Could you have designed a better Warbird?

[delcyros]

Don´t want to interfere but I still have the performance figures left out.

The performance model is based on the Ar-240 but with modifications.OK, so for this plane with 2x DB-601A and a full pressure height of 4500m we have a max speed: 618 km/h or 384 mph @ 4700m (including ram effects) - I will use subscript "1" for this model in the analysis below.
Power delivered is Thrust x Velocity. This is the power after the losses due to the gearbox and propeller inefficiencies.
P = T x V
At equilibrium straight-and-level flight (such as when maximum level speed is attained), Thrust is equal to Drag. Thus, the power required from the engine/gearbox/propeller combination is:
P(required) = D x V

Also, we know drag can be written as:
D = 1/2 x rho x V^2 x S x CD
where rho is local air density, V is true airspeed, S is wing area and CD is the aircraft drag coefficient.

With the power curves and the reported top speed of the Ar-240, it is possible to graphically extrapolate the speed / altitude curves:



Since the As-411 driven Ar-137 does only have a full pressure height of 3000m, we should take this into account as a reference altitude. At this altitude, the top speed of the Ar-240 was approx. 585 km/h or 363 mph and 2 x 995 hp were required from the Db-601A/B to achieve this speed.
Further when we study th wing area, we will see that the Ar-137 is just half the size of the Ar-240:

S[1] = 31,3m^2 -Ar-240
S[2] = 15,5m^2 -Ar-137

Unfortunately, the drag coeffcient is larger. In fact, the Ar-240 -thanks to the adoption of a high speed airfoil- was exceptionally clean aerodynamically. I got a CD-figure from rds-student, basing on wetted surface, frontal area and form factors beeing almost double as high as that for my Ar-240 simulation!
Ground attackers are not really aerodynamically efficient when relying on a high lift airfoil. Even with geared drives and center engines...

Ok-so far. This allows us to simplify top speed projections a bit. We have twice the CD but only half the reference area so both effects cancel each other out in this case, unless some fairly large changes to the model are made or compressibility effects are involved.
V[2]= V[1] * (sqrt^3(P[1]/P[2]))
= 585 * (sqrt^3(570/990))
= 510 km/h or 317 mph

300 mp/h at Bodenlader altitude is a reasonable top speed for an attacker
Performancewise, this is not stellar nor outstanding but it allows for a very fast speed at or close to Sealevel with an inferior engine. Quite an accomplishment (Anyone around interested to build a Reno Airracer with these center engines?).
The backside of the coin may be seen in a rather low service ceiling (ca. 7000m, of secondary importance for the mission profile). The high drag figure in combination with a high weight probably does not make for a good turner, too and single engine performance would be difficult -at best.



As a matter of fact, the drag coefficient is better than that of the Hs-129 (primarely due to missing nacelles and more thrust delivered by the prop installation) but not by much. Had the Ar-137 the same wingarea as the Hs-129, it would have been only able to hit 432 km/h at 3000m. Only a slight improvement over the Hs-129. The big improvement is the reduction in area and associated general drag reduction.

 

[Burmese Bandit]

Delcyros, what's the stall speed for your design? Especially at low levels?

 

[delcyros]

At Sea level and with flaps down, power on, the stall speed is 86.6 mph or 140 km/h at 4.781 kg with normal flaps and 135 km/h with split flaps (translates to a slightly higher CA-max).
At about sealevel with flaps up and power on, the stall speed raises to 118 mph or 190 km/h.
Finally the glide stall speed (flaps up, no power) is just under 210 km/h at sea level, not taking into account allowances for the ground effect.

 

[delcyros]

I owe You a side view. Here it is, with the BK 7,5cm gun in the heaviest version.



Compared to the normal plane several modifications are made:
A) removal of parts of the frontal armour
B) removal of the fuselage tank and bulkhead
C) Installation of the gun below the weapons bay and the loader in the weapons bay.
D) armoured bulkhead behind the engine section is enforced to 10mm (CoG -shift requirement)
E) installation of two smaller fuel tanks in the center section below and to the sides of the drive gear mechanism.

All these changes increase the gross weight to 5.250 kg (overload condition).
A good side effect is the larger drum for 75mm shells. Instead of 12 rounds, the loader contains now 16 rounds.

Top speed is significantly reduced, I consider a figure of about 450 km/h as reasonable, but it´s just a guess.

 

[Venganza]

Nice work, delcryos! Great to see the plane in side view. The pilot has an excellent view forward, although the rear view suffers. Is the 75mm jettisonable, in case of crashlandings? Are you sure the CG adjustments are sufficient to counteract the weight of the cannon - it still looks like it might be nose-heavy, which could be a problem when landing on mushy Ostfront airfields. Still, looking good!

Venganza

 

[delcyros]

It´s one good thing to have that much armour to deal with. Gives plenty of opportunity for CoG-changes. Do not forget that the engines are behind the CoG. Removing parts of the armour and fuel tanks fore and upgrading armour aft is the solution. Unfortunately, the gun cannot be dropped for the very same reasons. Removing almost 1000kg that far fore would drastically change the CoG to become tail heavy in combination with the measures outlined above. On the other hand, emptying the fuel tank or the ammo drum does not change the CoG. In this way, the Ar-137 is no different than the Hs-129B3.
The plane is in an overloaded condition and likely would require fine landing strips anyway.

 

[Burmese Bandit]

Delcyros, I would like to see a picture of your plane configured for the BK 5, and your thoughts on its ammunition supply. Also your comments on its handling thereof.

Furthermore, could you give us a top view?

 

[delcyros]

The BK-5 configuration appears to me the most reasonable tank hunter design. Only a small bulge is necessary. The belt design is unchanged (22 round closed belt). This gun fires normal APCBC rounds, which could be augmented by HE. Two MG 151 (either /15 or /20, depending on mission requirement) are placed to the sides with 100 rpg each. No special changes to the model are required. Gross weight is just under 4.800kg, top speed at about 500 km/h at 3000m.
The 50mm barrel extends through the lower cockpit and protrudes slightly over the frontal fuselage. A long recoil mechanism will help to dampen the 2.4t. recoil force.
Penetration performance exceeds any tank in service by 1940/41 to 1944.

The handling wouldn´t have been much affected as the reference weight and gross weight is pretty much the same. It´s just a bit slower, but not by much.

 

[VG-33]

The Russians tried 37s and 45s on the Yak.

Result: cracked engines. crashed aircraft.

Are you goind to breed some 451 other posts or show us soviet documents about Yak-9T MTTF/MTBR statistics? Some soviet and Normandie pilots didn't like it, it's true because it was heavier than a serial Yak-9. No mention about particular reliability problems, except maybe on prototype trials.

The Russians tried mounting the excellent NS-37 on the Stormovik (IMHO the best 37 of the entire war)

Not the entire war, in 1943 it clearly appeared on polygone and real tests results at the Koursk salient, that cumulative PTAB 1.5/2.5 was a much more effective weapon.

That was why the Russians suddenly demanded 40mm Hurricanes from Britain...to the intense embarrassment of the Brits, who had just discontinued that entire production line! Second hand Hurri Tankbusters were hurriedly rounded up and sent to the Russkies.

First, russians never asked for 40mm Hurricanes, it was a Churchill's direct proposal to Stalin, on his letter from the 12 april 1943. From the 60 promised Hurries IID only 46 reached USSR in october, the alternative others were mk IV ones without heavy guns, but ready to have them wing mounted frome spare parts.

Secund, since soviet pilots dislike Hurries from 1941, the unfortunate 25th and 11th ZAP pilots begun their training in the end of 1943 with a wise slowness as it was said Kamozin (chief) memors.

The regiment (246 IAP) moved to front as late as june 44, the 30, with the last 32 surviving Hurricanes (to the rough training and soviet pilots hate)! On july was on it's front-line airbase. Made virtually no recorded sortie against ennemy neither in the air neither on the ground. From 26/07 and 30/07/44 both regimental 1st engeener and commander send letters to the 215e division staff in order to change all those tired weared planes. Some complaints were justified, planes were only reaching 356* km/h on sea level, instead of official 427! On august, the 10, orders were given from the top to dismiss all Hurricanes in favour of Yak-1.

Thus ended the neither glorius, neither bloody Hurricane's IID carreer on the easter front. (Third and last point)

Regards

VG-33

* some 38 mph slower than Shturovik on that time!

 

[Venganza]

Very good, delcryos. Would the 50mm version have been capable of use from unprepared strips?

Venganza

 

[delcyros]

Definetely. It has enough performance and structural reserves for rough field service.

 

[Venganza]

Definetely. It has enough performance and structural reserves for rough field service.

So only the specialized 75mm version needed the prepared strips. I agree that the 50mm version is more plausible and useful, because of not needing prepared strips, being lighter, having less drag, etc. Perhaps you've covered this, but do you know how much lift that rather small wing is going to get from the propwash of those large, close-mounted (to the wing) propellers? By the way, what software are using to generate your nice drawings? If I try my hand at a "What if?" plane, I'd like to know a good drawing or engineering software to compose it in.

Venganza

 

[Burmese Bandit]

Excellent, Delcyros!

We now have three views of your aircraft, armament, handling, power, landing gear characteristics...and a performance chart as well. A very professional attempt, much better than what I could have done, and very succintly written as well.

My next proposal will be an HE-177 with five engines in a very high speed configuration, and a total all up weight comparable to the B-29.

 

[Burmese Bandit]

The He-177 I am designing has two engines in each wing, in tandem push-pull configuration as done by Dornier in their planes (most notably in the Dornier 26) and one engine in the fuselage behind the pilots cockpit. A shaft extends from that engine over the roof of the cockpit and exits in front of the cockpit, driving a propeller. This means that there is no physical communication possible between the pilot, flight engineer, navigator, and front gunner in the cockpit and the mid gunner, belly gunner and the tail gunner in the mid and rear section.

With five engines each driving one propeller the gearing problems, and oil fire problems of the historical HE 177 are instantly bypassed. Cooling wil be done by radiators in the wing leading edges, in the wing roots: there will be two radiators in all. They will be protected by a 12 mm armour plate top and bottom. The extra weight of armour will add about 4-6,000 lbs to the design, partly offset by losing the weight of two gearboxes. A total all up weight of 40,000kg or 90,000 lbs is envisaged.

With 7500 hp at least driving this design and basic drag lower than the original HE 177 frame even an extra 20,000 extra weight over the historical 177 should not slow my design down. As one or two of the five engines could be shut down in flight to save fuel this design should be able to cruise economically at a much slower speed that the historical 177, which means much greater range. The low drag airframe combined with the heavier weight means that my 177 design should be able to better the shallow dive speed of the historical 177.

The historical 177 was able to reach 420 mph in a full power shallow dive. My design should reach 450. This means that if my 177 is bounced from behind by fighters it has a good chance of escaping, and the rear gunner plenty of time to aim and kill the rear attackers.

 

[delcyros]

I sourced the net for programs.

rds-student is a basic and cheap tool for design, sizing, drag and performance analysis of general aircraft design, which I found to be helpful. Unfortunately, it´s not easy to get used to use.
Some model builders have very, very good airfoil and wing design tools for free.
Finally, Rick Robinson´s planebuilder and my own tabulated weight lists derived from my 1941 edition of Arado: Kriegsflugzeugbau helped me in my weight calculations.

All graphics were edited and made with Coral Draw 12.0, an older version of the series. It has some powerful tools for scaling and is easy to work with.

BurmeseBandit. If You want higher performance from Your He-177, then I suggest to use the fuselage engine as a slave engine to supply the air for the superchargers of the other engines (alike Hs-130e and Do-217P layout). Using this method You greatly increase the critical altitude and thrust output and hence improve the bombers high altitude speed by a considerable margin.

 

[Venganza]

Thank you for the info on the programs, delcryos! I'll take a look at them.

Venganza

 

[Burmese Bandit]

Is it Freeware? Or if you have to buy it, where and how much?

 

[Burmese Bandit]

Rereading the history of both aircraft design and air combat, I was struck by a sudden idea...

WHAT IF THE GERMANS HAD REVERSE ENGINEERED THE P-39 BUT WITH 3 30mm MK 108 INSTEAD OF A SINGLE 37mm M4 CANNON IN THE NOSE???

Let's see if this flies.

Since the Germans had access to captured P 39s from 1942 onwards and heard about and seen them from 1940 onwards the design would not be a problem.

In place of the Allison, mount a DB or Jumo engine.

From a weight point of view, the P 39 had one 96 kg M4 cannon in the nose plus two 29 kg Browning 50 cal machine guns. Total weight, 154 kg.

Three Mk 108 will weigh 180 kg. A little heavier, but in my opinion quite doable. The additional weight to the front will actually make the German P 39 handle better.

How will the guns be mounted? I would design two Mk 108 in a Zwilling configuration, and then put one 108 on top, staggered and back. Ammo feed would be top down for the top gun, set back, and standard left and right feed for the Zwilling.

The hole in the middle of the prop would need to be larger, but quite doable - this is a simple engineering problem.

Result?

1. High speed, maneuverable Bomber killer in the West. A one-pass kill machine. Also I suspect anything lighter and with less armour than a Sherman could be killed by the concentrated power of the three Mk 108 in the nose. With enough hits even the top armour of a Sherman could be vulnerable. And as for trucks, and armoured cars, goodbye. Heck, with a good burst I think even a LCI or LCT would be vulnerable!

2. High speed, maneuverable Stormovik killer in the east. Ditto. One pass one kill per Stormovik. And perhaps a T 34 killer too, although anything heavier would not be vulnerable. As for trucks... ditto as in west.

Opinions?

 

[riacrato]

I don't get it, do you want to have all 3 guns firing through the prop shaft? I very much doubt that's possible.

Remember you can't synchronize the MK 108, so you can't have it in cowling or wing root installations. I think a single MK 108 and 2 MG 151/20 would be a better setup anyways and easily enough to shoot down IL-2s in droves.

 

[Burmese Bandit]

And just why would that not be possible? I fail to see the slightest reason why. Mechanically, it is merely a matter of fitting a large, hollow fixed shaft through which the 3 barrels would shoot.

The second question you asked is: why would we want 3 Mk 108s anway?

And the answer is..

It is all about weight, and gun efficiency.

The P-39 was a wonderful aircraft, but there were three things wrong with it: it had poor high altitude performance, it had too little weight forward and too much weight back, and the rate of fire of its main gun was way, way too slow.

With a DB or Jumo, three Mk 108s, we take care of all three problems at once. Turning a 'good but flawed' design into an 'excellent' design.

Why three Mk 108s instead of a combination of other guns or one Mk 108 and some other gun combination?

Simple. If you look at Tony William's website, you will see that no gun designed and used in WW 2 comes even close to the gun weight/rate of fire/ammo power/ cost equation that shows how efficient the gun was. The MK 108 is the King of all guns. It has an efficiency factor of 9.7 on the Tony Williams Website. No other gun comes half as close.