-51D-10 (when the dorsal was introduced in production) and both were retrofitted to all Mustangs (P-51B-1 through P-51D-5) as field kits.

 

[drgondog]

But the extra power would cause the change?

Wayne - the extra torque of the Rolls was the root cause of directional stability issues with the P-51B-D. Additionally the 4 blade Hamilton Standard prop was 5" greater in diameter and obviously with greater mass distributed radially away from the thrust axis.

Rolls-Royce's conversion was found to have very poor stability, until they added fin area. NAA altered more than Rolls-Royce did in changing to the P-51B. IIRC teh wing was moved relative to the fuselage.

The only changes relative to the wing were as follows. The P-51A/MarkII version, when the P-51B/Mark III was redesigned, 'dropped' the wing approximately 7" to accomodate the new carb/lower cowl design and fare it into the wing. The main spar and wing did not change fore/aft along the X axis.

I'm not sure with the XII, but the thrust line for the Spitfire XIV did change - the engine was angled down to give a better view over the nose for the pilot, which in turn required a smaller diameter prop and the need for 5 blades to absorb the power.

I would have to check but the thrust axis of the P-51B/D probably did change from the A, and from the B/C/D to the H

The engine in the XII wasn't angled down, but I am unsure as to the relative position of the thrust line.

Additionally, the wing plan view (Root Chord to WS 61.5) changed from the B/C to the D series to accomodate new wheel cover doors and gear uplock re-design. Zero change to the airfoil, span, tip, flap, etc.

 

[GregP]

Adding power always decreases stability since the aircraft is most probably flying with the power it was designed for to start with.

According to Pete Law, several test pilots, and my aerodynamics courses way back when the earth cooled, adding blade area ahead of the CL will decrrease stability. The additional power wouldn't have any effect until the original power level was exceeded. That is, if the Merlin were to be operated at the Allison power level, then the only destabilizing force would come from the additional blade area ahead of the CL ... which is one of the factors in the case of the P-51B/C versus the P-51A.

The combination of extra blade area ahead of the CL, more power, and fuselage tank were enough to make it a real handful early in the escort flight. At takeoff, they could not fight effectively at all, and needed to at least burn off most of the fuselage tank before they were effective or very maneuverable.

 

[bobbysocks]

when based in england engaging the enemy shortly after take off was a slim chance. shortly after take off they would burn fuel in the fuse tank until it was down to ~35 gal ( ~50 min flight time ) and then switch to drop tanks. that 50 minutes was generally forming up and gaining alt while heading across the channel. now for 51s based in europe after d-day that is a different matter. they were closer to the front and actally in the case of y-29 they got into battle shortly after take off. in those planes did they even used the fuse tank as they really didnt need the range?

 

[barney]

Candidly I never heard of an elevator/horizontal stabilizer being the cause of any structural failure - although the fabric covered elevators were replaced by metal elevators beginning with the P-51D-10 (when the dorsal was introduced in production) and both were retrofitted to all Mustangs (P-51B-1 through P-51D-5) as field kits.

Google "T.O. 01-60J-18 dated 8 April 1944" to find many references to this modification.

 

[drgondog]

Google "T.O. 01-60J-18 dated 8 April 1944" to find many references to this modification.

Found many references but could not find the Technical Order itself. Still skeptical about solving the problem with a larger shank rivet. That would reduce shear stress on both a supporting doubler/spar as well as the shear panel - but don't see how that solves a structural failure that almost had to be due to bending.

 

[Milosh]

when based in england engaging the enemy shortly after take off was a slim chance. shortly after take off they would burn fuel in the fuse tank until it was down to ~35 gal ( ~50 min flight time ) and then switch to drop tanks. that 50 minutes was generally forming up and gaining alt while heading across the channel. now for 51s based in europe after d-day that is a different matter. they were closer to the front and actally in the case of y-29 they got into battle shortly after take off. in those planes did they even used the fuse tank as they really didnt need the range?

They took off with a full fuel load.

From Bodenplatte, pg 152: ".... went into dogfights with all tanks full."

 

[drgondog]

Adding power always decrfeases stability since the aircraft is most probably flying with the pwoer it was designed for to start with.

According to Pete Law, several test pilots, and my aerodynamics courses way back when the earth cooled, adding blade area ahead of the CL will decrrease stability. The additional power wouldn't have any effect until the original power level was exceeded. That is, if the Merlin were to be operated at the Allison power level, then the only destabilizing force would come from the additional blade area ahead of the CL ... which is one of the factors in the case of the P-51B/C versus the P-51A.

Greg - the problem I have with the notion that additional blade area/disk area causes an issue by and of itself is that there are too many contra-examples of going from 3 to 4 to 5 blades. The only two changes I see from a free body diagram is a.) increase in Thrust (potentially), and b.) a possible increase in the stream tube physics. For the latter we may be looking at upwash/downwash effects on the wing but other than that Possibiliy the Thrust axis does not change, thereby introducing zero ZY force vectors.

The combination of extra blade area ahead of the CL, more power, and fuselage tank were enough to make it a real handful early in the excort flight. At takeoff, they could not fight effectively at all, and needed to at least burn off most of the fuselage tank before they were effective or very maneuverable.

I have never flown a fully loaded 51. I have had many hours of interested 'shop talk' about the Mustang during WWII escort operations. Nobody considered it a 'real handfull' early in the escort flight simply because there were no high G maneuvers experienced in takeoff rool, formation assembly and climb out - and by the time they were crossing the channel the aft c problem was not a problem. As to fight effectively maybe JC Meyer was lucky when he shot down an FW 190 before his gear was fully retracted, then another one 5 miles out from Asch - all with full fuselage tank.

Fully understood that they were not high G manuevers - just a skillful pilot managing the fight with full understanding of his current limitations.

I would love to hear Pete Law's theory on the stability issues created by higher disk density - and help me understand why there seems to be no common ground for consistency?

 

[Milosh]

I think GregP has been reading too many of Gaston's posts.

 

[CobberKane]

Adding power always decrfeases stability since the aircraft is most probably flying with the pwoer it was designed for to start with.

According to Pete Law, several test pilots, and my aerodynamics courses way back when the earth cooled, adding blade area ahead of the CL will decrrease stability. The additional power wouldn't have any effect until the original power level was exceeded. That is, if the Merlin were to be operated at the Allison power level, then the only destabilizing force would come from the additional blade area ahead of the CL ... which is one of the factors in the case of the P-51B/C versus the P-51A.

The combination of extra blade area ahead of the CL, more power, and fuselage tank were enough to make it a real handful early in the excort flight. At takeoff, they could not fight effectively at all, and needed to at least burn off most of the fuselage tank before they were effective or very maneuverable.

You would have to to think that by the end of the war the torque effect of increasingly powerful engines was really pushing conventional airftames and airscrew systems to the limit and if the jets hadn't come along things like contra-rotating props would have become virtual neccessities. When the Tempest was in it's development stage the torque effect from the engine was so strong that test piolts would start facing in the opposite direction from the intended take-off run, do a sweeping torque induced 180 degree turn onto the runway, by which time there was enough airflow over the rudder to compensate and hopefully they would be pointing in the right direction. Not exactly ideal practice for the average pilot on a crowded runway!

 

[barney]

Found many references but could not find the Technical Order itself. Still skeptical about solving the problem with a larger shank rivet. That would reduce shear stress on both a supporting doubler/spar as well as the shear panel - but don't see how that solves a structural failure that almost had to be due to bending.

Sorry about that - try here
TO 01-60J-18 Reinforcement of horizontal stabilizer and fin - P-51B, P-51C, P-51D and P-51K

 

[bobbysocks]

i dont remember reading anything specific about separation of the eppenage that i can remember. i do recall is reading the terms "structural failure" and "catastrophic failure of the horizontal stabilizer" in relation to this subject which gave no specifics. so, i erroneously filled in the blanks as meaning separation. my main crux i was trying to get across was the dorsal fin is more often that not attributed to the streamlining of the fuse when the bubble canopy is used and not to fix the problems that existed prior to that. mention was made about the replacement of fabric with metal skins.... looking into to it a little more i found this...its the first time i heard of the crafts tendency to "porpoise"

Note on Dive Tests on 'Mustang IV'