B-17 and B-24: plausible upgrades?

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Thanks SR6.
So, are we saying that the B17 had reached the end of its development with the G series?
Cheers
John
 
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
 
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.
 
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GregP said:
.. 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.
Click to expand...

Which is the objective after all said and done.
 
About fuel consumption, kindly posted by SR6:


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.
 
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).
 
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.
 
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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.



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.
 
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.
 
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.
 
Greg, here is the data drom Joe Baugher:


Boeing B-17E Fortress



Boeing B-17F Fortress



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.
 
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.
 
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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.
 
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%.
 
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.
 
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