B-17 and B-24: plausible upgrades?

[Readie]

The B-17 started at 26,520 pounds empty, 37,000 pounds gross, 45,650 pounds (Y1B-17A)

Went to: 29,021 pounds empty, 39,320 pounds gross, 49,650 pounds maximum. (B-17C)

Jumped to: 32,350 pounds empty, 40,260 pounds gross, 53,000 pounds maximum (B-17E)

And finished at: 32,720 pounds empty, 55,000 pounds normal loaded, 72,000 pounds maximum. (B-17G)

A few structural modifications and beefed up landing gear along the way. Bomb Bay fit between wing spars so it was limited in length.

On lot of these big bombers (especially ones that grew in weight) you cannot stick what ever you want, where ever you want.

From the B-17 Manual:

A) B-17F airplanes, with modified landing gear
and added chord-wise wing tip tanks, can be flown up to
and including a gross weight of 64,500 pounds, with
the following restrictions:

(B) At 64,500 pounds,the extra wing tip tanks
must be full to obtain the effect of a relieving load on
the wings in flight. Care must be exercised in taxying
avoiding rough ground. Take-offs, above a gross
weight of 56,000 pounds may be made only on smooth
fields or prepared runways. All pivot turns on one
wheel, while taxying, will be avoided.
(C) All B-17 type airplanes, equipped with extra wing tip
chord-wise tanks, must be operated in accordance
with (B) preceding, whenever the wing tip
tanks are more than half full. Maximum permissible
indicated air speed of B-17F airplanes, with extra
wing tip tanks full, must be limited to 230 mph, when
loaded to 64,500 pounds. Maximum maneuver permissible
at 64,500 pounds; positive, 2.056; negative,
1.22; landing gear, 2.1.

A B-17 running light (40-45,000lbs) could hit 226mph at sea level (true and indicated speeds the same?) using max continuous power. A B-17 at 53,000lb could probably pull a 2.5 G maneuver and in the low 40s it could pull 3 "G"s.

Even if you increase the power you are starting to run in "placard" limits that restrict the actual performance increase.

Thanks SR6.
So, are we saying that the B17 had reached the end of its development with the G series?
Cheers
John

 

[Readie]

Boeing YB-40 Flying Fortress - Wikipedia, the free encyclopedia

This is a fascinating version too. I see that speed was a problem so, this may have been a candidate for more powerful in line engine. Maybe retractable guns too to save drag.

Cheers
John

 

[Shortround6]

I don't know if it had reached it's limit but you are getting into an area where any major improvements are going to need some major modifications to accommodate them and and that throws the easy estimate of the size of the improvement out.

The XB-38 was a modified "E" and ONE .30 cal gun in the nose (if it was ever installed for the tests?), in fact from looking at photos it may have had no tail guns installed (at least when pictures were taken) no waist guns and a remote control belly turret.


Trying to estimate the improvement on the "G" is a bit difficult as the "G" had a lot more drag than the "F" (or figures are given for much higher weight?)

At least one version of the B-17 was rated for a 20,800lb bomb load but both the bomb load and the flight "profile" were rather useless. 8 1600lb armor peircing bombs inside and a pair of 4000lb demolition bombs out side. Drag was horrendous, fuel limited, and flight maneuvers severely restricted.

Say you take your "72,000lb" B-17 and up the installed engine weight by 1200lbs and up the landing gear and structural weight by several hundred pounds (say 300?) do you now have a 73,500lb plane with the same restrictions (and payload=fuel+bombs) as the 72,000lb "G"? of course your empty weight is 1500lbs greater so less than max load flights may suffer a bit.

Weights for the XP-38 were supposed to be 34,748 pounds empty, 56,00 pounds gross, 64,000 pounds maximum.

Note the 2400lb increase in empty weight over the parent "E". at 64,000lbs gross that is 2400lbs less fuel or bombs. It better get more range for the same fuel.

 

[Readie]

I don't know if it had reached it's limit but you are getting into an area where any major improvements are going to need some major modifications to accommodate them and and that throws the easy estimate of the size of the improvement out.

The XB-38 was a modified "E" and ONE .30 cal gun in the nose (if it was ever installed for the tests?), in fact from looking at photos it may have had no tail guns installed (at least when pictures were taken) no waist guns and a remote control belly turret.


Trying to estimate the improvement on the "G" is a bit difficult as the "G" had a lot more drag than the "F" (or figures are given for much higher weight?)

At least one version of the B-17 was rated for a 20,800lb bomb load but both the bomb load and the flight "profile" were rather useless. 8 1600lb armor peircing bombs inside and a pair of 4000lb demolition bombs out side. Drag was horrendous, fuel limited, and flight maneuvers severely restricted.

Say you take your "72,000lb" B-17 and up the installed engine weight by 1200lbs and up the landing gear and structural weight by several hundred pounds (say 300?) do you now have a 73,500lb plane with the same restrictions (and payload=fuel+bombs) as the 72,000lb "G"? of course your empty weight is 1500lbs greater so less than max load flights may suffer a bit.

Weights for the XP-38 were supposed to be 34,748 pounds empty, 56,00 pounds gross, 64,000 pounds maximum.

Note the 2400lb increase in empty weight over the parent "E". at 64,000lbs gross that is 2400lbs less fuel or bombs. It better get more range for the same fuel.

I guess every design reaches the stage where an 'improvement' does not necessarily give the sought exponential increase in performance / usability.
Thank you for your facts and figures, I hadn't considered many of them when I thought that the simple application of more power would 'do the trick'
Cheers
John

 

[GregP]

Good post Shortround. It's starting to look like U-2! Then U-2 had severe limitations in flight but did the job very well.

The B-17 was a workhorse until the war ended, despite any limitations. I don't really think the Allison, Merlin, or anything else would have made the B-17 all that much faster or better. It was a good design as designed and to make it a significant notch better would have taken another design. Most aircraft perform about as they do regardless of modifications made.

One example that was noticeably better was the difference between the F9F Panther and the F9F Cougar. The difference was the swept wing. The fuselage and tail were essestially the same. It made a speed difference of 75 mph. The modifications we've been talking about are engines, drag elimination, and some weight savings. They just won't make all that much of a difference. The cruise speed delta of about 40 mph for the Allison engine variant is nice, but hardly significant when the attackers come in at 400 mph. Now if you could put on an engine that would cruise at 1,200 HP, that would make a real difference.

A 300 mph B-17 would have been wonderful ... but the engines would have burned fuel fast enough to preclude bombing Berlin from England. That's why "what-ifs" are not my favorite subject. Almost everyone forgets the real-life consequences of what may LOOK like easy changes. The specific fuel consumption of high output radials is fairly well known and upping the power from 750 - 850 hp at cruise to 1,200 hp materially affects the range. Real performance changes can be had from reducing weight, drag, or both. Adding power without reducing drag or weight will help but will also reduce the range. It's OK if the range is already more than enough but, if the range is critical at all, adding power won't help much since you can't get there and back again.

 

[Readie]

.. Adding power without reducing drag or weight will help but will also reduce the range. It's OK if the range is already more than enough but, if the range is critical at all, adding power won't help much since you can't get there and back again.

Which is the objective after all said and done.

 

[tomo pauk]

About fuel consumption, kindly posted by SR6:

At MAX cruise (lean condition) the Allison is good for 795hp at 63 gallons an hour. The Cyclone is good for 750hp at 62.5 gallons and hour.

At MAX continuous (rich) the Allison is good for 1100hp at 113 gallons and hour and the Cyclone is good for 1000hp at 103 gallons an hour.

At max cruise (lean), the V-1710 produces 6% more power, while using 1% more fuel per hour. So the distance covered in an hour should be better for V-1710 engined plane, since such plane should cruise faster. That is even if we don't account for improved streamlining of the former.

At max cruise (rich), the Cyclone uses some 10% less fuel to make 10% less power. It is also a less streamlined affair than V-1710. So yes, the V-1710 onboard will consume more, but the plane with it can make more miles in a same time than a Cyclone-engined one.

 

[Shortround6]

True, Tomo, but the size of the bombbay and the size/type/number of bombs is pretty much fixed without redoing the wing-center fuselage.

I don't agree with Greg, the Allison power does add a bit of speed but I doubt the cruise goes up by 40mph IF both planes have the same nose/guns/weight and use about the same power. a 10% increase in power is good for about a 5% increase in speed if the drag stays the same. A 40mph increase in cruise is over 20%. The Allisons are more streamlined but they didn't reduce the size/cross section of the nacelles and unlike the P-36 to P-40 conversion the engines, even four of them, are a smaller percentage of the total drag.

And unless you increase the gross weight of the airplane, if you want to carry the same bomb load to Berlin you have to do it with 2400lbs less fuel (400 gallons).

 

[tomo pauk]

deleted - messed up my math

 

[GregP]

Actually Shortround, I was using the numbers supplied by history, not doing any calculations. The reported specifications for the XB-38 say it cruised at 226 mph ... I didn't claim that. They also say the top speed was 327 mph. Again, not my claim, just a number from a reference.

You are bringing up things about drag and guns. etc. that the reports on the XB-38 don't cover. Since you are doing a "what if," claim anything you want and nobody can prove or disprove it. I really don't think all the things you can think up are covered in the reports of the day, so we are both left wondering what really might be true.

Personally, I don't even know if the cowlings were mated up to the stock B-17 firewalls or if maybe they made newer, smaller firewalls since they were chaning the engines anyway. If I were in charge, I would have if only to see what effect the lesser drag had. I wasn't in charge but firewalls aren't all that tough to make. I calculate a 10% increase in power to be worth about a 3.1% increase in speed if drag is the same, so obviously I think the drag of the XB-38 was lower than the drag for a radial powered unit. One reason may well be smaller cowlings mated to a smaller firewall ... I can't really say unless I, or WE, get access to some of the design data on the aircraft. Personally, I haven't seen that to date. Then again, I also haven't looked for it.

Since they only built one and it crashed, we are both left speculating a bit. If you happen to have information about the fiitment of the XB-38, or the final configuration, maybe you could share that. I don't have any such data at this time from which to form a basis for a sound opinion, just the reported historical performance numbers. The historical reports say 327 mph max, 226 mph cruise and I have no reason to doubt the claimed numbers. They are also relatively meaningless in importance since they didn't produce and deploy the XB-38.

I DO think there was room for improvement in the B-17 airframe. Whether or not the Allison or Merlin would be the basis for improvement I can't say, but the British tried going between powerplant types on several designs and none seemed to make a huge difference unless the installed power was also very different. Likely as not, the B-17 would have followed suit with that regard even if V-12's were used in production.

 

[wuzak]

Actually Shortround, I was using the numbers supplied by history, not doing any calculations. The reported specifications for the XB-38 say it cruised at 226 mph ... I didn't claim that. They also say the top speed was 327 mph. Again, not my claim, just a number from a reference.

You are comparing the XB-38, which was based on a B-17E, with the later B-17G. The performance comparison you should be making is between the XB-38 and the B-17E/F. It is well known that the nose turret on the B-17G cost it performance, particularly in maximum speed.

Personally, I don't even know if the cowlings were mated up to the stock B-17 firewalls or if maybe they made newer, smaller firewalls since they were chaning the engines anyway. If I were in charge, I would have if only to see what effect the lesser drag had. I wasn't in charge but firewalls aren't all that tough to make. I calculate a 10% increase in power to be worth about a 3.1% increase in speed if drag is the same, so obviously I think the drag of the XB-38 was lower than the drag for a radial powered unit. One reason may well be smaller cowlings mated to a smaller firewall ... I can't really say unless I, or WE, get access to some of the design data on the aircraft. Personally, I haven't seen that to date. Then again, I also haven't looked for it.

The V-1710s were definitely hooked up to the standard B-17 firewalls. There is a picture of the V-1710 on the XB-38 in Vees for Victory, the engine without cowling. It has a lot of spare space around it.

The V-1710s used the same turbo in the standard location as for B-17s. Not sure about the intercoolers, but since the chin on the nacelles didn't include coolant radiators I would surmise that there were intercoolers in there.

 

[yulzari]

As has been mentioned, the key limit to the B17 is the size of the bomb bay. Pimp it up how you may it can only carry just so much inside.

Along time ago on another forum it was suggested that early (pre tail turret) B17s with high altitude optimised Bristol Hercules engines and the Type 442 Wellington B Mark VI pressurised cabin could make a viable huge PR machine capable of vast photo coverage at heights above interception. Someone else reckoned it would take the Luftwaffe about 3 weeks to strip something (an Me 109T with a DB605 was suggested) down that could reach it with a single 20mm cannon.

 

[GregP]

As for comparing variants, the B-17E, F and G and all the rest cruised at about 180 mph. The E might have been a bit faster if it was flying alone, but bomber streams of B-17's cruised at about 180 mph as a matter of record.

I've seen that picture of the XB-38 as I happen to own that book. I can't tell if the firewalls are stock or not, and when the fitment is finished, there isn't much room left at all for anything else to be stuffed into the cowling. The book SAYS they were stock, but where did Dan get that data? The radiators are buried in the wing leading edge according to the book, and there are some good pics supporting that. Whatever the case, the reported cruise speed was 46 mph faster than the reported cruise of wartime B-17's. Had the XB-38 been produced and deployed, it might well have also cruised at 180 mph just to fly with it's radial engine brothers, I can't say since it didn't happen. It is interesting that Dan Whitney has the information to write a few paragraphs about the XB-38 but supplpied no performance data in his book. I had to get the reported performance from other references.

I'm not making wild claims and I'm not trying to change the reported numbers ... they are what they are. If you want to compare the XB-38 to anything, have at it in good health. The reported numbers are from a total population of only one, so it was certainly the fastest XB-38 ever produced, being the only one.

 

[wuzak]

Greg, here is the data drom Joe Baugher:

Specification of Boeing B-17E Fortress:

Four Wright R-1820-65 Cyclone radials rated at 1200 hp for takeoff and 1000 hp at 25,000 feet.

Performance:
Maximum speed 318 mph at 25,000 feet.
Cruising speed 195-223 mph.
Landing speed 70 mph
Service ceiling 36,600 feet.
Normal range 2000 miles with 4000 pounds of bombs.
Maximum range 3300 miles.
Initial climb rate 1430 feet per minute.
An altitude of 5000 feet could be attained in 7 minutes.

Dimensions:
Wingspan 103 feet 9 3/8 inches
Length 73 feet 10 inches
Height 19 feet 2 inches
Wing area 1420 square feet.

Weights:
32,350 pounds empty
40,260 pounds gross
53,000 pounds maximum.

Fuel: Normal fuel load was 2490 US gallons, but extra fuel tanks could be installed which raised total fuel capacity to 3612 US gallons.

Armament: Specified defensive armament was as follows: one 0.30-inch machine gun which could be mounted on any one of six ball-and-socket mounts in the extreme nose. One Sperry No. 645473E power turret in the dorsal position with two 0.50 Browning M2 machine guns with 500 rounds per gun. One Sperry No. 654849-J power turret in ventral position with two 0.50-inch Browning machine guns with 500 rounds per gun. One 0.50-inch Browning M2 machine gun is each of the two waist windows, 400 rounds per gun. Two 0.50-inch M2 Browning machine guns in the tail position, with 500 rounds per gun.

Maximum bomb load was 26 100-pound bombs, or 16 300-pound bombs, or 12 500-pound bombs, or 8 1000-pound bombs, or 4 2000-pound bombs.

Boeing B-17E Fortress

Specification of B-17F:

Four Wright R-1820-97 Cyclone radials rated at 1200 hp for takeoff and 1000 hp at 25,000 feet.
For brief intervals, a war emergency power of 1380 hp could be delivered.

Performance:
Maximum speed 299 mph at 25,000 feet, 325 mph at 25,000 feet (war emergency).
cruising speed 200 mph. Landing speed 90 mph
An altitude of 20,000 feet could be attained in 25.7 minutes.
Service ceiling 37,500 feet.
Range 1300 miles with 6000 pounds of bombs, maximum range 2880 miles.
A range of 4420 miles at 5000 feet could be attained with 3612 gallons of fuel.

Dimensions:
Wingspan 103 feet 9 3/8 inches
Length 74 feet 9 inches
Height 19 feet 1 inches
Wing area 1420 square feet.

Weights:
34,000 pounds empty
40,437 pounds loaded
56,500 pounds maximum.

Fuel: Normal fuel load was 2520 US gallons, but extra fuel tanks could be installed which raised total fuel capacity to 3612 US gallons.

Armament: Specified defensive armament was as follows: six ball-and-socket mounts in the nose and front windows. One 0.30-inch M2 machine gun with boxes for 5100 rounds of ammunition. One Sperry No. 645473E power turret in dorsal position with two 0.50 Browning M2 machine guns with 500 rounds per gun. One Sperry No. 654849-J power turret in ventral position with two 0.50-inch Browning machine guns with 500 rounds per gun. One 0.50-inch Browning M2 machine gun is each of the two waist windows, 400 rounds per gun. One ball and socket mount was fitted to the roof of the radio operator's compartment for a 0.50-inch Browning M2 machine gun. Two 0.50-inch M2 Browinging machine guns were installed in the tail position, with 500 rounds per gun. Later production batches (B-17F-70 to 130-BO, B-17F-35 to 65-DL, and B-17F-35 to 50-VE) had a single 0.50-inch machine gun installed in a cheek mount on each side of the nose. Additional flexible machine guns installations were often mounted in the nose in the field, firing from sockets cut into the side or from windows cut into the side of the nose.

Boeing B-17F Fortress

Specification of Boeing XB-38:

Four Allison V-1710-89 liquid-cooled V-12 engines, rated at 1425 hp at 25,000 feet.

Performance:
Maximum speed 327 mph at 25,000 feet.
Cruising speed 226 mph.
Service ceiling 29,700 feet.
Range 2400 miles with 3000 pounds of bombs, 1900 miles with 6000 pounds of bombs.
Maximum range 3600 miles.

Dimensions:
Wingspan 103 feet 9 3/8 inches
Length 74 feet 0 inches
Height 19 feet 2 inches
Wing area 1420 square feet.

Weights:
34,748 pounds empty
56,000 pounds gross
64,000 pounds maximum.

Armament: Armament was the same as that of the standard B-17E, namely one 0.30-inch machine gun which could be mounted on any one of six ball-and-socket mounts in the extreme nose, one Sperry No. 645473E power turret in the dorsal position with two 0.50 Browning M2 machine guns with 500 rounds per gun, a remotely-controlled power turret in ventral position with two 0.50-inch Browning machine guns with 500 rounds per gun, one 0.50-inch Browning M2 machine gun is each of the two waist windows, 400 rounds per gun, and two 0.50-inch M2 Browning machine guns in the tail position, with 500 rounds per gun.

Vega XB-38


So, the XB-38 was 9mph faster at 25,000ft than the B-17E, 2mph faster than the B-17F (when it was using WER).

On the face of it the XB-38 was 26mph faster than the B-17F's cruise speed, but the cruise speed of the E is between 195mph and 223mph - that is between 3mph and 31mph slower than the XB-38. The range would, probably, be the difference between cruise settings - maximum cruise, best economy cruise.


I do think that the cruise speeds employed by B-17s in combat were the result of formation flying and the need to maximize range.

 

[wuzak]

Double post.

I would like to correct something I said earlier.

The XB-38 had the oil coolers in the chin position in the nacelles. It used the intercoolers in the standard position, though the ducts were opened up slightly to give greater cooling capacity. The turbo wasn't exactly the same as used in production B-17s at that time, but it was a B-series, so it fit into the standard position.

 

[wuzak]

The V-1710-89 was rated at 1425hp Military power @ 54inHg MAP and 1600hp @ 60inHg MAP WER. Using WER could give a few more mph over the recorded results.

A quick estimate gives 340mph.

 

[wuzak]

Was just looking at a test report on the B-17F http://www.wwiiaircraftperformance.org/B-17/B-17F_41-24340_FS-M-19-1470-A.pdf and found, interestingly, that the inboard and outboard engines had different critical altitudes by 1500ft! 27,800ft for inborad engines, 29,300ft for outboard engines.

 

[GregP]

I don't really have any quarrel with any of your post above, Wayne. I have the speed for the standard E as a bit slower, but a few mph either way isn't going to make or break it. And I also think you are right, the 180 mph was a formation cruise speed ... and that's where they cruised on most missions regardless of whatever speeds they were capable of.

In any case, they didn't proceed with the V-12 B-17, so it's pretty much a moot point. I like the XB-38's looks but, in the end, only the radial variants saw service. I prefer the two-row Pratt 1830 to the single-row Wright 1820, but that's personal preference only. The specific fuel consumption is better for the Pratt by about 20%.

 

[wuzak]

http://www.wwiiaircraftperformance.org/B-17/B-17G_Standard_Aircraft_Characteristics.pdf

This characteristics data sheet for the B-17G shows a "high speed" mission profile. Cruising speed is listed as 214kt @ 10,000ft (246mph @ 10,000ft) and combat radius 595 miles with a military load of 10,000lbs. Ammunition is listed as 5970 rounds of 0.50". I assume the weight of the ammo is included in the military load?

The speed curves show that with maximum power the G could do 282kt = 325mph.

 

[wuzak]

This chart, from http://www.wwiiaircraftperformance.org/B-17/B-17G_Standard_Aircraft_Characteristics.pdf, seems to be showing a 10,000lb load could be carried on a 600nm (690 mile) radius mission at an average speed of 214kt (246mph).

If it could do that, why didn't it?