B-29 & Altitude

[Zipper730]

Don't forget propeller efficiency falls off with altitude.

You mean the lower speed of sound?

A main consideration may be wing lift.

The cruise speeds seem above the stall speeds

You also have the needs of formation flying in which the formation has to travel at the speed of the slowest aircraft and not only the slowest aircraft in the squadron/group but you have to plan the flight with the possibility that the slowest plane is on the outside of the formation on about 1/2 of the turns the formation makes.

Strange how such common sense variables have a way of being forgotten.

I have heard and read that B-29 operations started "high" but bombing accuracy was not all that great, sometimes due to heavy cloud cover, so they started coming in at 30,000 - 31,000 feet, and then started a slight descent to bomb at higher airspeeds and lower altitudes. The lower release altitudes materially increased bombing accuracy.

So they were starting out at 33,000 or even greater?

In the last few months, I have heard they were routinely releasing at 18,000 - 25,000 feet most of the time (mostly low 20's, but sometimes releasing as low as 10,000 feet or so) while moving along at 335 mph or faster in the descent. It made interception difficult and a second firing pass VERY difficult since most Japanese interceptors had top speeds near 350 - 370 mph, making for a very good shot for the B-29 tail gunners when closing speeds were low.

That's a smart move, the Germans did the same thing with their Greifs, though they could dive faster

 

[wuzak]

You mean the lower speed of sound?

Yes, and the lower density of air makes teh props less efficient as well.

The cruise speeds seem above the stall speeds

I should hope so!

 

[Shortround6]

Both propellers and wings have the same problems with air density. Propellers work by accelerating a mass of air to a higher speed and directing it rearward in relation to the aircraft. If the air at altitude"X" is 1/2 the mass/weight per cubic ft as sea level air a prop of a given diameter and pitch will only have 1/2 the thrust. Somewhat simplified but you get the idea.
Wings also provide less lift in less dense air at the same speed. Stalling speed goes up as air density falls. At a plane's "service ceiling" the plane is actually flying on a very narrow speed range or edge. Stalling speed has actually risen to a point (and power fallen) where the plane, flying straight and level has enough power to climb 100ft per minute. Bank/turn and the plane cannot maintain altitude. Cut the throttle below the power needed for level flight (and obviously there is precious little "extra" throttle) and the plane will either decend or stall/spin.
Just about all planes had a CALCULATED max ceiling because it took too long to climb from the service ceiling to the max ceiling, last few minutes of climb would be under 10 feet per minute of climb rate. And it took very skillful piloting as the max speed and the stall speed were almost identical.
Ceilings for formation flying (and small formations at that) were several thousand feet below service ceilings and combat ceilings were even lower.

 

[Greyman]

The Merlin III was a single speed engine.

The throttle is part closed at sea level and then gradually opens up, maintaining the boost level, until it reaches critical altitude - known as full throttle height (FTH) in the UK - at which point the throttle is fully open. Above that altitude the boost cannot be maintained, so power falls away.

As the boost levels for cruise and maximum power the FTH is very similar.

I'm not sure what the curve at the left is.

I get that aspect - but why is the critical alt speed lower in the Spitfire chart while being higher in the Lancaster chart (M & S ratio)? Maybe without knowing the engine settings for sure it's not much use.

Leftmost curve is best climb speed.

 

[GrauGeist]

I have heard and read that B-29 operations started "high" but bombing accuracy was not all that great, sometimes due to heavy cloud cover, so they started coming in at 30,000 - 31,000 feet, and then started a slight descent to bomb at higher airspeeds and lower altitudes. The lower release altitudes materially increased bombing accuracy.

In the last few months, I have heard they were routinely releasing at 18,000 - 25,000 feet most of the time (mostly low 20's, but sometimes releasing as low as 10,000 feet or so) while moving along at 335 mph or faster in the descent. It made interception difficult and a second firing pass VERY difficult since most Japanese interceptors had top speeds near 350 - 370 mph, making for a very good shot for the B-29 tail gunners when closing speeds were low.

That's a smart move, the Germans did the same thing with their Greifs, though they could dive faster

A couple of things here:
The B-29s started using lower altitudes because the Jet Stream and subsequent air currents over Japan proper made for terrible bombing accuracy from higher elevations. So the bombing altitudes were reduced to improve accuracy.

Regarding the He177, it was relegated to shallow dives only, in order to increase speed for a bombing run. Even though it had originally been mandated to be dive-bomb capable, that order was rescinded as it was structurally not able to perform that role.

 

[wuzak]

I get that aspect - but why is the critical alt speed lower in the Spitfire chart while being higher in the Lancaster chart (M & S ratio)? Maybe without knowing the engine settings for sure it's not much use.

Leftmost curve is best climb speed.

If I understand what you are asking correctly, the answer is the differences in boost between max weak (cruising) and all-out power.

Higher boost lowers the FTH. The Lancaster is running slightly more boost at cruise, but a lot more at maximum power.

If you look again at the Spitfire chart, http://www.spitfireperformance.com/spitfire-I-rae-12lbs.jpg, you can see a couple of lines drawn in pencil on the lower right. These correspond to +12psi boost and +16psi boost. You can see that the FTH is substantially below the FTH for maximum weak mixture (red line) and the FTH for maximum power (blue line).

For maximum power the FTH has gone from 18,500ft with +6.25psi bost to 13,000ft at +12psi boost and just under 7,500ft for +16psi boost.

The Lancaster has a 20-series Merlin, which had a much improved supercharger and intake system compared to the III. These improved FTH, improving performance at altitude.

 

[Greyman]

For maximum power the FTH has gone from 18,500ft with +6.25psi bost to 13,000ft at +12psi boost and just under 7,500ft for +16psi boost.

Right - I was getting hung up on the +6.25 boost figure. Everything fits if I compare the +12 or +16 boost Merlin III to the Lancaster figures.

 

[Shortround6]

The Merlin 45 and the Merlin XX used the same supercharger except for the 2 speed drive.
Merlin III and Merlin X used the same supercharger.
Be wary of comparing speeds at altitude between planes as the speed affects the critical altitude due to RAM.
A Hurricane and Spitfire have different critical altitudes (FTH) due to different speeds let alone a bomber.
This can easily be seen in looking at the charts for even the same plane for climb and for level speed.
a given boost level (say 6lbs) can be held several thousand feet higher in level speed vs climb speed due to the forward speed creating a higher pressure in the intake duct before the carburetor. a Spitfire had a higher critical altitude by around 1500ft than a Hurricane in level flight due to it's 40mph higher speed creating higher pressure in the intake duct.

 

[Greyman]

The Merlin 45 and the Merlin XX used the same supercharger except for the 2 speed drive.
Merlin III and Merlin X used the same supercharger.
Be wary of comparing speeds at altitude between planes as the speed affects the critical altitude due to RAM.
A Hurricane and Spitfire have different critical altitudes (FTH) due to different speeds let alone a bomber.
This can easily be seen in looking at the charts for even the same plane for climb and for level speed.
a given boost level (say 6lbs) can be held several thousand feet higher in level speed vs climb speed due to the forward speed creating a higher pressure in the intake duct before the carburetor. a Spitfire had a higher critical altitude by around 1500ft than a Hurricane in level flight due to it's 40mph higher speed creating higher pressure in the intake duct.

Quick Merlin III graph to your point:

Battle, Defiant, Hurricane, Spitfire --- based on A&AEE --- 6.25 boost, 3000 rpm

 

[Shortround6]

Thank you, I would be a bit suspicious of the Battle's line though.
The Merlin III was rated at 1030hp at 16,250 ft at 6lbs boost.

 

[Greyman]

Good point - looking at the Battle data it isn't from a normal-style Boscomb Down test and doesn't have a special data point for the critical altitude ... just a table with figures every 5,000 feet.

 

[stona]

This was a departure from precision bombing to area bombing and the beginning of the firebombing campaign.

You have to be wary when you use terms like that.Nobody precision bombed, even in perfect conditions, from 30,000' + in WW2.
Cheers
Steve

 

[Shortround6]

Most countries couldn't Precision bomb even in perfect conditions (like practice in a cloudless sky, no flak) even from 20,000ft.
They may have gotten good enough results under such conditions to make them think it was possible but there is a long, long way from possible to practical.

 

[GrauGeist]

"Precision Bombing" by heavy bombers at altitude, was achieved by saturating the target area, counting on the probability that several *may* hit the intended target.

The only Precision bombing that happened during the war (aside from dive bombers), was done by light/medium bombers during a high-speed, low altitude run.

 

[Greyman]

I think a couple of Lancaster Squadrons would disagree with you there.

 

[GrauGeist]

I think a couple of Lancaster Squadrons would disagree with you there.

If you're referring to the dam raids, they had a mix of results.

The Sorpe dam had two bombs dropped on it during the attack, but were off their mark. The dam remained intact.

The Eder dam had three attempts, the first missed it's mark, doing more damage to tje bomber than the dam, the second bomber found their mark but the dam held. It was the third bomber that finally breached the dam.

The Mohne dam took a tremendous effort by the bombers, finally being succesful in the end.

On the otherhand, the Mosquito raid on the Gestapo headquarters in Aarhus, is an excellent example of a successful precision raid. The precision raid on Gestapo headquartes (Shell House) in Copenhagen was technically a success, as the primary target was completely eliminated, however, an accidental strike on a building nearby killed over 100 civilians, diminishing the mission's success.

 

[gjs238]

Then there are the Tallboy and Grandslam raids.

 

[Shortround6]

The Lancasters did do some good work with Tallboys and Grandslams but the dropping altitude was something under 20,000ft wasn't it?

 

[wuzak]

The Lancasters did do some good work with Tallboys and Grandslams but the dropping altitude was something under 20,000ft wasn't it?

Between 12,000ft and 18,000ft I believe.

 

[wuzak]

Other Lancaster precision raids - Augsburg and Le Cruesot with the associated attack on Montchanin led by Gibson.