Compressibility problems - Aircraft of World War II

MaxQ · 11-05-2007, 01:19 AM

I'v read that quite a few US fighters ran in to compressibility problems during dives. Except for the Me262 I don't recall if any other German fighters had this problem, or other fighters from differant countries.
MaxQ

drgondog · 11-05-2007, 10:16 AM

Max - virtually all of the high performance a/c would enter compressibility in a dive, speed dependin on Critical mach number of the wing. The complications that arose were formation of a shock wave at the point where the velocity was greatest over the wing - leading to separation, change of center of lift, turbulent flow over stabilizer, etc

all bad things.

the Me 262 you used as an example would start to mask the horizontal stabilizer effectriveness and move the center of lift aft (IIRC) which in turn would cause the nose to tuck - but at a higher diving speed than the 51 or 47 or 38.

mad_max · 11-07-2007, 01:56 AM

The P-51 had a limiting safe Mach of .80-.81 depends if British or American data.

Atfirst the P-38 was around .65 until dive recovery flaps were installed, then it was
around .75 from what I gather.

The P-47 had a limiting safe Mach number of around .79.

Most Luftwaffe fighters had a limiting safe Mach number of about 0.75 (Bf-109, Fw-190A), while the Dora comes out to 0.77.

The Spit's was a safe Mach number of 0.83.

The Me-262's limiting safe Mach is .83.

Sure most could be dove at higher mach, but your taking a chance of structural
failure and/or uncontrolled flight.

So yes the 262 had a slightly higher dive speed. Over the Mustang of approx. 14 mph at
30,000ft and 20,000 ft. At 10,000 ft approx. 5 mph. All from Mach tables.

Disclaimer....the above can change with further information.

drgondog · 11-08-2007, 12:18 AM

Quote:

Originally Posted by mad_max

The P-51 had a limiting safe Mach of .80-.81 depends if British or American data.

Atfirst the P-38 was around .65 until dive recovery flaps were installed, then it was
around .75 from what I gather.

The P-47 had a limiting safe Mach number of around .79.

Most Luftwaffe fighters had a limiting safe Mach number of about 0.75 (Bf-109, Fw-190A), while the Dora comes out to 0.77.

The Spit's was a safe Mach number of 0.83.

The Me-262's limiting safe Mach is .83.

Sure most could be dove at higher mach, but your taking a chance of structural
failure and/or uncontrolled flight.

So yes the 262 had a slightly higher dive speed. Over the Mustang of approx. 14 mph at
30,000ft and 20,000 ft. At 10,000 ft approx. 5 mph. All from Mach tables.

Disclaimer....the above can change with further information.

The primary issue wasn't stucture - it was the aerodynamic forces which caused unwanted control inputs and change in attitude that made airframe structure inadequate.

At .82-.85 the Me 262 intial shock wave/compressibility effects caused the horizontal stabilizer to start becoming ineffective and the Center Lift moved toward cg, then aft causing a pitch down that would not correct..

At .78-.81 the Mustang started a severe yaw to right and even the boosted trim tab on the rudder became less effective at controlling that yaw- somewhere in the dive the 51 tail was in danger of failure.. You DID NOT want to use trim to get out of a compressibility dive.

Etc, etc

mad_max · 11-08-2007, 11:52 AM

These limits are supposed to be max. limits for tactical control. Not the max. that
was higher, but dangerous to perform. My fault if anyone thought this was the Max.
Mach numbers that a specific airframe could achieve.

11-05-2007, 01:19 AM	#1
MaxQ Junior Member Join Date: Nov 2007 Location: CT Posts: 3	Compressibility problems I'v read that quite a few US fighters ran in to compressibility problems during dives. Except for the Me262 I don't recall if any other German fighters had this problem, or other fighters from differant countries. MaxQ

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11-05-2007, 10:16 AM	#2
drgondog Senior Member Join Date: Jun 2006 Location: oregon Posts: 3,940	Max - virtually all of the high performance a/c would enter compressibility in a dive, speed dependin on Critical mach number of the wing. The complications that arose were formation of a shock wave at the point where the velocity was greatest over the wing - leading to separation, change of center of lift, turbulent flow over stabilizer, etc all bad things. the Me 262 you used as an example would start to mask the horizontal stabilizer effectriveness and move the center of lift aft (IIRC) which in turn would cause the nose to tuck - but at a higher diving speed than the 51 or 47 or 38.

11-07-2007, 01:56 AM	#3
mad_max Senior Member Join Date: Oct 2006 Posts: 110	The P-51 had a limiting safe Mach of .80-.81 depends if British or American data. Atfirst the P-38 was around .65 until dive recovery flaps were installed, then it was around .75 from what I gather. The P-47 had a limiting safe Mach number of around .79. Most Luftwaffe fighters had a limiting safe Mach number of about 0.75 (Bf-109, Fw-190A), while the Dora comes out to 0.77. The Spit's was a safe Mach number of 0.83. The Me-262's limiting safe Mach is .83. Sure most could be dove at higher mach, but your taking a chance of structural failure and/or uncontrolled flight. So yes the 262 had a slightly higher dive speed. Over the Mustang of approx. 14 mph at 30,000ft and 20,000 ft. At 10,000 ft approx. 5 mph. All from Mach tables. Disclaimer....the above can change with further information.

11-08-2007, 11:52 AM	#5
mad_max Senior Member Join Date: Oct 2006 Posts: 110	These limits are supposed to be max. limits for tactical control. Not the max. that was higher, but dangerous to perform. My fault if anyone thought this was the Max. Mach numbers that a specific airframe could achieve.