I don't understand how some planes ended up being so fast

[pbehn]

I don't join cliques Tomo. My argument is based purely on the P51. It needed all the fuel that could be put inside and outside to do the job as a long range escort and it needed a 1750BHP Merlin engine to get it all off the ground and up to altitude. An airframe with more drag needs more fuel and power The P51 was already heavier as an airframe than a Spitfire with a 1940 Merlin it just wouldn't have been a practical proposition.

 

[pbehn]

QUOTE="InlineRanger, Here's my understanding of the topic:

Maximum speed = thrust - drag. Thrust is engine power x propeller efficiency. At maximum speed, drag is almost entirely parasitic. The larger the difference between thrust and drag, the higher the maximum speed.

Your equation is for Force, not velocity.

When Thrust=Drag, acceleration =0. At that 'point' (for that altitude, MP, RPM), Vmax is attained. That said, deriving actual Thrust Hp is very complicated, particularly when Ram air and exhaust Thrust for the engine is required to solve. Total Drag is reasonably straightforward for level flight in incompressible flow range <0.3M. As flight speeds approach Mcr the drag rise factor is a Major delta. Incremental Drag factors over Zero Lift drag for say, climb or turn, must take into account RN, Form Drag due to AoA, pressure Drag due to airframe components immersed in prop vortex, Cooling drag, etc.

The determinant for achieving max velocity (and increase climb and Turn) is Power achieved - Power Required = T*V-D*V and is quite a bit more complicated analysis with Conventional aircraft.

You maximize engine power by increasing the oxygen mass flow rate, increasing compression ratio and reducing parasitic losses. You increase propeller efficiency by reducing span loading (low disk loading, high aspect ratio blades), maximize lift distribution (elliptical loading), and keeping wave drag to a minimum (tip speed below critical mach, thin airfoil/sweep to increase critical mach).

Parasitic drag is defined by the well known equation drag = zero lift cd * .5 * air density * velocity^2 * wing area. In reality, this is just an approximation that assumes all aircraft share similar fuselage to wing proportions. A more relevant metric would be the wetted area drag coefficient and wetted area. So drag can be reduced by flying higher and/or in hotter temperatures (reduce air density), reducing the wetted area, and streamlining.

CD= (CDp1+DeltaCDp1+DeltaCDp2)*CD/CDinc + Cdi where CDp1 is a f(RN) and will decrease with Velocity and density
CDp1 is minimum Parasite Drag Coefficient of the aircraft in zero lift at a particular RN and is comprised of wind tunnel measured component characteristics of the airframe (wing, fuselage, empennage, cockpit enclosure, carb duct, radiator duct and exhaust stacks for P-51)
'Delta' CDp1 are CD of individual components such as external antenna, machine gun ports or fairings, leaks and surface roughness)
Delta CDP2 are increments due to Angle of Attack and expressed as CL. Included are Cooling Drag/Net internal Flow losses during climb, external load items such as fuel tank/bombs, and Immersed items in slipstream (fuselage/empennage/cockpit, etc plus 1/3 of the wing) --------> for the latter, wind tunnel data are power off. Calculations of Power Required mandate accounting for the free stream dynamic head in the increased velocity slipstream behind the prop. The usual method is to apply increased drag for increased velocity over those components assuming velocity is constant throughout the slipstream. This calculation is extrapolated for RN in same fashion as CDp1

CDi is the Induced Drag component due to lifting surfaces, including Wing, H.Stab/Elevator and slats if ya got em'. There is also a small delta component due to washout of leading edge which changes the chordwise Lift distribution.

For Power Required calcs, the momentum loss in Carb air is also considered a loss in Power Available.

CD/CDinc is the compressibility factor over CD for incompressible flow.

For your THp data, you need to correct Static Hp with following:
Adiabatic temp rise
Friction Hp of Engine
Manifold Temp
Aircraft True air speed.

The Jet Thrust is a factor of;
Outside air pressure
Total Stack exhaust area
Engine charge consumption, slugs/sec
Ratio outside static air pressure to manifold pressure

With these data in hand the next step is to calculate prop efficiency - normally taken from Manufacturer's data

Then, make corrections to Power Available by deriving Power losses due to Slipstream, engine air momentum loss and corrections to Thrust Hp prop efficiencies

During the analysis phase, particularly for a two stage/two speed supercharged engine - the Power Available and Power Required for each desired Weight and stores condition and altitude are calculated to point to achievable Velocity and climb and range performance as function of MP and RPM.

Flying higher reduces thrust (reduced oxygen mass flow rate, lower propeller efficiency due to decreased critical mach) and also reduces critical mach (drag divergence occurs at a lower TAS). Supercharging (mechanical, turbine) and oxygen injection (compressed/liquid O2, nitrous oxide) help restore mass flow rate. Larger, slower turning propellers and/or increased blade count mitigate propeller efficiency losses at altitude.

The wetted area can be reduced by increasing thrust/weight ratio, increasing wing loading, using a low moment coefficient airfoil (reduce tailplane size), decrease cg range (reduce tailplane size), reduce area forward of wing quarter chord (reduce tailplane size), and reducing protuberances (canopy, antennas, lights).

Wetted area is what the term implies ---------> the total surface area top to bottom, side to side, of the airframe. The Wetted Area Drag coefficient is simply based on Total Drag divided by wetted area while the conventional CD is derived by dividing Total Drag by Wing Area.



The wetted drag coefficient can be reduced by using a long chord wing (higher reynolds number), reducing pressure gradients (laminar flow wings, long shallow canopy, generous radii at stagnation points), reducing interference drag (area ruling fuselage/wing/tailplane intersections), removing protuberances (antennas, lights), reducing surface roughness, decreasing cooling drag.

To the extent that Total Drag of the studied airframe can be reduced by any/all of your proposed changes will alter Wetted Surface Area, the Wetted Drag Coefficient will also change but;
1.) increasing chord without changing wing area will move the needle on RN but not change Wetted Area or Wetted Drag Coefficient unless the Wing change to alter chord also changes the CDp1 of the wing. This is true for P-51H wing in contrast to P-51/A/B/C/D/K
2.) All of your cited factors are functions of Parasite Drag (Form drag, interference drag, friction drag, etc) - independent of Wetted area. If you maintain Wetted Area but say, change the Parasite Drag CDp1 by making an airfoil change that achieves lower Drag as function of CL, then Wetted Area is same but Total Drag is reduced so both Total Drag and Wetted Drag reduce. If you change aspect Ratio while maintaining Oswald Efficiency but make no change to Wing Area - you may or may not change parasite drag but will reduce Induced drag - so possible change Wetted Area Drag to same extent Total Drag is altered.

All this is memory driven and of all the things I miss, I miss my mind the most.

AAAHHHHHHRRGGGG

Like a fool I googled Oswald efficiency number, then Reynolds number. Within minutes I am thinking about viscous flow in closed channels. After five minutes I have more coefficients and ratios than I can wave a stick at and stuff too complicated to copy and paste.



You are not fooling me Drgondog, it is black magic and witchcraft because I now know less than I did before I switched my computer on.

Great post, I will read it a few times to get my head around it.

 

[tomo pauk]

I don't join cliques Tomo. My argument is based purely on the P51. It needed all the fuel that could be put inside and outside to do the job as a long range escort and it needed a 1750BHP Merlin engine to get it all off the ground and up to altitude. An airframe with more drag needs more fuel and power The P51 was already heavier as an airframe than a Spitfire with a 1940 Merlin it just wouldn't have been a practical proposition.

Sorry if you feel called out, it was an attempt of a joke in no way directed at you.
I was trying to name a few real aircraft that possesed long range combined with useful performance, while using just one engine of modest power. The engines actually being lower power than weakest Merlin. Neither of the aircraft I've listed was using the ground-breaking aerodynamics, either.
P-51 with 1940 Merlin would've been even better than P-51 or P-51A.

 

[pbehn]

Sorry if you feel called out, it was an attempt of a joke in no way directed at you.
I was trying to name a few real aircraft that possesed long range combined with useful performance, while using just one engine of modest power. The engines actually being lower power than weakest Merlin. Neither of the aircraft I've listed was using the ground-breaking aerodynamics, either.
P-51 with 1940 Merlin would've been even better than P-51 or P-51A.

I thought you were joking. The first Mustang 1 was powered by a 1220 BHP Allison, well above the 1940 Merlin which was 1030-1100.

 

[tomo pauk]

The first Mustang 1 was powered by a 1220 BHP Allison, well above the 1940 Merlin which was 1030-1100.

I'm afraid you're wrong here.

On 100 oct fuel, in 1939, the Merlin III was rated for 1300 HP at 9000 ft, 5 min limit. At rated altitude of 16250 ft it was making 1030 HP. There is 48 total of V-1710s delivered in 1939, on 100 oct fuel they do 1040 HP at 14300 ft. There is no overboost ike the Merlin was rated, per factory. There is also no Merlin X (2-speed S/C) equivalent of the V-1710.
In 1940, slightly improved Merln XII (1-speed S/C) and much improved Merlin XX* are in production and in service. 1280-1400 HP.
V-1710, power-wise, remains at 1939 figures at altitude. Still no overboost - no factory-approved nor otherwise. Prototypes of the V-1710-39 are installed on the NAA-73 and P-40, power is 1150 HP at 12000 ft, altitude power over 15000 ft is in the ballpark of the older V-1710-33 of 1940. The -33 is rated 1090 HP at ~13200 ft.

Power charts:
Merlin III (and Merlin 60)
V-1710-39 (a.k.a. V-1710 F3R, on the Mustang I/Ia/(X)P-51 and P-40s; taken from manual)
Merlin 20 series (includes Mk.XX)
Interestingly enough, the V-1710-33 gives a tad more power at altitude than DB-601A in 1940.

*The advent of the Merlin XX will not go unnoticed by the USAAC/AAF brass in the USA, where Packard got a license production deal after the deal with Ford went thorugh the floor.

 

[Shortround6]

We seem to suffering from thread drift.........

There are a number of fighters that could possibly fly the distances we are talking about, the problem comes in with either escorting bombers or simply surviving in enemy airspace at the speeds and altitudes required to get the long ranges specified.

Numbers are all over the place for the Zero. However from "Zero" by Motorbooks International and authored by Robert Mikesh

We have some figures, since they are copies of US translations of captured wartime documents they may or may not be accurate
BTW I would note in the link you gave that US intelligence on the Zero is a little off. Like they have the wrong engine in the Model 32.

Anyway fuel consumption figures for the Sakae 12 are given as (after the US converted liters to gallons and KPH to Knots)
16.4 US gallons per hour at 180 knots
24.0 US gallons per hour at 190 knots
26.15 US gallons per hour at 200 knots
91.14 US gallons per hour at max rated power.

Unfortunately altitude is not given. Now if you can survive and/or perform escort duties while flying at 180 knots (207mph/334kph) all well and good. However if you need to fly much faster the range goes to hell in a handbasket. As does the range if you are forced to use max rated power for very long.
Flying range with combat time.
after 10 minutes 1025NM
after 20 minutes 900NM
after 30 minutes 774NM
so 10 minutes of combat was worth 125NM (144 statute miles/ 213KM)
Please note that flying at 190kts or above will shorten the range by about 1/3. roughly 10% increase in speed needs 50% more fuel per hour.
Also note that the British found that Spitfire II & Vs needed to cruise about 300mph (260kts) to improve survivability even on fighter sweeps over the low countries and France. It taking about 2 minutes to accelerate from a speed in the low 200mph range to full speed at which point the German attackers have pretty much done whatever they were going to do and left the area.
How good the A6M2 model 21 with it's single speed supercharger would have been over Europe in 1940/41 is questionable even if not restricted to low cruise speeds.
For the Sakae 21 the fuel figures change to:
21.98 US gallons per hour at 180 knots
24.43 US gallons per hour at 190 knots
26.97 US gallons per hour at 200 knots
114.92 US gallons per hour at max rated power.

Forget escorting B-17s, could any of these 1940/41 planes (or low tech 1942/43 planes) actually succeed in escorting even Whitley's/Wellingtons or on the other side He 111s in daylight on long range missions?
Granted tactics evolved and pre war or 1939/40 thinking may have been that cruising alongside or just over the bombers at the same speed was good enough. Actual combat might soon consign that thinking to the rubbish bin but then the escort "range" of these early fighters plummets if they adopt higher cruise speeds and "S"ing over the bombers.

 

[Shortround6]

I'm afraid you're wrong here.

On 100 oct fuel, in 1939, the Merlin III was rated for 1300 HP at 9000 ft, 5 min limit. At rated altitude of 16250 ft it was making 1030 HP. There is 48 total of V-1710s delivered in 1939, on 100 oct fuel they do 1040 HP at 14300 ft.

You are correct but then so is Pbehn, in the sense that the early Merlins were NOT rated at the higher powers for take-off and if trying to fly top cover for a bomber group/formation flying at 15-18,000ft the Merlin's power was pretty much down to 1030-1100hp.

 

[tomo pauk]

You are correct but then so is Pbehn, in the sense that the early Merlins were NOT rated at the higher powers for take-off and if trying to fly top cover for a bomber group/formation flying at 15-18,000ft the Merlin's power was pretty much down to 1030-1100hp.

I'm not sure if my point came through. Basically - in each year we pick, and currently our interest is 1940 and 41, Merlin has more power than V-1710 at any altitude. Best Merlin also beats best DB 601 in 1939-41.
During several years when the early Merlins were in service, there was no in-service V-1710.

 

[Shortround6]

That may be true, however the real question is does the Merlin have the power to lug around the extra weight of the "escort" fighter and remain competitive with the "local" interceptor even if the "local" interceptor uses a lower powered engine?

Part of this is theory as some of the actual aircraft suffered from some problems. Like being able to beat 109Es might not be a good benchmark as the 109E seems to be flying around with a small parachute attached in regards to drag.
If you can build 350mph+ Spitfires or 350mph P-40s with their large wings using 1000-1100hp engines then what could you get from a small fighter (short ranged) using the same engines?
Granted 109Fs were not much as bomber busters but Spitfire Is, IIs and Vs trying to play escort with several hundred more pounds of fuel and fuel tank aboard against early 109Fs might be biting off more than they can chew.
Designing and building escort fighters that depend on your opponent screwing up his defensive fighters is not very good planning or strategy.

 

[drgondog]

As all of you know, the discussion of Range compared to Combat Radius for operational planning purposes is far apart on just basic assumptions. The Range is the optimal, controlled, performance data for which the conditions of STP, zero winds aloft, minimal warm up and taxi time, perfect climb out to the stated altitude for which the test was designed, perfect throttle and altitude and RPM management including changes as fuel burned and a/c became lighter. Never happened except in extreme Ferry role.

'Book' Combat Radius for planning purposes presented a set of assumptions for warm, take off, climb, cruise, fight, cruise and RTB with 30 minute reserve.

Real Combat Radius "Depends'. For AAF the SOP for Fighter Operations differed significantly between ETO and CBI, for example in specific parameters as: Altitude, External Load, type of mission (Sweep, Close escort - Penetration, Target Escort, etc, etc.)

So, to clarify for decomposing a frag order from 8th AF HQ to Bomb Division to Fighter Wing to Fighter Group the general Plan breaks down to Group/Squadron assignments for that mission. When the Fighter Group receives the Frag Order, the information presented is a.) type of mission i.e. Ramrod bomber escort - Target, Map Co-ordinates for R/V, time of R/V, Position in Bomber stream for assigned Box/Boxes.

The Group Ops, Weather and Intelligence staff co-ordinate the flight path, winds aloft and waypoints, taking into consideration various flak concentrations desired to avoid. Based on the PLAN to arrive at the Rendezvous point at same time as the Bomb Wings, then the entire route timings are backed out based on the Cruise Speed and mission Load and Performance Charts right to the time of Start Engines for the Entire Group of three squadrons plus spares back in Jolly Old.

As an aside - from SE time is the project plan to back out times for a.) get line crews up for breakfast and out to line, b.) wake up pilots, feed them, brief them, draw equipment, catch ride to parked a/c, perform pre-flight - and hop in to perform cockpit/panel check, Battery/Generator On- bring the systems to life and watch the instruments as crew chief looks over your shoulder, adjust the seat, hook up oxygen, etc - then Start Engine.

The Engine warm up time, taxi and form up like gaggles of Geese from the squadron revetments to position on Transient position, element or flight take offs, circling the airfield as individual elements form into flights form into Flight position behind squadron lead, circling as number tow and three squadrons assemble and start climb to cruise altitude. Once cruise altitude is reached the guys with least fuel consumption is Squadron/Group CO. The other bastards behind him have to jocky throttle to maintain wing/formation integrity and usually use more than 5% greater fuel during the cruise process. Combat is accounted for but entirely depends on dropping externals to clean up, engage at Military Power for 15 minues, WEP for 5 - and have enough fuel for straight line cruise at a specific altitude - to RTB with a 20-30 minute reserve.

What really happens is that every pilot pretty well knows how much internal fuel he needs to return Radius "X" based on the winds experienced - plus reserve. He knows how much internal fuel he used to get to Cruise altitude, and how much he has when he punches his tanks.

The Story of 8th AF would have been different if the tactical thesis was to perform bomber escort at 15,000 feet rather than 25K

 

[tomo pauk]

...

Part of this is theory as some of the actual aircraft suffered from some problems. Like being able to beat 109Es might not be a good benchmark as the 109E seems to be flying around with a small parachute attached in regards to drag.
If you can build 350mph+ Spitfires or 350mph P-40s with their large wings using 1000-1100hp engines then what could you get from a small fighter (short ranged) using the same engines?
Granted 109Fs were not much as bomber busters but Spitfire Is, IIs and Vs trying to play escort with several hundred more pounds of fuel and fuel tank aboard against early 109Fs might be biting off more than they can chew.
Designing and building escort fighters that depend on your opponent screwing up his defensive fighters is not very good planning or strategy.

1st half of 1941 will mean historically Merlin 45 for the UK and DB 601N for the Germans. At the rated alt of 18000 ft (5.5 km) and +9 psi boost, Merlin 45 has 1210 HP, vs. 1100 PS for the 601N (one minute rating for the DB!). Spitfire V acting as LR fighter does not need to absolutely have several hundreds pounds worth of cannons & ammo installed.
Short range fighter is a disadvantage for Germany in 1941 if the RAF deploys LR fighters. Defending fighters in different parts of the occupied Western Europe can't support each other, situation is much worse than with different Groups of the RAF FC during the BoB. So the attacker has numerical advantage, plus it is already at altitude and speed, plus there are actual bombers than needed to be killed.

 

[pbehn]

We are at cross purposes in the discussion. The engine development went pretty much in parallel with the Alison and Merlin. When the Mustang Mk 1 first flew The Spitfire had the MkV in production with a 1450BHP engine. My point was that in 1940/41 there was nothing to base a long range escort fighter on. By the time of Pearl Harbour in Dec 1941 less than 100 Mustangs had been manufactured. It is a massive leap of faith to base a strategy on what this airframe may do, with a better engine and better fuels which are still on the drawing board or in the lab.

When discussing a raid to Berlin as an example, it is stated elsewhere that forming up of bombers took hours not minutes. The escorts therefore have to meet the bombers and escort for a set period then hand over to another group. The mission is therefore a complex calculation of take off, climb, cruise to rendezvous, escort, combat and cruise home. Six hour missions were not uncommon, the issue is therefore a plane that can stay in the air for 6 hours and be competitive with the enemy between hours 2 and 5 (or similar).

My contention is that with a 1940 Merlin a P51 would not be a plane to base a strategy on. Those who saw the promise of the Mustang /P51 saw it as a prospect worth looking at not a certainty.

 

[tomo pauk]

We are at cross purposes in the discussion. The engine development went pretty much in parallel with the Alison and Merlin. When the Mustang Mk 1 first flew The Spitfire had the MkV in production with a 1450BHP engine. My point was that in 1940/41 there was nothing to base a long range escort fighter on. By the time of Pearl Harbour in Dec 1941 less than 100 Mustangs had been manufactured. It is a massive leap of faith to base a strategy on what this airframe may do, with a better engine and better fuels which are still on the drawing board or in the lab.

Non-turbo V-1710 was lagging at least two years behind the Merlin. For a short war the ww2 was (technology-wise; it was endless war for people at the receiving end), that is big chunk of time. What Merlin offered in 1937, V-1710 offered in 1939. It was 1942 when V-1710 went better than Merlin III in altitude power - 5 years to beat it, (while the Merlin started being manufactured in the excellent 2-stage supercharged variant). And it still was not as good as Merlin XX or 45. As per 'it was nothing to base a long escort fighter on in 1940/41' - it was, in Japan, Italy, USA, Germany and the UK.

When discussing a raid to Berlin as an example, it is stated elsewhere that forming up of bombers took hours not minutes. The escorts therefore have to meet the bombers and escort for a set period then hand over to another group. The mission is therefore a complex calculation of take off, climb, cruise to rendezvous, escort, combat and cruise home. Six hour missions were not uncommon, the issue is therefore a plane that can stay in the air for 6 hours and be competitive with the enemy between hours 2 and 5 (or similar).

My contention is that with a 1940 Merlin a P51 would not be a plane to base a strategy on. Those who saw the promise of the Mustang /P51 saw it as a prospect worth looking at not a certainty.

As before - P-51 with Merlin from 1939 would've been a great fighter. See the P-51 and P-51A - 390-410+ mph on 1150-1125 HP. linky

 

[pbehn]

1st half of 1941 will mean historically QUOTE].

Tomo, not to take your post out of context but please bear in mind in the first half of 1941 Roosevelt was having difficulty getting the USA engaged in another European war, Germany had a non aggression pact with Russia, London was still enduring the Blitz and Pearl harbour hadn't yet happened. With Pearl Harbour and the declarations of war between the Axis and Allies discussion of strategic bombing went from theoretical to logistical almost overnight, but that is December 1941. Only a year before the USA was looking at the possibility of the UK falling to Germany and launching bombing raids across the Atlantic. The term long range escort is a convenient construct for what was actually possible, if the British Isles were a hundred miles from France not twenty one then the whole thing would have been impossible

 

[pbehn]

As before - P-51 with Merlin from 1939 would've been a great fighter. See the P-51 and P-51A - 390-410+ mph on 1150-1125 HP. linky

I am not talking about just a fighter but one that hauls full fuel tanks additional rear tank and external tanks up to 25,000 ft.

 

[InlineRanger]

Brute force.

Voodoo holds the record now, with maybe 1000hp less.

Brute force ignores exponential drag rise. You can't get 500mph putting 4500hp on a Cessna... but yes, brute force.

Is Voodoo's record unofficial? What mph? I thought Rare Bear still holds the FAI piston speed record?

Your equation is for Force, not velocity.

Ah, I agree. Power Available - Power Required is the relevant metric.

That said, deriving actual Thrust Hp is very complicated, particularly when Ram air and exhaust Thrust for the engine is required to solve.

Good points, especially about the exhaust thrust/carb momentum changes.

Total Drag is reasonably straightforward for level flight in incompressible flow range <0.3M. As flight speeds approach Mcr the drag rise factor is a Major delta.

Other than the propeller tips, the critical mach of WWII airfoils seems to be ~.72 to .74 mach, with drag divergence occurring a few tenths of a mach later. So we're talking ~530-550mph at Reno race density altitudes. I can see drag divergence happening at lower speeds during maneuver/climb, but does incompressible flow have any other relevant effects I'm not thinking of during maximum speed flight?

With these data in hand the next step is to calculate prop efficiency - normally taken from Manufacturer's data

Wouldn't it be ideal to analyze the propeller as the wing it is rather than rely on a third party? Most commercial/old military propellers seem inefficient with wide tips/non elliptic loading.

To the extent that Total Drag of the studied airframe can be reduced by any/all of your proposed changes will alter Wetted Surface Area, the Wetted Drag Coefficient will also change but;
1.) increasing chord without changing wing area will move the needle on RN but not change Wetted Area or Wetted Drag Coefficient unless the Wing change to alter chord also changes the CDp1 of the wing. This is true for P-51H wing in contrast to P-51/A/B/C/D/K

Increasing the wing chord increases reynolds number, which decreases the airfoil's parasitic drag coefficient, right? I should have said a low aspect ratio has a lower parasitic drag coefficient than high aspect ratio due to reynolds effects.

2.) All of your cited factors are functions of Parasite Drag (Form drag, interference drag, friction drag, etc) - independent of Wetted area. If you maintain Wetted Area but say, change the Parasite Drag CDp1 by making an airfoil change that achieves lower Drag as function of CL, then Wetted Area is same but Total Drag is reduced so both Total Drag and Wetted Drag reduce. If you change aspect Ratio while maintaining Oswald Efficiency but make no change to Wing Area - you may or may not change parasite drag but will reduce Induced drag - so possible change Wetted Area Drag to same extent Total Drag is altered.

This is just semantics, though isn't it? The parasite drag coefficient is in reference to some area, usually the wing. My understanding is that it can also be expressed in terms of wetted area, which is more difficult to calculate but more relevant.

All this is memory driven and of all the things I miss, I miss my mind the most.

Thanks for making me think.

 

[drgondog]

Is Voodoo's record unofficial? What mph? I thought Rare Bear still holds the FAI piston speed record?
Simple answer Yes, official, but for the FAI piston speed record, Voodoo had to average 1% over Rare Bear for the 'book' The last run was a slow 510 (IIRC). The first was 550 IIRC the 6 run average was 531+ to Rare Bear 528.

Other than the propeller tips, the critical mach of WWII airfoils seems to be ~.72 to .74 mach, with drag divergence occurring a few tenths of a mach later. So we're talking ~530-550mph at Reno race density altitudes. I can see drag divergence happening at lower speeds during maneuver/climb, but does incompressible flow have any other relevant effects I'm not thinking of during maximum speed flight?

If we take Mcr as that region in which Cd (total) increases by 0.002 above low speed CD, but Drag Divergence dCD/dM= 0.10, then most WWII airfoils stepped into Mcr near .6-.62, with full blown Drag Divergence at ~ .65 for similar T/C of 14-16%. It is only in this discussion that Incompressible flow has much meaning for practical aero. At Reno speeds all of the 500 mph racers are in full blown Drag Divergence well before 500mph - the Mustang wing was the lucky accidence of max T/C at 45% which delayed the drag rise region due to the reduced velocity gradient from nose to Max T/C (when compared to conventional 23-25% max T/C) and 'disturbeth not' the CMac compared to the others - so no violent Mach Tuck when CP moved aft with shock wave.



Wouldn't it be ideal to analyze the propeller as the wing it is rather than rely on a third party? Most commercial/old military propellers seem inefficient with wide tips/non elliptic loading.

I would think not - For the same reason I would be inclined to rely on NAA for Mustang Aero analysis, I would as an NAA aero rely on HamStd for their prop analysis, unless I wished to set up my design team with prop specialists and subcontract development and production? The Military props are migrating more and more to swept paddle blade with pointed tips to address the balance between delaying transonic drag divergence while stlll generating the desired performance - but take this comment with appropriate salt dose. I was never a 'prop' guy.

Increasing the wing chord increases reynolds number, which decreases the airfoil's parasitic drag coefficient, right? I should have said a low aspect ratio has a lower parasitic drag coefficient than high aspect ratio due to reynolds effects.
Yes to increasing Reynold's number for same velocity and density altitude. The P-51H wing compared to P-51A/B/C/D/K and earlier Mustangs is an example of slightly larger chord. As to increasing Parasite Drag? If the Wing Are is maintained by shortening span proportionately, with the same airfoil, the only Drag difference should be Induced drag due to slightly smaller AR and perhaps tip effects due to change in Tip/Root Chord ratio. The friction drag should be same, form drag would be examined in wind tunnel. I can confirm that P-51H Parasite Drag was slightly lower at Same RN but it also had a newer NACA 66,2-18155 airfoil so the extrapolation of cause and effect is not clear. It also a straight LE wing with constant washout, whereas the Early Mustangs through P-51B/C had a cranked LE with one washout to WS 61, the D/K had a greater crank and different washout from C/L to WS 61.

Now, all that said, the effect of RN is not an effect - it is a non scalar value used to compare CDs for comparable RN as well as to plot CD as function of Velocity. So, all I can tell you is that RN was only important to me to extrapolate more information from wind tunnel results for drag on a scale model.

This is just semantics, though isn't it? The parasite drag coefficient is in reference to some area, usually the wing. My understanding is that it can also be expressed in terms of wetted area, which is more difficult to calculate but more relevant.

The Cd, Cl and CMac are all a function of Wing Area. Wetted Drag coefficient is a function of total area. The latter is useful when comparing against another airframe, for Kentucky windage of design efficiency. An example of that would be Mustang vs Bf 109. The P-51 was huge compared to 109 but much lower drag, including wetted area drag which pointed out why the meticulous high quality manufacturing process and airframe design - smooth surface, flush rivets, tight gaps between sheet metal, no protubrances, second order curves, windshield/canopy design, etc - of the Mustang was so novel at the time. NAA does not get near as much credit as they deserve for the high quality/high quantity production techniques.

Thanks for making me think.

Unfortunately you made me think, also - hate when that happens.

 

[wuzak]

Is Voodoo's record unofficial? What mph? I thought Rare Bear still holds the FAI piston speed record?
Simple answer Yes, official, but for the FAI piston speed record, Voodoo had to average 1% over Rare Bear for the 'book' The last run was a slow 510 (IIRC). The first was 550 IIRC the 6 run average was 531+ to Rare Bear 528.

The numbers show they didn't have enough to officially beat Rare Bear's record, but I understand the categories were redone, so Voodoo hold the record for its category now.

 

[tomo pauk]

Tomo, not to take your post out of context but please bear in mind in the first half of 1941 Roosevelt was having difficulty getting the USA engaged in another European war, Germany had a non aggression pact with Russia, London was still enduring the Blitz and Pearl harbour hadn't yet happened. With Pearl Harbour and the declarations of war between the Axis and Allies discussion of strategic bombing went from theoretical to logistical almost overnight, but that is December 1941. Only a year before the USA was looking at the possibility of the UK falling to Germany and launching bombing raids across the Atlantic. The term long range escort is a convenient construct for what was actually possible, if the British Isles were a hundred miles from France not twenty one then the whole thing would have been impossible

No quarrels with this quoted post. IMO it points into political realities, not technical ones.

I am not talking about just a fighter but one that hauls full fuel tanks additional rear tank and external tanks up to 25,000 ft.

I was talking about both existing and feasible long range fighters - with plenty of both internal and external fuel.

 

[Shortround6]

We have pointed out the large increase in power the Merlin got for a small increase in weight due to better supercharges and fuel that helped make the "escort fighter" possible.

Part of our trouble is not having a good definition of "escort fighter" for the beginning of the war. We don't need P-51B performance in 1940/41 because A, the enemy fighters aren't that good, and B, the bombers aren't B-17s, they can't fly as high or as far carrying 4-5000lbs of bombs.

The US is sort of a special case as they had some of the longest distances to deal with and the most practice in designing long range aircraft in the 30s. Just getting around the US was major problem. In 1939 the US only had 17 air bases and 4 air depots in the entire country. This is one reason for the overload tank in the P-36, just getting from base to base required longer ranges than the European air forces needed.
Then look at what the US was buying for bombers and what they were planning.
Even a B-18A Bolo was supposed to fly 1150 miles with 2496lbs of bombs, granted at around 167mph. and it wasn't what the Army wanted, it was what they could afford.
The Army also wanted 7000ft paved runways for it's bombers. It couldn't afford many of them either in 1938-40.
The B-15 and B-19 showed where the Army wanted to go and no single engine fighter of the time could hope to get anywhere near their range. Granted this is an extreme.

For the British things are both a bit simpler and bit more complicated. Their bombing force, as it existed in 1939-41 was somewhat shorter ranged. Leaving the Whitley out of it as it was almost always considered a night bomber, you have the Wellington and Hampden as the main bombers with the Blenheim and Battle hanging around the edges. Since even the last two could operate (in theory) over a 400 mile radius (1000 mile range - 1 hour at cruising speed reserve/allowance and then divide by two for radius) you are trying to design a 400 mile radius fighter in 1938-40. that or tell the air staff and the politicians doling out the money that "Yes we have bombers that can strike 400-500 miles from base but our escort fighters can only cover them to a 300 mile radius." Hardly a good answer.
British have tiny airfields and are several years behind in adopting constant speed propellers so while in theory it may be possible to design an escort fighter with good performance from a technical point of view, in practice it was much harder.

Please remember up until the shooting started there were all manner of restrictions on what were allowable landing speeds and even how many pounds per square in the tires were allowed to put on the turf/sod airfields. The Whirlwind had to get special dispensation for being 10% over the limit.
Please note that even a P-39 had a higher approach and landing speed than the Whirlwind which was much criticized for it's landing speed and the feared restriction on number of fields it could operate from.
any pre war work on a British escort fighter would have to take all this into account in initial design stages even if a lot got thrown out by the time it entered service.