Cruising speed?

[Just Schmidt]

One of the many things I've learned after years of mostly lurking on the forum, is how relative a term maximum speed is. Or rather how difficult it is to compare maximum speeds considering that it is dependent on altitude, fuel, state of airframe, limited time overboost and so forth.
This has finally prompted me to consider just how ignorant I am about the term cruising speed, that is, how it is determined when it is quoted in publications. Quite often I see it qualified in different ways, like 'max cruise' ,'economic cruise' or 'max continuous cruise', as just two examples. Apart from the nominal aspects, I doubt that I really understands the underlying physics.
I do suspect that cruising speed is even more arbitrary (as dependent on more chosen variables) than maximum speed, which at least focuses exclusively on the fastest speed the aircraft could achieve in level flight. Conversely, cruising speed might be a more useful figure, that is if it is in a meaningful way standardized. I am especially interested in the relationship between cruising speed and range, and will try to break the problem up into more specific questions.

1: Is cruising speed ideally (or usually) determined by the speed at which the aircraft in theory will achieve the longest range?

2: If so, will the most economic speed equal the slowest the aircraft can go without loosing height?

3: If not, is it then the speed at which the aircraft can attain a certain range of the aircraft at a given load (ideally quoted in the same set of specifications)?

4 (Which may be asking for opinion): is the quoted values for cruising speed likely to be useful for any kind of comparizon between aircraft, especially in the crude single value form I oftenmost come across?

I'd be very grateful for any specific answers to the questions I did manage to come up with, or conversely any feedback that can enlighten me to what questions I ought to have formulated, and why.

 

[swampyankee]

For all aircraft, there is a speed, which varies with altitude and weight, where the efficiency is greatest; this is where lift over drag(L/D) is greatest,and where the aircraft will have the greatest range. There's a different, lower speed for minimum power required, where the aircraft will have greatest endurance. There's a third speed, speed times L/D, which is a good approximation of the best compromise between efficiency (best L/D) and productivity, and is considerably greater than the speed for best L/D.

For the questions

1: depends. For general aviation aircraft and many commercial, best range speed (max L/D) is too slow. For something like the U-2, cruise speed is set by not stalling and not having buffeting due to local shocks.

2: no, the best range airspeed is greater than the least power airspeed.

3: that depends on how the spec is written, but that is probably the major reason.

4: what sort of comparisons? To compare relative airframe efficiency? Productivity? Combat capability? For the first, rate of climb would be better. For the last, some of the last generation piston airliners and bombers cruised nearly as fast as many successful WWII fighters' demonstrated max speeds. (See Republic XF-12 Rainbow - reconnaissance for the most extreme case)

 

[Just Schmidt]

4: what sort of comparisons? To compare relative airframe efficiency? Productivity? Combat capability? For the first, rate of climb would be better. For the last, some of the last generation piston airliners and bombers cruised nearly as fast as many successful WWII fighters' demonstrated max speeds. (See Republic XF-12 Rainbow - reconnaissance for the most extreme case)[/QUOTE]

Thank you!

One aspect of the comparison is whether the specifications as usually given is actually apples to apples.

One of the reasons I find this interesting, is that the survivability of a bomber or reconnaissance aircraft will be dependent not only on the ability to outsprint an intercepting fighter (even if rarely a practical proposition), but also on time to target. Plus of course other factors like armament and height.

Of course chases over extended distances are also relevant, for fighters as well. Basically I suppose I'm just trying to get a fuller picture of all the parameters that make the better aircraft in the context of armed conflict.

 

[Shortround6]

Thank you!

One aspect of the comparison is whether the specifications as usually given is actually apples to apples.

One of the reasons I find this interesting, is that the survivability of a bomber or reconnaissance aircraft will be dependent not only on the ability to outsprint an intercepting fighter (even if rarely a practical proposition), but also on time to target. Plus of course other factors like armament and height.
[/QUOTE]

Having difficulty responding to threads.

as far as apples to apples goes please see link;
http://zenoswarbirdvideos.com/Images/P-40/P-40FOIC.pdf

On some aircraft there are several such pages in the manuals depending on weight and or external loads (drag).

Note that 9000ft the P-40 can cruise anywhere from 325 miles at an IAS of 261mph burning 115 (?) US gallons an hour to 700 miles at an IAS of 183mph burning 38 US gallons per hour.

As has been noted range or cruise speed can depend on how the specification is written. For the US the range in the right hand column was important pre-war or early war as it affected the deployment of the aircraft from one part of the US to another. It was in no way a requirement or indicator of ability to escort bombers.
Other, smaller countries sometimes specified endurance/range at higher speeds for "operational use".

"best" range should achieved at the lowest drag.

which is not the lowest fuel burn. Many planes could stay in the air for a slightly longer period of time by going to a lower speed than the one that gave the best range. However the increase angle of attack needed by the wing for lift (the induced drag) meant that the efficiency was lower.

As an example of this the F4F-3 had a max range at an average cruise speed of 180mph. (speed could be lowered a bit in the later stages as fuel burned off) while speed for maximum endurance was an average of 160mph.

 

[GregP]

Another thing to consider is cooling. Air-cooled radial usually had their best cruise speed significantly lower than max speed. By way of example, a Grumman F8F Bearcat can get to 421 mph or 455 mph, depending on variant, or close to those numbers. But when cruising around on patrol, they usually cruised at 165 - 185 mph for best economy.

For a direct comparison, a Boeing B-17G could reach some 287 mph top speed, but cruise was 182 mph. The XB-38 was a B-17 converted to Allison V-1710 power. It reached 327 mph and cruised at 225 mph or so, some 38 - 40 mph faster than radials did. They only built one. But liquid-cooled engines in general usually, but not always, cruised at a larger fraction of top speed. The liquid-cooled fighters would have to S-turn above the slow bomber formation to stay in contact since they cruised much faster than the bombers did.

There is a big debate in here about the usefulness of top speed. Many say top speed is life. The actual fighter pilots I have spoken with discount top speed and rarely ever got there except maybe in a dive. The reason is simple. Engines will run at maximum power, but are under great stress. Only an idiot would put his single-engine fighter under great stress some 500+ miles away from home unless there was a driving need to do so. Most said they never used full power unless engaged in combat. So, they hardly ever got to top speed. I'd estimate the time spent at top speed is less than 1% of a fighter's flying lifetime. But when top speed was needed, it was usually a life or death need, and everyone would break the wire gate on the throttle to go into WEP when required. They just didn't stay there longer than required for the combat before throttling back somewhat to ease the engine stress. That engine was your ticket home, usually over very non-survivable, cold water for a good chunk of the trip home. Take care of your engine and it would get you there and back. Thrash it and it still most likely would get you home, but most people didn't want to bet their lives on it and were as easy on their engines as they figured the situation warranted. Sometimes they were wrong and got caught going slow with a first-pass victory for the bad guys. So, keeping a good eye on the sky was essential, especially at cruise.

 

[Shortround6]

Actually the cruise figures for the B-17F are closer to the ones shown on this chart.

http://zenoswarbirdvideos.com/Images/B-17/17FOIC.pdf

The XPB-38 not being a converted "G"

Please note that max cruise at 20,000ft is 183 IAS which is around 256mph true, however this is at max continous and the plane is burning 413 US gallons per hour, it is also useless for formation flying as when the formation turns left or right the planes on the outside of the formation have no reserve power left to speed up to maintain formation.

The "F" will cruise at 151 IAS at 20,000ft (211mph true) at max lean power burning 284 gallons per hour. This is about 750hp per engine.

The 8th air force cruised B-17s at about 180mph because of the formation considerations and to get the range to payload they wanted.
Faster cruise might require more fuel meaning a lower bomb load to some targets. The bombers never flew a straight line to the targets or on the return. They took 1/2 hour or more after take-off to form up the formations and flew dog leg fight paths too and from the target, including sometimes large detours around certain cities (flak zones) or over the North Sea.

For more B-17F performance figures see;
http://www.wwiiaircraftperformance.org/B-17/B-17F_41-24340_FS-M-19-1470-A.pdf

But please take careful note of the weights in the right hand column.

Bomber weights can vary an enormous amount even with the plane in "clean" condition and speeds (and ceilings) can be over over the place as a result.

 

[pbehn]

Surely cruising in a plane is much like cruising in a car. I don't care about fuel when I am in town, on a long journey I may take more notice, especially with a car full of people and luggage. In Saudi Arabia I became slightly paranoid, crossing the desert I filled up at every station because my gas guzzling V8 didn't go far on a tankful and there aren't many stations.

Calculations can be done for most economical in terms of distance and time but then the reality of performing a mission kicks in. A Mosquito could perform a recon mission where it had huge onboard fuel so could cruise at almost any speed that the engine would allow for sustained performance. as a bomber it didn't have so much so flew more economically. Then again as a bomber sometimes it had to fly with an escort, with P51s as escort it reduced its cruising speed to the best speed for the P51, about 20MPH less. Again in the bomber role many missions were performed at low level so regardless of the economical height and speed the height was governed by enemy radar and the speed was as high as possible so long as you got there and back on the fuel load.

 

[Shortround6]

A Mosquito could perform a recon mission where it had huge onboard fuel so could cruise at almost any speed that the engine would allow for sustained performance. as a bomber it didn't have so much so flew more economically. Then again as a bomber sometimes it had to fly with an escort, with P51s as escort it reduced its cruising speed to the best speed for the P51, about 20MPH less.

This may require a bit of explanation or qualification:

Type of Mosquito and type of Mustang for instance. Or if the Mustangs were early "B"s with defective mixture control linkages?

 

[pbehn]

This may require a bit of explanation or qualification:

Type of Mosquito and type of Mustang for instance. Or if the Mustangs were early "B"s with defective mixture control linkages?

SR I was recalling a mission by Mosquitos to Norway. I was just making a general comment about why cruising speeds may vary. On a cross channel mission not requiring external tanks the positions may well have reversed. Similarly the P51 had all sorts of cruising speeds, to a rendezvous, while escorting a bomber group, when returning home, on a cross channel raid, loitering about waiting for a V1. etc etc etc.

It may have been the Shellhouse raid to Copenhagen.

 

[Shortround6]

On one particular mission at one point in time it was very likely true.
However a Mustang D could fly at speeds comparable if not higher at 2700rpm than a Mosquito XVI could using 3000rpm (depending on height) leading one to really question what the Mustangs were doing as their engines could always use a lower power setting for the same speed. Granted the Mosquito held more fuel however:
A Mustang D with no rear tank and a pair of 75 gallon drop tanks can cruise slightly faster over about the same distance as a Mosquito XVI can with 2000lbs inside and a pair of 50imp gallon drop tanks.

Mosquitos without two stage engines????

 

[pbehn]

On one particular mission at one point in time it was very likely true.
However a Mustang D could fly at speeds comparable if not higher at 2700rpm than a Mosquito XVI could using 3000rpm (depending on height) leading one to really question what the Mustangs were doing as their engines could always use a lower power setting for the same speed. Granted the Mosquito held more fuel however:
A Mustang D with no rear tank and a pair of 75 gallon drop tanks can cruise slightly faster over about the same distance as a Mosquito XVI can with 2000lbs inside and a pair of 50imp gallon drop tanks.

Mosquitos without two stage engines????

Hey SR I wasn't making a point about relative performance, it was just choice of optimum cruising speeds. It may well have been that the Mustangs would have to ditch the drop tanks even if they had fuel and complete the attack and return home on internal fuel. I believe most of the mission was at low level.

 

[Shortround6]

Something still doesn't make sense.
It is 435 miles from Edinburgh to Bergen. A "D" Mustang with under wing racks empty is rated as being able to fly 410 miles on 100 gallons of fuel at 300mph at sea level. At 5000ft the speed goes to 323 but the fuel burn goes up at the same rpm and boost settings.

The target may very well have not been Bergen but then there is a lot of Scotland North and East of Edinburgh.

I know you were try to make a point about optimum cruising speeds. It is just that once statements like Mosquitoes needing to throttle back so Mustangs could keep up are put out on the net without any qualifiers they sometimes take on a life of their own.

Look at the effort needed to try to counter the idea that Mosquitoes could carry the same bomb load as a B-17 to Berlin.

It is true BUT only if you use the 4000lb cookie AND average the GP bomb loads (5000lbs) and incendiary bomb loads (around 3200lbs) of different B-17s flying the same mission.

The British could have been using Mustang IIIs (or Mustang Is with Allisons?) The US Army spent months trying to straighten out an Early P-51 B with a defective mixture control linkage, auto rich in cockpit was still full rich at the carb giving some rather strange speed, fuel consumption results. How many similar planes got into squadron service?
A lot of variables without even questioning which Mosquito.

 

[wuzak]

SR I was recalling a mission by Mosquitos to Norway. I was just making a general comment about why cruising speeds may vary. On a cross channel mission not requiring external tanks the positions may well have reversed. Similarly the P51 had all sorts of cruising speeds, to a rendezvous, while escorting a bomber group, when returning home, on a cross channel raid, loitering about waiting for a V1. etc etc etc.

It may have been the Shellhouse raid to Copenhagen.

The Copenhagen raid was done with Mosquito FBVIs and, I believe, Mustang IIIs.

 

[wuzak]

On one particular mission at one point in time it was very likely true.
However a Mustang D could fly at speeds comparable if not higher at 2700rpm than a Mosquito XVI could using 3000rpm (depending on height) leading one to really question what the Mustangs were doing as their engines could always use a lower power setting for the same speed. Granted the Mosquito held more fuel however:
A Mustang D with no rear tank and a pair of 75 gallon drop tanks can cruise slightly faster over about the same distance as a Mosquito XVI can with 2000lbs inside and a pair of 50imp gallon drop tanks.

Mosquitos without two stage engines????

I have seen the claim that Mosquitoes had to slow down in order to stay in formation with escorting P-51s on long range reconnaissance missions.

The issue being that the Mustang would have to have external tanks to get the range that a PR Mosquito could without having external tanks.

A PR.XVI or B.XVI using Merlin 76/77s (later production versions) could cruise at 2850rpm and +7psi (max continuous or weak mixtire - never sure which or if they are the same thing) at over 350mph TAS at 20,000ft+.

A B.XVI with maximum internal fuel has 540UKG on board. WIth two 50UKG drop tanks that was a total of 640UKG, or about twice a P-51D's maximum fuel. But with less effect on drag.

A PR.XVI could carry 760UKG internally and 100UKG externally. It seems quite possible that a Mosquito PR.XVI could use a higher engine cruise rating (and therefore speed) and match the range of a P-51D with maximum fuel.

 

[Shortround6]

From a manual on the late model Mosquitoes: https://ww2aircraft.net/forum/attachments/mosquito-mks-viii-ix-and-xvi-pdf.45891/

Mosquito Mks VIII, IX, and XVI.pdf

which lumps a bunch of them together it seems that they could do about 349mph true at 20,000ft using 2650 and 7lbs boost and got 2.4 A.M.P.G. speed at sea level under same conditions was 285 mph and 2.1 A.M.P.G.

They could cruise faster but it was recommended to use higher boost and not increase rpm.

At 2650rpm and 7lbs boost fuel consumption was figured at about 143 imp gallons per hour
At 2850rpm and 12lbs boost fuel consumption was figured at about 210/230 imp gallons per hour
At 3000rpm and 15lbs boost fuel consumption was figured at about 260/280 imp gallons per hour
At 3000rpm and 18lbs boost fuel consumption was figured at about 280/300 imp gallons per hour

the split figures are for M.S. gear and F.S. gear. 2850rpm and 12lbs was the climb rating.

Granted if you can break out certain model/types of Mosquitoes they might do a bit better. The P.R. ones were noted as running a bit lighter and could cruise higher on the outward part of the mission.

I would note that a P-51D with a pair of 75 gallon drop tanks could cruise at sea level at 282mph using 2550rpm and 44in (7lbs boost) and used 78 US gallons per hour. or about 4.3 AMPG using imp gallons.
At 20,000ft it could do 355mph at 2700rpm and 46in (8lbs boost) using 93 US gallons per hour for 4.58 AMPG using imp gallons.
The Mosquito has amazingly low drag for a twin engine plane but it needs twice the fuel of a Mustang to go about the same distance at the same speed (or within a few percent).

It had a nominal range of 1130 miles on 310 US gallons of fuel at these speeds. adjust fuel as you see fit for rear tank and warm-up/take-off allowance and reserves. 330 gallons is wing tanks and 75 gallon drop tanks. I have allowed 20 gallons which isn't enough but shows that the Mustang had enough range for many missions. I am also using the 9800lb to 11200lb range chart.

It would take a pretty unusual set of circumstances for Mosquitoes to have to slow down for the P-51.

BTW with the tanks gone the P-51D could do 279mph at sea level at 2150rpm and 39.5in (4 1/2lbs ?) using 61 US gallons an hour. I am sure the Mosquito could do better running light than the above figures but I doubt the difference would be as dramatic.
for the Mustang 2500rpm and 43in (6 1/2 lbs?) gave 300mph at sea level without tanks.

 

[pbehn]

It would take a pretty unusual set of circumstances for Mosquitoes to have to slow down for the P-51.
.

It was, but that was my point. The P 51s may well have been travelling at below optimum too. The point being that regardless of calculations other factors over ride the best cruising speed.

German Jet Encounters
"The Mosquito met the fighter escort as planned; but now heavily loaded with l,000 gallons of fuel, flew at a severe speed disadvantage. Geary attempted to maintain economical cruising speed but outpaced the P-51s and was forced to throttle-back to continue flying formation with them. The Mustangs had long-range drop tanks and were also fully loaded. Once involved with enemy action, they would jettison their tanks, and therefore, were attempting to conserve and obtain maximum range from their fuel supply. This exacerbated the problem. It was a very-long flight to the Polish border, and on three occasions Geary throttled-back and did not receive the mileage planned."

 

[Just Schmidt]

Thank you very much everybody, as expected I'm getting some new insight here.

In an attempt to reciprocate just a little, I'd like to point out that Copenhagen is in Denmark. Danes remembered the Shellhouse raid quite well as, tragically, one of the first mosquitoes crashed in a school. Some of the following aircraft targeted the school instead of the gestapo headquarter, mistaking the burning building for the target.

Numerous similar pin point attacks were made against gestapo headquarters in occupied Europe (I positively recall Denmark, Norway and the Low Countries), as far as i understand all by the fighter bomber variants of the mosquito at low level.

 

[pbehn]

In an attempt to reciprocate just a little, I'd like to point out that Copenhagen is in Denmark. Danes remembered the Shellhouse raid quite well as, tragically, one of the first mosquitoes crashed in a school. Some of the following aircraft targeted the school instead of the gestapo headquarter, mistaking the burning building for the target.
.

JS Maybe reading my posts you could get the impression that I thought Copenhagen is in Norway, that is not the case I have been to Copenhagen and various places in Norway. There was a great thread about a painting and reunion of Operation Carthage veterans here, I will try to find it.

The painting is here maybe AIrframes can help.
The Bombing of the Shellhus on March 21, 1945

 

[Shortround6]

Thank you very much everybody, as expected I'm getting some new insight here.
.

Might I suggest going here : Other Mechanical Systems Tech.

And checking out some flight manuals/pilots notes.

The American ones often have the most detail on different cruise conditions.

 

[GregP]

Actually, Shortround, it was close to 165 - 185 mph.

Not because of the B-17F, but because of the large formation of B-17s on large-formation missions. The entire group has to slow down so the poor guys at the end of the line can maintain position, They use large power changes relative to the two guys on either side of the lead aircraft. That from B-17 pilots and anyone else who escorted large bomber formations. I have no doubt the aircraft itself, by itself, could cruise at the indicated speeds ... alone. Say, even flying from San Francisco to Hawaii in a group of 3 or 4.

But I have grave doubts about that cruise speed in a formation of 300+ B-17s, and I KNOW it is about that on 1,000-plane raids. I'd choose 185 mph most of the time for larger formations.

Regardless, the escorts had to S-turn above the bombers because even slow cruise for the fighters was faster than a gaggle of cruising B-17s, even if they were empty.