Author Topic: Escorts could only stay with bombers for 20-40 minutes?  (Read 15548 times)

Offline FLOOB

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Re: Escorts could only stay with bombers for 20-40 minutes?
« Reply #30 on: October 19, 2016, 12:05:02 AM »
"You don't pay escorts to stay, you pay them to leave."

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Offline Vraciu

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Re: Escorts could only stay with bombers for 20-40 minutes?
« Reply #31 on: October 19, 2016, 12:10:24 AM »
"You don't pay escorts to stay, you pay them to leave."

(Image removed from quote.)

LOL.

I note that DragonDog's dad is Burt Marshall.  I recently finished Bud Fortier's book I which he wrote of Marshall extensively and highly.   Very cool. 
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Offline GScholz

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Re: Escorts could only stay with bombers for 20-40 minutes?
« Reply #32 on: October 19, 2016, 04:35:04 AM »
Ponies are special. Their laminar-flow wings are less efficient than more conventional profiles outside of the drag bucket. To go anywhere far a pony needs to stay in the drag bucket.
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Offline GScholz

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Re: Escorts could only stay with bombers for 20-40 minutes?
« Reply #33 on: October 19, 2016, 04:35:55 AM »
"With the first link, the chain is forged. The first speech censored, the first thought forbidden, the first freedom denied, chains us all irrevocably."

Offline drgondog

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Re: Escorts could only stay with bombers for 20-40 minutes?
« Reply #34 on: October 19, 2016, 08:39:11 AM »
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 sorry. I must have missed the memo about the Post question? If the question was, as I understood it, was "Escorts could only stay with the bombers for20-40 minutes"?

I confess that I thought the question was ETO/8th AF bombers, not torpedo bombers.



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).

I'm pretty sure I mentioned that the variables were altitude, GW, airspeed and drag conditions (racks, tanks, bombs, etc) that move the CD=CL point from optimal 'clean'.

Mr GScholz just posted a CD vs CL plot that explains the key factor for discussion of flight test planning. He posted the 2415 vs 66-415 with same T/C for rough wing drag efficiency snapshot. Although it says nothing about RN and the Drag (total Parasite Drag= wing+Fuse+empennage+cowl+exhaust stacks, external stores, etc). It says a great deal about the CL range for most efficient cruise conditions.  The 2315 has a pretty well defined bottom CL for which the CD is a minimum, not so the 66-415 (on NACA.NAA 45-100).

On a P-51 (or any other fighter) this plot is essential for Performance calcs and usually start at RN=~2x10^^6 and slopes DOWNWARD as a function of true airspeed until it flattens out near 20-30x10^^6. Reynolds Number is a function of density, Mean Aero Chord of the wing, Velocity in fps and inversely proportional to absolute viscosity.

The reason I bring all this crap to the discussion is that CL is absolutely critical to the discussion. At the CL range of 0.25 to 0.5 Drag is at its lowest for the Wing. CL has to be examined at L=W for level flight and solved for Velocity for that range and then calculate Induced Drag for the velocity range above - then plot CDtotal = CDparasite+CDinduced

With this in hand the test pilot can test plan changes in velocity for a specific altitude and unaccelerated flight for the drag parameters stated above. He will have a real good idea for the velocity envelope for test.

Next he (They) need to calculate Powere Required for level flight and start tweaking MP and RPM to find maximum efficiency of the engine-prop system consistent with safety factors for engine performance and life.

What you see in the flight tests shown on spitfireperformnce.com generates the tables that you will see in the future Pilot's Manual in the table for Range/speed per MP/RPM settings as a function of total fuel on takeoff, internal fuel, for Gross Weight and altitude comparisons.

The ones plotted are the foundation for developing Combat Radius with mission segment variables (warm up/taxi, takeoff/climb to cruise altitude, Cruise to RV, Drop externals and Fight, Return at optimal cruise speed to letdown - and have 30 minutes of fuel reserve.

The combat radius calc have to take into account the drag conditions inbound with external tanks/ordnance for inbound leg to Combat profile, the jump in fuel consumption for WEP and MP which taps your internal fuel reserve and gives you a new Gross Weight/less Drag and different MP/RPM for new maximum range which has to be managed to be carried home with internal fuel remaining.



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.

To quibble the point, going at the recommended speed. Speed/MP/RPM vs altitude is the first step to pick 'close to best case for range' before headwinds/tailwinds/etc have to be taken into account.

Respectfully, diverting from that setting range to lower airspeed/RPM/MP to minimize fuel consumption per hour only extends the time before you have to crash land out of fuel - but you won't travel as far as the other gent that flies to maximize miles/gallon
 

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.)

----

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.

----

Respectfully, if the data is presented in toto, there are many points in which less airspeed at the recommended MP/RPM settings - all of them. Go back to picking the optimal CL as a function of airspeed and gross weight. Lower airspeed for that airplane from the recommended 'bottom' of the drag bucket drives the required angle of attack higher to sustain level flight ----------> Higher Induced Drag value from optimum ------> higher total Drag from optimum.
Nicholas Boileau "Honor is like an island, rugged and without shores; once we have left it, we can never return"

Offline Vraciu

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Re: Escorts could only stay with bombers for 20-40 minutes?
« Reply #35 on: October 19, 2016, 08:57:19 AM »
Ponies are special. Their laminar-flow wings are less efficient than more conventional profiles outside of the drag bucket. To go anywhere far a pony needs to stay in the drag bucket.


The Mustang wing was not a true laminar flow wing.  It was a noble attempt but could not be manufactured with the tolerances needed to achieve much, if any, laminar flow.





« Last Edit: October 19, 2016, 09:08:53 AM by Vraciu »
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Offline Vraciu

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Re: Escorts could only stay with bombers for 20-40 minutes?
« Reply #36 on: October 19, 2016, 08:59:35 AM »
Again, I am not saying lower speed is always better.  It's not.   I have stated that REPEATEDLY.

But my original point remains valid and is backed up by the numbers. 
« Last Edit: October 19, 2016, 09:02:21 AM by Vraciu »
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Offline drgondog

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Re: Escorts could only stay with bombers for 20-40 minutes?
« Reply #37 on: October 19, 2016, 10:30:38 AM »
I am not trying to cause indigestion in this debate.

Vraciu - what is your specific point that you felt you have made in fuel consumption versus miles per gallon optimization?  And thank you for your comments re: Bud Fortier.
Nicholas Boileau "Honor is like an island, rugged and without shores; once we have left it, we can never return"

Offline drgondog

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Re: Escorts could only stay with bombers for 20-40 minutes?
« Reply #38 on: October 19, 2016, 01:02:18 PM »
(Image removed from quote.)

This is worth taking about. First is it a 2D section lift and drag plot from wind tunnel result at a specific Reynolds number -  with no induced Drag so the entire Drag component is Parasite Drag. The introduction of a finite wing reduces the Max L/D as Induced Drag is introduced. For a very large AR the comparison is close.

You can note that the range of 66-415 superior CL/CD ratio (over 2315) is much broader than the 0.25 to 0.5 CL range. All things equal this a/c will exhibit far better Drag reduction across most of the performance envelope until in the lower speed range where CL has to increase due to angle of attack increase to maintain level flight.

Vraciu's comment that laminar flow was not achieved is true in a practical sense, although the Mustang surface prep on flush rivet construction, filling, sanding, priming and painting did delay boundary layer separation. That said slower velocity gradient of the 'laminar' flow airfoil delayed pressure gradient induced separation ---- and a fundamental reason why a 'fat wing' NACA/NAA 45-100 Low Drag wing had delayed shock wave formation over conventional airfoils (like the 2315). 

That said the actual real life low drag result was achieved - just not to expected theoretical laminar flow objectives. As a contrast - The CDparasite of the P-51 at RN=9x10^^6 =~ .017 while the F4U and F6F comparable parasite Drag values, all derived from full scale wind tunnel testing, is ~ 0.26 and 0.27 respectively.  That is mostly wing and both the F4U and F6F had, IIRC 23015 section airfoils (Ditto P-38 and FW 190 and F8F). The Total Drag comparisons between the latter five fighters varied primarily due to Induced Drag via AR.

To toodle back to our discussion, looking at the respective Total Drag vs Velocity plots for each fighter yields a plot of CDparasite which increases from CDzero lift (low) and a CDinduced which starts high and decreases non-linearly from low speed to high. When you look at each plot, where the two curves cross is CDminimum.

THAT is the Golden strike zone for fine tuning speed vs engine settings to get maximum mileage per pound of fuel - for That altitude, That GW, That External Drag condition and it changes every minute as fuel is consumed, lowering GW and reducing AoA required for level flight.. and so on.

Why does the F4U, et al - Never-  get the same range attainable, per pound of fuel carried, nor cruise at the same relative high speed for that Golden Strike zone, you ask? They all start with 50% more parasite drag - which shifts the plot to the left (Lower airspeed at the CD=CL bottom of the plot) - AND only the Allison had essentially the same (or better) fuel consumption for same MP/RPM- all the rest had gas guzzling powerful radials.   
Nicholas Boileau "Honor is like an island, rugged and without shores; once we have left it, we can never return"

Offline icepac

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Re: Escorts could only stay with bombers for 20-40 minutes?
« Reply #39 on: October 19, 2016, 07:40:51 PM »
More than one race plane has had around 100 pounds of bondo removed from the plane when they decide it's cracking and could come off.

They are changing the profile of the wings.

Offline Vraciu

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Re: Escorts could only stay with bombers for 20-40 minutes?
« Reply #40 on: October 19, 2016, 08:11:26 PM »
The Mustang is fast because it is slick.  The attempt at laminar flow drove the design to be streamlined.   The big brute radials had more power than the Mustang which really is in need of more horses in that Merlin.

Mustang empty weight is nearly 1500# less than the Corsair and has a higher fuel fraction.   I don't know if a Corsair gets its range (1050 miles) with drop tanks or not, but the Pony does (1650 miles).

The Corsair is extremely fast and slick in its own right. 

« Last Edit: October 19, 2016, 09:14:23 PM by Vraciu »
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Offline drgondog

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Re: Escorts could only stay with bombers for 20-40 minutes?
« Reply #41 on: October 19, 2016, 08:29:26 PM »
Vraciu - all the F4F, F6F and P-38 were able to outmuscle terrible drag with HP.
So, what? has nothing to do with CL=CD to pick optimal cruise speed.
Nicholas Boileau "Honor is like an island, rugged and without shores; once we have left it, we can never return"

Offline Vraciu

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Re: Escorts could only stay with bombers for 20-40 minutes?
« Reply #42 on: October 19, 2016, 09:13:31 PM »
Vraciu - all the F4F, F6F and P-38 were able to outmuscle terrible drag with HP.
So, what? has nothing to do with CL=CD to pick optimal cruise speed.


You're claiming the Mustang had long range because of its wing.   That's simply not the case.  Fuel fraction and a slick slick fuselage with an efficient cooling system did it. 

It was also more advanced in design and tech than any of those you mention.   

The F6F and F4F were very very slow relatively speaking.  The 38 actually had more range than the Mustang, to boot.   The Mustang "laminar flow" wing is an overrated legend.   A wive's tale. 
« Last Edit: October 19, 2016, 09:16:02 PM by Vraciu »
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Offline icepac

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Re: Escorts could only stay with bombers for 20-40 minutes?
« Reply #43 on: October 20, 2016, 05:07:30 PM »
The radial engines themselves were 20 inches wider than the merlin but I'm not sure if they counted the stacks or not.

Also the air cooled engines needed a little bit of space between the engines and the cowling.

The frontal area of a pilot in the sitting position is very close to the frontal area of the mustang until you figure in the doghouse.

Offline DaveBB

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Re: Escorts could only stay with bombers for 20-40 minutes?
« Reply #44 on: October 20, 2016, 05:15:58 PM »
At 400mph at 25,000 feet, the P-51's prop produced 1000 pounds of thrust.  Meanwhile, it's radiator produced 375 pounds of thrust. Meredith Effect.  While most aircraft had to dedicate approximately 10% of engine power to cooling, the P-51 only needed to dedicate 1-2%.
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