Escorts could only stay with bombers for 20-40 minutes?

[Vraciu]

A funny story I remember from the above site.  The Mustang drivers figured out that if you tucked in tight behind the #3 engine the P-51 would stay in place hands off.   Sometimes the B-29 guys would get uncomfortable and jockey the throttle tossing the Pony out of the bubble.  Lol. 

[MiloMorai]

This site has some good info. 

http://www.506thfightergroup.org/mustangsofiwo.asp

Great article, thanks for posting.

[Vraciu]

Great article, thanks for posting.

Glad you enjoyed it. 

[drgondog]

I came across an article that said escort fighters could only stay with bombers for 20-40 minutes due to the higher speed and zig-zagging pattern the fighters had to fly.  The article implied this was P-51s escorting B-24s and B-17s.  It went on to say that multiple fighter squadrons had to be used to escort the bombers on different parts of the missions.

Few questions:
(1) Is this true?
(2) Why didn't the escort fighters simply slow down to a cruise speed and conserve fuel?

This series of points isn't directed to you but to the many questions raised.

For a really long mission, carrying 108 gallon externals, the P-51B/D FREQUENTLY escort 2 1/2 + hours from Rendezvous to Break escort for . As an example the 355th FG flew escort to the lead boxes of 1st BD to Zwickau on May 12, 1944 for 8th FC, F.O. 337. The escort R/V was made at 1200 hrs S. Bonn, escorted to target at 1350, then broke escort (relieved by 20th FG P-38s) over Eisenach at 1405.. total mission duration was 1026 to 1558.

Another example - 8th FC F.O. 456 to Munich area on July 19, 1944. Mission from 0644 hrs to 1245. R/V at 0837 with BE at 1200 near Antwerp. In between there was a fight around Munich at 0930.

The optimal cruise speed for maximum range (not duration) was ~ 260mph TAS with 110 gallon externals at 25000 feet. After tanks dropped optimal cruise increased to 295/300mph TAS at 25000 feet. Right at 4-5 miles/gallon for squadron/flight leaders.

8th AF Doctrine from February 1944 forward was a three relay system with P-47s performing the short range legs (Penetration/Withdrawal) and P-38 performing intermediate Penetration sometimes as well as intermediate Target support, with P-51B/D doing Target support but also long Penetration, Target and Withdrawal as given above.

Even after the P-38J's received 55 gallon LE tanks they basically were limited to a Gardelegen, Magdeburg, Augsburg, Freidrichshafen combat radius.

Essing above and/or to either side was SOP in two flight sections- covering each other. Sweeps out in front or flanking 50 miles along bomber stream were variations of a theme - both for Penetration and Target escort as independent Group tasked to 'Hunt'.

From personal observation (from Logbook), in 72 missions my father flew fifteen 6+ hour missions, six 7+ hour missions and one 7:55 hour mission - the latter while leading the last Shuttle FRANTIC VII mission from Steeple Morden to Piryatin, Ukraine after meeting the B-17s near Stettin, Poland.

As to 'slowing down' to bomber speed (~210 mph TAS for B-17 at 25000 feet), the range  was dramatically decreased as miles per gallon at the less than optimal RPM and Boost reduced to the 3+ mpg range for the duration of that actual escort leg. The second reason as addressed before is that the time to accelerate from say 200 mph to 400 mph for combat took some time and exposed the fighters to attack. "Tiny specks' become Big very rapidly when closing at 250+ Mph from the rear.

[Rich46yo]

Interesting. I knew we had developed navigation aids by the end of the war but never knew it was called "Uncle Dog". Nor did I ever know we used it in a B29 mother plane.

[Vraciu]

I'm not buying it. 

The fuel flow at 210 mph will be a fraction of that used for 300 mph.  This will vary with weight but I do not see a Mustang being on the backside of the power curve at 210 mph.   No way. 

[DaveBB]

Ok, the math of why the P-51 cruised most efficiently at 260mph is explained on pages 57/58.  Below 260mph, induced drag increases.  Above 260mph, parasitic drag increases.

https://www.princeton.edu/~stengel/MAE331Lecture6.pdf

Strangely enough, a pilot on F-16.net said the best cruise speed for the F-16 was right at 200mph.  But that's comparing pistons to turbines.

[Vraciu]

Ok, the math of why the P-51 cruised most efficiently at 260mph is explained on pages 57/58.  Below 260mph, induced drag increases.  Above 260mph, parasitic drag increases.

https://www.princeton.edu/~stengel/MAE331Lecture6.pdf

Strangely enough, a pilot on F-16.net said the best cruise speed for the F-16 was right at 200mph.  But that's comparing pistons to turbines.

Then 260 it is but not 300. 

Induced drag will vary with weight also. 

I think it depends on the goal.   Max range will give you better mpg, but weaving over the stream means you are working at cross purposes.   I can see how max endurance at the expense of range would be a benefit depending on the mission. 

I don't hop the Atlantic in my bird at max endurance, but if there is a storm over the field the latter becomes vital. 

I can see how 225 will save gas over 260...    Wish I had those charts.   

[Serenity]

Strangely enough, a pilot on F-16.net said the best cruise speed for the F-16 was right at 200mph.  But that's comparing pistons to turbines.

The Hawk is 200-ish give or take a couple knots for max range. 180 for max endurance IIRC (we cruise looking more at the AOA gauge than airspeed)

[drgondog]

I'm not buying it. 

The fuel flow at 210 mph will be a fraction of that used for 300 mph.  This will vary with weight but I do not see a Mustang being on the backside of the power curve at 210 mph.   No way.

I'm not sure if that remark was directed to me.  That said, look to the flight envelope at which CL/CD is maximum to plan a flight test to adjust key variables RPM and Manifold Pressure settings which a.) minimize #fuel/distance traveled .  The exercises for determining loiter is focused on minimum fuel flow and is found at (CL)^^3/2 divided by CD.

Rich and Randall are on the right track. Essentially look to the region in which Induced Drag= Parasite Drag for the Gross weight and altitude and speed for that region of the Drag vs Velocity Plot.

For an 8th AF escort profile only one scenario really calls for best Loiter settings - namely when fighters got to R/V place and bombers were late.

Summary - the key interest for determining Combat Radius - all things equal (GW, External stores, internal ammo and fuel load), in the Flight test program, is looking at fuel flow vs airspeed as a function of RPM and MP. At this point (in time) the Power Required = Power Available for those setting to maintain altitude.

The outcome is Gallons (or weight) per mile traveled until you find the settings that give you the 'minimum' #/mi ratio.

That said, as GW changes with time based on internal fuel (and changes as step function when external load dropped), the Breguet equation for Range = (prop eff)/(BHP) times CL/CD times the Integral of dW/W from starting gross weigh to final gross weight - where dW is the change in weight as a function of fuel consumption along the course.

With that equation the design team is looking to get the largest possible prop efficiency, the lowest possible pounds of fuel per mile, at lowest Required BHP possible for L=W, the highest Wo to W1 fuel fraction and flight at highest L/D.

I'm having brain farts re: posting images but go to the P-51D Flight Test Reports in Mike Williams site spitfireperformance.com for the development of P-51 Range and Endurance data for the Operating manual.
 

[FLS]

I'm not buying it. 

The fuel flow at 210 mph will be a fraction of that used for 300 mph.  This will vary with weight but I do not see a Mustang being on the backside of the power curve at 210 mph.   No way.

210 TAS at 25,000 ft is about 140 IAS.

[Dobs]

Have at it....

https://drive.google.com/file/d/0B-kwhapBgCtYMW9HdWVSV1VNSTg/view?usp=sharing

[drgondog]

I'm not buying it. 

The fuel flow at 210 mph will be a fraction of that used for 300 mph.  This will vary with weight but I do not see a Mustang being on the backside of the power curve at 210 mph.   No way.

DaveBB said "Ok, the math of why the P-51 cruised most efficiently at 260mph is explained on pages 57/58.  Below 260mph, induced drag increases.  Above 260mph, parasitic drag increases."

The only comment worth making is that all the optimal cruise settings were Different for different GW, different external stores, and different altitudes. At lower altitude, for same gross weight and external stores conditions the entire Induced Drag/Parasite Drag plot shifts to the left so that the bottom of the drag curve is at a lower airspeed.

In the attached report at 25K for take off weight of 9600+ pounds racks only the optimal cruise speed is 303mph with 29"MP/2050 RPM and 52 gal/hr fule flow.

Add another 1400 pounds for two 110 gallon externals shows optimal cruise at 281mph TAS with 32"MP/2250 RPM and 57 gph ---------------> higher total drag, more power required for level flight, higher AoA for greater Induced Drag (than clean Mustang).

In this example the inbound cruise is 281mph, make RV, Ess around the bombers, drop tanks and automatically increase speed to 303mph straight line when you reduce throttle to 29" and decrease RPM to 2050.

With the Mustang as the center of discussion, the first fact you have to nail a.) the Gross Weight, and b.) the external stores condition.

At 25000 feet the chart I point you to below is Appendix B is: http://www.wwiiaircraftperformance.org/mustang/P-51D_15342_AppendixB.pdf

The values tabulated are those for the P-51D Manual for Range as a function of GW, external stores (or racks only), Max Continuous Power, Military Power and WEP at different altitudes.

The summary report is:
http://www.wwiiaircraftperformance.org/mustang/p51d-15342.html

The important point for Vraciu is that for the aircraft to sustain level flight, there is very little leeway in the Drag bucket at 25,000 feet (and you know this) to lower speed and fuel flow because you also don't have much to play with to maintain Power Available = Power Required for level flight at the lower airspeed.

So the Max Loiter fuel consumption is lower than best cruise setting as expected but the angle of attack and Induced drag increases exponentially as the velocity decreases.

To your point, so what? - reduced fuel flow for greater loiter time is good as the bomber stream is zipping along at 2/3 the TAS of the Mustang? Why not drop even further on MP and RPM?

Well three reasons - First, to Fighter pilots in ETO with LW having altitude advantage nearly always, means that occasionally to throttle up ASAP is far better starting at 300 TAS than 220mph.  The second reason is that Merlins are grumpy around 1600 RPM and MP below 30". The last reason is that the range at optimal cruise and higher airspeed gave the escort a lot of leeway.

[Vraciu]

Techniques used in the ETO were different than the Pacific. 

Oversquaring was a big one that increased efficiency.   There were others.

ETO Mustangs were not flown as efficiently as they could have been IMO.

The penalty in range for endurance is sometimes worth the price.  It depends on the situation and variables involved. 

[Vraciu]

Also note at 5,000' with drop tanks there is a 34+ percent increase in fuel efficiency (thus range AND endurance) flying at a TAS of 187 mph vs 298 mph -- 5.29 MPG vs 3.48 MPG.  (Indicated speeds will be fairly close to TAS at this low of an altitude.)

Which makes my exact point.

I'm not buying it. 

The fuel flow at 210 mph will be a fraction of that used for 300 mph.  This will vary with weight but I do not see a Mustang being on the backside of the power curve at 210 mph.   No way.

By interpolation 210 MPH TAS will yield close to 5 MPG or an increase of more than 30 percent over 300 MPH TAS. 

Now, these numbers will change with altitude but will be generally similar.    At 25,000' 280 MPH TAS is more efficient than 300, etc. etc.   I will make a terrible guess and say shave 80 or 90 MPH off TAS to get IAS at 25,000 (it has been a long day so my mental math may be wrong).

Going faster is, generally speaking, only helpful in terms of range when you are on the backside of the power curve--or close to it.    Then we start accounting for headwind vs. tailwind, temperature, weight, configuration/loadout, and so on which will cause the most efficient speed to change. 

All things equal, there is not a single point on the cruise chart where the slowest LISTED speed is less efficient than the higher one(s).

(I am presuming a typo at 321 and 331 MPH at 25,000' with drop tanks where fuel mileage bucks the trend mysteriously.)

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Bomb racks-only fuel flow doubles between 300 MPH TAS and 400 MPH TAS at 25,000' (~210 IAS vs. ~310) -- 52 GPH vs. 100 GPH. 

With drop tanks the fuel flow nearly doubles from 281 MPH TAS to 357 MPH TAS (~200 IAS vs. 270 IAS) -- 57 GPH vs. 100 GPH.   Range AND endurance suffer dramatically as a result.

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