Fw 190A-3/U7 - a concept too cool to be ignored?

[tomo pauk]

I was reading about the Fw-190 recently, and one minor version popped up as an interesting one: the Fw 190A-3/U7. The plane was featuring reduced armament protection, but, most interestingly, it was equipped with external engine air intakes. Apparently, all of this enabled it to increase the high altitude performance. Here is n post from our forum:

Hi Broncazonk,

>It's July 1943, and you have just replaced Adolf Galland as General der Jagdflieger.

Maybe of interesting in this context: In August 1942, the first prototype of the Fw 190A-3/U7 made its first flight. In the current issue of Flugzeug Classic, there is an article by Dietmar Hermann on this type.

The gist of it: The Fw 190A-3/U7 was down in weight to 3660 kg from the 3850 kg from a standard A-3. This was achieved by eliminating pilot back and head armour, the cowl guns and by using non-sealing fuel tanks. Additionally, forward-facing "cheek" intakes were used to exploit ram effect.

This resulted in a decrease in turn radius from 1450 m at 10 km altitude for the A-3 to 1250 m for the A-3/U7. The Höhenjagdflugzeug, which would have a slightly enlarged wingspan (12.3 m) and wing area (20.3 m^2) - adding another 40 kg to the weight of the A-3/U7 - would have decreased turn radius to 950 m.

For the longer wings, the safe pull-out load multiple decreased from 6.93 for the A-3 to 6.0 for the (lighter) Höhenjagdflugzeug.

The Fw 190A-3/U7 with enlarged wing achieved an initial climb rate of 18 m/s, a ceiling of 11.9 km and a top speed of 694 km/h @ 7.4 km at "take-off power". Sea level speed was 534 km/h at take-off power.

The article is a bit contradictory with regard to the A-3/U7 - it seems it had the enlarged wing, but the above comparison suggests otherwise. Three prototypes were built.

Hermann reports that the full throttle height increased from 6400 m for the A-3 to 7400 m for the A-3/U7 thanks to the external intakes. Reportedly, this means 60% ram efficiency compared to 22% ram efficiency for the standard variant. Unfortunately, he omitted the full throttle heights for climb, or there'd be a good way to cross-check it.

Anyway, someone once posted a later chart on the Fw 190A-6, showing 652 km/h @ 6300 m for internal intakes versus 655 km/h @ 6850 m for external intakes. The gain above full throttle height appears to be around 10 to 15 km/h top speed, quite a bit less than the figures mentioned in Hermann's article.

Regards,

Henning (HoHun)

So, what's the take - Germans were wrong not to deploy a similar Fw-190 (plus bigger wing, minus the armor reduction) en masse, or they did the right thing leaving it at 3 prototypes built?

 

[ShVAK]

Hmm... sounds like a cool idea. The question is whether the cost of this development made sense over the Dora. Fw 190A-3/U-7 probably handled better but the D models were substantially faster at medium to high altitude, and could carry better armament and more armor.

 

[davebender]

Fw-190A doesn't need cowl mounted machineguns. However you need the protective armor and self sealing fuel tanks.

 

[tomo pauk]

Hmm... sounds like a cool idea. The question is whether the cost of this development made sense over the Dora. Fw 190A-3/U-7 probably handled better but the D models were substantially faster at medium to high altitude, and could carry better armament and more armor.

For the Jumo-213 to enter combat, LW needs to wait until 1944 (D-9 was historically fielded in late 1944, ie. too late to influence anything). The Fw 190A-3/U-7 was flying already in August of 1942.
The better armament is a moot point. Let's say that the U-7 enters service in 1943:
-the most likely targets are the B-17s and, in second half of 1943, the P-47s. Having extra 2 HMGs + their ammo are more likely the burden, than the asset. So maybe having a 3rd cannon (cowl position) instead of 2 MGs makes more sense? The armor need to stay as same as for the regular A-3, along with self sealing tanks. So the weight loss should be some 150 kg vs. the A-3, for the normal take off weight?

Fw-190A doesn't need cowl mounted machineguns. However you need the protective armor and self sealing fuel tanks.

Agreed.

 

[davebender]

That's a job for the 3cm Mk108 cannon. Three (1 in hub. 1 in each wing.) will make short work of B-17s.

 

[riacrato]

Fact remains many pilots liked the cowl guns for aiming purposes. I know, i know: different trajectories an all but it's still the case.

Iirc there were some field modified Fws with external air intakes, the performance increase at altitude came at the cost of less perfor.ance down low. Dietmar Harmann wrote an article on it somewhere.i'll try to dig that up later. Always seemed like a worthwhile mod to me.

Edit: i see hohun already quoted the article. Too bad hes not around anymore.

 

[cimmex]

That's a job for the 3cm Mk108 cannon. Three (1 in hub. 1 in each wing.) will make short work of B-17s.

It is impossible to install a hub cannon in a radial engine driven plane
Cimmex

 

[Tante Ju]

So, what's the take - Germans were wrong not to deploy a similar Fw-190 (plus bigger wing, minus the armor reduction) en masse, or they did the right thing leaving it at 3 prototypes built?

I think not, since they already had an overall much more capable high altitude fighter in their hands, in production and with the units, the Bf 109G with GM-1...

 

[davebender]

Why can't a radial engine be designed for a hub cannon?

 

[cimmex]

At a radial engine the prop axis is always in line with the crank axis. At V-engines the prop axis has an offset to the crank axis by the reduction gear and is placed between the cylinder banks so a cannon can be placed between the banks and fire through the hollow prop-shaft.
Cimmex

 

[cimmex]

At a radial engine the prop axis is always in line with the crank axis. At V-engines the prop axis has an offset to the crank axis by the reduction gear and is placed between the cylinder banks so a cannon can be placed between the banks and fire through the hollow prop-shaft.
Cimmex

 

[Shortround6]

Why can't a radial engine be designed for a hub cannon?

You might get a single row radial to have the gun go through the propshaft. IF you can offset the propshaft from the crankshaft enough to clear the crankcase.

Other wise you need to get the cannon shells to make right angle turns as they travel through the crankshaft

Every successful two row radial used staggered cylinders so there is no space to sneak the cannon barrel though the cylinders no matter what you do with the reduction gear.

 

[tomo pauk]

I'll return again to the part of the HoHun's post:

The gist of it: The Fw 190A-3/U7 was down in weight to 3660 kg from the 3850 kg from a standard A-3. This was achieved by eliminating pilot back and head armour, the cowl guns and by using non-sealing fuel tanks. Additionally, forward-facing "cheek" intakes were used to exploit ram effect.

This resulted in a decrease in turn radius from 1450 m at 10 km altitude for the A-3 to 1250 m for the A-3/U7. The Höhenjagdflugzeug, which would have a slightly enlarged wingspan (12.3 m) and wing area (20.3 m^2) - adding another 40 kg to the weight of the A-3/U7 - would have decreased turn radius to 950 m.

The Fw 190A-3/U7 with enlarged wing achieved an initial climb rate of 18 m/s, a ceiling of 11.9 km and a top speed of 694 km/h @ 7.4 km at "take-off power". Sea level speed was 534 km/h at take-off power.

Hermann reports that the full throttle height increased from 6400 m for the A-3 to 7400 m for the A-3/U7 thanks to the external intakes. Reportedly, this means 60% ram efficiency compared to 22% ram efficiency for the standard variant. Unfortunately, he omitted the full throttle heights for climb, or there'd be a good way to cross-check it.

Anyway, someone once posted a later chart on the Fw 190A-6, showing 652 km/h @ 6300 m for internal intakes versus 655 km/h @ 6850 m for external intakes. The gain above full throttle height appears to be around 10 to 15 km/h top speed, quite a bit less than the figures mentioned in Hermann's article

The U7 was, let's say, cheating - it was lacking not just cowl LMGs, but also the outer wing cannons (MG/FFM, in this particular case), wing cannon bulges, and was lacking 4 out of 6 chutes for the empty casings - speed gain of some 15 km/h just because of that?
We can use the figures from the manual of the A-5/A-6 (660 km/h @ 6300 m) for an assessment. Deletion of the all but the wing root cannons provides the speed of cca 675 km/h. Adding the external wing intakes makes the plane going faster, above 6850 m, those 10-15 km/h. So the two easy undertakings provide speed gain of some 20-25 km/h above 6850 m. Basis being, for example, 640 km/h at 8300 m/ 26770 ft - we now have 660-665 km/h there. Thus beating the D-9, but not the P-47/-51B-K. Not until the 801S is installed, with same intakes...

Of course, I'm looking forward for corrections suggestions.

 

[wuzak]

You might get a single row radial to have the gun go through the propshaft. IF you can offset the propshaft from the crankshaft enough to clear the crankcase.

View attachment 212854

Other wise you need to get the cannon shells to make right angle turns as they travel through the crankshaft

Every successful two row radial used staggered cylinders so there is no space to sneak the cannon barrel though the cylinders no matter what you do with the reduction gear.

Not the best arrangement for cooling a radial, though.

Also, the intakes, supercharger, etc, at the back of the engine would have to be designed to allow for the cannon.

 

[tomo pauk]

The chart showing the speed achieved with internal ("Ansaughutzen innen") and external intakes ("Ansaughutzen aussen"), by otherwise standard 109A-6:

Speed gain, at altitudes from 6800 m above, ranges from 15 to 10 km/h. The A-6 from the chart weights some 440 kg more than the A-3/U7, the extra 4 weapon openings chutes adding to drag (speed loss by some 10 km/h only because of that, vs. the U7?).
The standard A-6 (internal intakes) is faster under the full throttle height, no wonders there.

 

[tomo pauk]

Now let's go back to the A-3/U7. It was claimed (HoHun kindly provides an excerpt from the magazine):

The Fw 190A-3/U7 with enlarged wing achieved an initial climb rate of 18 m/s, a ceiling of 11.9 km and a top speed of 694 km/h @ 7.4 km at "take-off power". Sea level speed was 534 km/h at take-off power.

The article is a bit contradictory with regard to the A-3/U7 - it seems it had the enlarged wing, but the above comparison suggests otherwise. Three prototypes were built.

Hermann reports that the full throttle height increased from 6400 m for the A-3 to 7400 m for the A-3/U7 thanks to the external intakes. Reportedly, this means 60% ram efficiency compared to 22% ram efficiency for the standard variant. Unfortunately, he omitted the full throttle heights for climb, or there'd be a good way to cross-check it.

Looking at the BMW-801D table, we can see that, at speed of 700 km/h, the full throttle height for Notleistung, high gear, was 7.1 km (at 500 km/h it was 6.5 km), apparently with 100% ram efficiency. So IMO there was no way the A-3/U7 was capable to peek out at 7,4 km with claimed 694 km/h.
As for the A-6 with external intakes, it peaked out at some 6850 m, making 655 km/h. Going by the table, the max obtainable FTH for that speed is circa 6.9-7 km, so the A-6 was able to exploit the ram effect rather well.

BTW, for our members speaking good German: the term "durch strahl schub" - is that "exhaust thrust" by any chance?

 

[Denniss]

BTW, for our members speaking good German: the term "durch strahl schub" - is that "exhaust thrust" by any chance?

Yep
Why should the A-3/U7 not be able to get 7.4km FTH? Doesn't look that way off and depends on aircraft condition.

 

[tomo pauk]

You can take a look at the table for the post #16. Ideally*, the plane need to fly 700 km/h in order to have it's FTH at 7.1 km; the FTH increase with speed. Now since the A-3/U7 was reported to make 694 km/h, the FTH cannot be at 7.4 km, but lower.

*why ideally? It's unrealistic to expect that an airplane/engine combo has an ideal intake system that would harvest 100% of theoretically available ram effect. The A-6 with external intakes would, at speed of 655 km/h, have the FTH at 6.9 - 7 km, but it's a tad lower (check the graph at post #15). Hence the A-3/U-7 was maybe able to achieve the 7 km?

 

[stona]

The Luftwaffe obviously didn't consider the type worth developing. I've read that it had only a 8-10 kph speed advantage over a standard A-3 "at altitude" whatever that is supposed to mean.

The three prototypes built all ended up with the Geschwader Stab of JG 2.

Steve

 

[tomo pauk]

I doubt that it was only 8-10 km/h faster. The subtype had only 2 cannons vs. 4 cannons and 2 LMGs = less drag weight. The fuel tanks were not protected, too. The weight went down to 3660 kg, ie. lighter than the A-1. I've provided a chart (post # 15 here) that shows the A-6 with external intakes, but otherwise standard, was ~15 km/h faster at 7 km vs. the A-6 with internal intakes, above 6.5 km. The speed difference thins to ~10 km/h at 10 km of altitude.
The worth of external intakes was not earth shaking, but it was deemed worthwhile enough to be featured in manuals for the A-5/A-6 and A-8. The manuals clearly state that the speed can be increased above FTHs, but also decreased under FTHs.
The external intakes were also to be found in Ju-188E-1, and seem like they were doing a good job (1440 HP at 6 km, at 300 km/h - just like it was noted in the table from post #16):