B-29 & Altitude

[Zipper730]

If I recall the aircraft could maintain takeoff rated power to 35,000 feet, why did the aircraft cruise all the way down at 31,000-33,000 feet?

 

[wuzak]

Don't forget propeller efficiency falls off with altitude.

 

[Shortround6]

A main consideration may be wing lift. The higher you go the less lift at a given speed. 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. Unless the "slow" plane has a margin of speed/lift in hand it will not be able to keep up with the formation.

 

[rinkol]

I would add that the early R-3350 engines were problematic. Increasing the duration of the climb to reach higher altitudes would have exacerbated engine reliability issues. Also, at higher altitudes, the air is thinner and cooling becomes more of a problem.

 

[Shortround6]

There are B-29 manuals in the manuals section of this web site.

B-29 and P-61 Flight Manuals

Which have got climb to altitude charts and fuel used and other information that show why operations were seldom, if ever, conducted at the limits of an aircraft's performance envelope.

 

[BiffF15]

It could be due to weight. Modern airliners except maybe the 787, takeoff and initially climb to their most efficient altitude for their current weight. After fuel is burned they will step climb several times always seeking the lowest fuel burn by cruising at the most efficient altitude for their given weight.

Cheers,
Biff

 

[Greyman]

Lower engine settings result in a lower critical altitude, no?

So while best speed at full (take off) power is attainable at 35,000 - best speed at cruise settings will be a few thousand feet lower.

 

[FLYBOYJ]

Lower engine settings result in a lower critical altitude, no?

So while best speed at full (take off) power is attainable at 35,000 - best speed at cruise settings will be a few thousand feet lower.

Probably - but you'll have to add a few more variables in there like air temp and density altitude. Some of this is captured in the flight manuals, there may also be a requirement to use a "whiz wheel," basically a circular slide rule to calculate this, usually done by the flight engineer. I think there was one developed for the B-29.

Edit:
Green Flight Engineers Computer
Made in USA by Alex. E.S. Green during WWII.
For maximizing the fuel range of B-29 Gun during long over-water missions.

Obviously its not circular!

 

[GregP]

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 from former crew members at the museum during talks at the monthly events, but I have also seen in print before, though no specific reference comes to mind just now. So far, I haven't gone back and dug it out of primary sources myself.

 

[wuzak]

Lower engine settings result in a lower critical altitude, no?

So while best speed at full (take off) power is attainable at 35,000 - best speed at cruise settings will be a few thousand feet lower.

Generally speaking, lower engine boost means higher critical altitude.

I was unsure if this applied to turbo engines, but looking some data from teh P-38 it appears if it is so.

P-38J Performance Test

In section II you can see the critical altitude for 70inHg MAP is 19,800ft and for 60inHg is 24,000ft.

 

[GregP]

Makes sense because boost will drop with increasing altitude anyway. Eventually even the best pressure ratio will not keep up with the decreasing atmospheric pressure drop.

 

[Greyman]

Generally speaking, lower engine boost means higher critical altitude.

I don't think that example is entirely representative with what we're talking about. The two speed curves are two maximum 'all-out' speeds at full RPM - just with two different boost pressures. Similar to the maximum 'all-out' curves seen by Merlin III fighters pre and post 100 octane.

Get a chart showing actual cruise settings and you'll see what I was originally referring to.

This chart from wwiiaircraftperformance illustrates both my points:

http://www.spitfireperformance.com/spitfire-I-rae-12lbs.jpg

 

[Shortround6]

Basically the turbo was supposed to supply sea level pressure to the carburetor. At 20,000ft the air pressure is 13.75in so dividing that into 30 in(I rounded up to make things easy) gives a pressure ratio of 2.18 to 1 for the turbo. At 24,000ft the pressure has dropped to 11.59in for a pressure ratio of 2.58 in the turbo, RAM is not included. Against the increased boost you have to balance back pressure and the P-38J/L maxed out at about 40-41in of back pressure at around 26,000ft with the turbo governed at 26,400rpm or back pressure was about 4 times what an open exhaust was. at lower altitudes the waste gate was open to a greater or lesser extent, Turbine rpm was lower and back pressure was lower.

 

[BLine22]

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 from former crew members at the museum during talks at the monthly events, but I have also seen in print before, though no specific reference comes to mind just now. So far, I haven't gone back and dug it out of primary sources myself.

In March of 1945, General Lemay ordered a change in tactics, missions were flown at night at 5000ft. The aircraft were stripped of much of the defensive armament to maximize the bomb load of M-69 incendiary cluster bombs. This was a departure from precision bombing to area bombing and the beginning of the firebombing campaign.

 

[wuzak]

I don't think that example is entirely representative with what we're talking about. The two speed curves are two maximum 'all-out' speeds at full RPM - just with two different boost pressures. Similar to the maximum 'all-out' curves seen by Merlin III fighters pre and post 100 octane.

Get a chart showing actual cruise settings and you'll see what I was originally referring to.

This chart from wwiiaircraftperformance illustrates both my points:

http://www.spitfireperformance.com/spitfire-I-rae-12lbs.jpg

http://www.wwiiaircraftperformance.org/Merlin_46_47_Power_Chart.jpg

I see what you're saying now. The lower rpm at cruise mode reduces the critical altitude. Notice that maximum cruise power (2650rpm, +7psi boost) has a higher critical altitude than maximum all-out power (3000rpm, +16psi boost), but lower than maximum normal power (3000rp, +9psi boost)

 

[Greyman]

That must be what I'm seeing with some of the Lancaster charts I have - getting confused here myself. My brain doesn't do engines.

Lancaster II (Herc VI) +8 boost 2900 rpm
Lancaster II (Herc XVI) +2 boost 2400 rpm
Lancaster III (Merlin 28) +12 boost 3000 rpm
Lancaster III (Merlin 28) +4 boost 2650 rpm

Quick graph - definitely not pixel-perfect. Just trying to get a point across ...

 

[Shortround6]

What you have going on in the graph is several things.
The lower "leg" of the graph is the throttle being opened until you reach the 1st (lower) angle on the right. The supercharger can make more boost/flow more air but it may wreck the engine. once you reach the angle the supercharger is maxed out.
The next "Leg" is the air thinning faster than the supercharger can cope with, power is falling off as is boost.
until the left hand "angle/corner" which is were the supercharger is shifted to high gear and we repeat the part closed throttle until critical height is reached and then the power fall off.
Centrifugal Superchargers consume power in relation to the square of their of their speed. And the work they do is proportional to the square of their speed. Change the gear ratio for 7:1 to 10:1 for example and the power required doubles. the airflow may not exactly double but the combination of airflow and pressure rise will equal the power input minus the inefficiency of the supercharger (most superchargers being around 70% efficiency, the other 30% going into heating the intake charge over and above the heat generated by simple compression).

Now in cruise mode you have the impeller turning slower. Low gear in the Merlin 28 was 8.15:1 so at 2650rpm the impeller was turning 21600rpm instead of the 24450rpm at full throttle. Supercharger takes about 78% of the power to turn at 2650rpm. It is heating the air less and giving a bit denser charge and allows the throttle to be opened a bit more reducing pumping losses. (trying to suck through pinched straw
In high gear the supercharger is turning 28470rpm at 3000rpm.

It might have been possible to get 12lbs boost at 2650rpm in low gear if the throttle could have been opened all the way at 4000ft or under.

It might not have been good for the engine however.

Don't know if this helps or if you already know all this.

 

[wuzak]

Basically, cruise power critical altitude for both gears is at a higher altitude than maximum power critical altitude.

 

[Greyman]

Anyone know what we're seeing with the Spitfire I (Merlin III) chart (ie: the 6,000 ft lower critical alt)? Assuming the labels in the chart correspond with the Pilot's Notes engine settings:

+6.25 boost, 3000 rpm
+2.25 boost, 2600 rpm

Hurricane chart similar:
http://www.wwiiaircraftperformance.org/hurricane/hurricane-I-raechart.jpg

 

[wuzak]

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.