Post by Defender on Nov 21, 2014 11:07:21 GMT -5
There's not much that hasn't already been said in this thread but just in case there's now confusion developing I thought I might contribute some comments on the low blower v. high blower issue, based on research the Connie Team did and what I've read elsewhere.
First of all let's dismiss the speed factor. The 1950's airlines bought the DC-7/7B and the Super Connies primarily to get from A to B faster than their competitors. Cost of fuel, maintenance etc and economical cruise settings went out the window. So you went as high as needed and the high blower got you up to higher altitudes and better true airspeed.
But the Starliner and DC-7C were bought specifically to allow a better payload over a longer range so long range cruise settings became much more important. They were no faster than the earlier versions with the same payload.
But although a long range aircraft could also benefit from a higher TAS at higher altitudes there are at least two limiting factors which affect range, you will use extra fuel and possibly lose some TAS climbing to that altitude, and the blower itself will absorb more engine power and so result in a higher cruise fuel consumption. Consequently at the initial HB altitudes, let's say 15,000 to 17,000', the extra fuel used, both climb and cruise, can outweigh the TAS benefit.
So let's look at examples using the real Starliner's cruise settings. These are the nautical miles covered using 1,000lbs of fuel, at weight 130,000 lbs, 50% fuel remaining.
Engines at 1,600 BHP (no improvement using higher power at that weight)
FL130 LB 94.0
FL140 LB 95.0
FL150 LB 95.0
FL160 LB 95.7
FL150 HB 91.6
FL160 HB 92.6
FL170 HB 93.4
FL180 HB 94.3
FL190 HB 95.2
This means that even at 50% tanks, more than half the trip gone, there is still little or no benefit in climbing above LB limits until light enough to climb to 19,000' or above without using too much climb fuel.
Of course, as discussed earlier in this thread there are several other factors, not related to performance that might affect the choice of flight level.
Bill
First of all let's dismiss the speed factor. The 1950's airlines bought the DC-7/7B and the Super Connies primarily to get from A to B faster than their competitors. Cost of fuel, maintenance etc and economical cruise settings went out the window. So you went as high as needed and the high blower got you up to higher altitudes and better true airspeed.
But the Starliner and DC-7C were bought specifically to allow a better payload over a longer range so long range cruise settings became much more important. They were no faster than the earlier versions with the same payload.
But although a long range aircraft could also benefit from a higher TAS at higher altitudes there are at least two limiting factors which affect range, you will use extra fuel and possibly lose some TAS climbing to that altitude, and the blower itself will absorb more engine power and so result in a higher cruise fuel consumption. Consequently at the initial HB altitudes, let's say 15,000 to 17,000', the extra fuel used, both climb and cruise, can outweigh the TAS benefit.
So let's look at examples using the real Starliner's cruise settings. These are the nautical miles covered using 1,000lbs of fuel, at weight 130,000 lbs, 50% fuel remaining.
Engines at 1,600 BHP (no improvement using higher power at that weight)
FL130 LB 94.0
FL140 LB 95.0
FL150 LB 95.0
FL160 LB 95.7
FL150 HB 91.6
FL160 HB 92.6
FL170 HB 93.4
FL180 HB 94.3
FL190 HB 95.2
This means that even at 50% tanks, more than half the trip gone, there is still little or no benefit in climbing above LB limits until light enough to climb to 19,000' or above without using too much climb fuel.
Of course, as discussed earlier in this thread there are several other factors, not related to performance that might affect the choice of flight level.
Bill