U.S. "Super Prop" Fighters P-51H, XP-72, and moreJun 11, 2021
Greetings, I'm Greg, the arrival of fighter aircraft near the end of World War II caused the demise of piston-powered fighter aircraft, however it did not happen suddenly due to uncertainties about the performance of the aircraft, especially takeoff performance range and engine longevity, many manufacturers kept some eggs in the piston engine basket until about 1946, so there was a period at the end of World War II when they began to A series of new airplanes emerge that I will call
superaccessories as an example. Here is a mustang model p51h at first glance. Like a common WWII p51d, however, it is a totally different aircraft, having a longer and deeper fuselage, a different tail, smaller landing gear, many weight saving techniques in construction and a wing completely new.
You may notice the leading edges of the wings are straight all the way. from the wingtip to the fuselage, which is different than on a previous 51. It also featured the upgraded Packard Merlin engine, the v1650-9 equipped with water injection that allowed it to achieve over 2,200 horsepower through 90 inches of manifold pressure. There was also a new
propeller to drive. That power and top speed of the P51H were
morethan 470 miles per hour. There is some calculated data from North America to support a top speed of 490 miles per hour at very low weights. I couldn't find any real flight test data to support that.
The P-51H was produced before the end of the war, but it was too late to see combat in World War II. He continued in military service, but was overshadowed by the newer fighter jets in the republic aircraft, they were also very busy coming. In fact, it was not long after the first flight of the P-47 Thunderbolt in May 1941 that they unveiled the design of an aircraft that would become known as the XP-69. be powered by a liquid-cooled 42-cylinder
supercharged turbo radial engine, that's right, a liquid-cooled radial. We don't know much about this aircraft because Republic records were destroyed in the 1980s.
We do know that it was intended to have counter rotation. supports four 50 caliber machine guns and two twin 37 millimeter cannons, it was to use a new naca wing profile,
moreor less the same as would be seen later on the p51h that we just talked about, it was to have a canopy with a top part of bubble that is not. Shown here because this is just a mockup, the xp-69 was canceled and I don't have any original sources saying why, but I think we can safely assume it was due to engine problems with the R216 liquid cooled radial engine. right.
The plane was designed around this engine and I believe the engine had serious problems. I based this on the fact that no production aircraft ever flew with that engine and there were other aircraft in development that were designed for it, but as with the XP-69, those developments also went nowhere after the Republic XP-69 will advance with three other super accessories, the P47H, P47J and XP72, all of which are based on the tried and true P47. I'll start with p47h, which was a p47d15 with an inverted one. v16 liquid cooled made by chrysler performance data for this aircraft is sparse and original test data appears to be non-existent.
Secondary sources put its top speed at approximately 490 miles per hour, but I believe that is a calculated number the plane flew. There is a video. on youtube with color images of in-air color tuning, but it appears it didn't get far enough along in its development to generate meaningful test data. The plane probably had engine problems, as that engine never made it to production. This may have been a loss for Republic Aircraft, but in a strange way, I think it actually worked out for the Chrysler corporation. This was their first hemi engine or at least their first liquid cooled hemi engine.
There was an air-cooled tank engine built as a joint venture with the Continental I. I'm not sure which came first, but both had hemispherical combustion chambers. In any case, it seems that they really stuck with this combustion chamber design, as it became common on Chrysler Corporation engines starting in 1951. This design was not unique to Chrysler, but it was very unusual. in American cars and gave them an advantage over Ford and GM. Then in 1964, a marketing genius at Chrysler trademarked the word Hemi, and to this day that word is synonymous with power. Now modern Chrysler Hemi engines aren't really Hemis, which means they don't have any. true hemispherical combustion chambers, but hey, there's a marketing plan at work here and they own that trademark, so they can call anything they want hemi and the combustion chambers in the new Dodge Challenger v8 and its mates are somewhat hemispherical, so the legendary hemi engine has its beginning in a p47 Thunderbolt strangely they called the top model of the Challenger Challenger Hellcat.
I think Challenger Thunderbolt would have made more sense. The next contender was the XP-47j, which was essentially a fully optimized P47 Thunderbolt. It still had the Thunderbolts r2800 engine. although now at 2,800 horsepower, this was in 1943 and at a time when a normal Thunderbolt had around 2,000 or maybe 2,300 horsepower depending on exactly what time of year you're looking at, the XP-47J also had changes aerodynamic and was lighter than a typical p47 it was the first piston-engined fighter to exceed 500 miles per hour in level flight its actual top speed was probably about 505 miles per hour at altitude the plane certainly had a lot of performance but the project The XP-47J was abandoned.
In fact, it didn't fail, I think by any reasonable performance standard it was a success, but I also think it's obvious why they didn't move forward. The j model represented the end of the p47 development line with the r2800 engine. There are no more improvements. in performance they were realistic, not everything was effort in vain, since that 2800 horsepower engine saw combat in the p-47 mic and november variants and much of the aerodynamic work was transferred to the next project we are going to talk about . that brings us to the xp-72 as with these other republic aircraft there just isn't much out there, the destruction of republic records was a real loss but at least with planes that entered service like the p47 we have data on testing and reports from others, but for prototypes and experimental things, we have almost nothing.
Still, people have been asking for information on the xp 72, so I'll do my best here and by the way, these airplanes I'm covering, this isn't exactly in chronological order. um a lot of this development was simultaneous with the other aircraft anyway regarding the xp 72 let's start with the engine it's the pratt whitney r 4360 wasp major this is a 28 cylinder four row engine with about 43 62 cubic inches of displacement which is just shy of 71.5 liters so this is a huge engine any four row radial will present cooling challenges sometimes even two row radials have these issues to minimize this every The row of seven cylinders was slightly offset from the row in front, this was combined with deflectors.
It helped cool and gave the engine its corn on the cob nickname because the configuration looks like a cup of corn. You can notice that the intake space for cooling air at the front is quite small, especially for an engine as large as the German fw 190. The xp-72 has an engine driven cooling fan to help, this allowed them to keep the engine very well covered for aerodynamics and still have enough cooling for the engine. The big r4360 had a history of cooling problems in several aircraft and although they only built two xp-72s. I have never read that they had a cooling problem so I suspect this fan arrangement worked quite well on my fw 190 series.
I am talking about the cooling system for the bmw radial and that fan specifically, some people have hinted that the fan cooling and the fw190 were a waste of time due to the power requirements of the drive. They don't have any kind of mathematical evidence or anything from the time to really back this up, but they say it must be that way because no one else copied it. well first of all I don't think that's evidence of anything, there are many reasons not to use a cooling fan, however the xp 72 does have one and it's obvious that Republic knew what Focal Wolf had been doing and concluded At least in this case, sacrificing a little engine power to drive a cooling fan and trading it for an aerodynamic advantage was worth it.
The exhaust outlets and bonnet fins also look very similar to the fw190a8, the outlets are on the side of the bonnet. not completely or mostly as they normally were on American radial
fightersup to that point, the exhaust stacks were angled sideways to provide forward thrust, this was possible because the XP-72 does not have a turbocharger like the P -47, but instead the xp-72 is designed with a two-stage mechanically driven supercharger system in the front, the engine was connected to the engine, it was a centrifugal supercharger much like that of any other radial engine fighter aircraft from World War II, so there was a second supercharger behind the cockpit driven by a long driveshaft, this gives us the two stages, the first stage which is the rear supercharger and the second stage in the engine, so far it could be On second thought, then, did the F4f Wildcat have a second boosted supercharger?
The Corsair and Hellcat also had it on a separate axle. Now in those cases the shaft was much shorter, but a second remote supercharger certainly wasn't very special at this point in airplane development, what was special at least for any American radial engine airplane was that this rear-mounted supercharger rear was driven by a variable speed fluid coupling much like the one we see used on the Messerschmitt 109. The Bell P63 King Cobra also had a similar
propulsion system for its primary stage supercharger, very similar to that of the XP-72. So it seems that Daimler-Benz was right with that variable speed drive, but the United States adopted it too late to fully develop it.
In fact, the same can be said for the p63, now that this variable speed supercharger allowed the XP-72 engine to run. It retains full manifold pressure over a wide altitude range without acceleration losses, just like the P47's rear-mounted turbocharger. It's unclear why Republic decided to use a mechanical supercharger for the aft-mounted supercharger instead of an exhaust-driven turbo, but I believe it's because at the time no exhaust-driven turbocharger existed that was large enough to do the job, in any case the mechanical supercharger had the advantage of being able to retain exhaust thrust for propulsion and still avoid acceleration losses, I'm sure it did.
It costs more to drive than a turbo, but as we have seen in other episodes probably not much more. I have a video that covers these concepts, the concept of exhaust driven turbos, mechanical superchargers along with reducing acceleration losses and driving power requirements. I'll put the link for that in the description if you're interested, of course there was a large intercooler between the stages, as there was on the p-47, the engine was initially configured with 3450 horsepower, even without the meaning of the second supercharger . without the aft mounted supercharger it was still 3000 horsepower, post war development on other aircraft had this engine up to 4300 so there was a lot of development potential which again I think is probably why the xp was chosen 72 instead of the XP 47J of the two powered the two prototypes, the second had contra-rotating propellers, the production version was to use contra-rotating propellers with 3,600 horsepower, giving it a safely calculated maximum speed of over 500 miles per hour in tests, the plane made 490 miles per hour with the non-counter-rotating four-blade propeller, it is not clear if it had its second supercharger installed, it also had an incredible climb speed of over 5,200 feet per minute.
There was no lack of firepower, it could carry six 50 caliber machine guns, which I am sure They have been changed to eight. I say this because they went to six for the p-47 microphone and then went back to eight for November. The six-gun configuration was not popular with pilots. I had two other weapon configurations I could use with 450 and dual. 37 millimeter guns, which I think would have probably been the best configuration or armament of all the 437 millimeter guns. That 37 millimeter cannon will destroy with a single shot. Believethat any plane in the sky might not be like a B-29 or something like the US Army Air.
The Army was so impressed with the XP-72 that they placed an order for a hundred of them right after seeing test flights; However, the order was canceled as soon as it became clear that the war was going to be won with the P51 and P47 and of course, the planes were arriving so there was simply no reason to build the XP-72 powered aircraft. per propeller were starting to hit a brick wall in terms of speed there were three problems first the piston engines were nearing the end of their development potential the The rate of improvement was going to slow down even today, with the exception of electronic controls.
Almost all of the features of a modern automobile engine existed before or during World War II. These include dual overhead cams, four valves per cylinder, turbos, superchargers, intercoolers, roller bearings and/or followers. Nitrous oxide 150 octane multistage supercharge fuel and even variable valve timing had already been developed, although it had almost no use in airplanes because airplanes operate in a very narrow rpm range due to constant speed propellers, so the Variable valve timing and airplanes never went anywhere during We're talking about this era, but it had been invented, there were patents dating back to the early 1900s, the potential improvements in piston-engine airplane propulsion were going to be small. in the post 1945 world compared to what they had been in I'm not saying there won't be improvements, things will certainly improve, things have become more advanced, but nothing like the advances they had seen in piston engines. from, say, 1900 to 1945.
Now, on the other hand, it was very clear that the new airplanes had a lot of room for development, so the next problem we're going to have to talk about is that speed limits are the resistance and of course this affects both jet and propeller aircraft. This is not a p-47 chart, it is a generic aerodynamics chart for naval aviators, but the point is to show. how total drag increases quite a bit with speed at high speeds total drag is mostly made up of parasitic drag that parasitic drag increases as this has the square of the speed increases in other words at 200 miles per hour you will have four times as much parasite The drag you had at 100 miles per hour, so at very high speeds, every increase in speed causes a disproportionately high increase in drag and we need to compensate for that drag with thrust, which brings us to the third and probably biggest problem for the propeller-driven aircraft, which is that for a given amount of power, the thrust of the propeller will decrease with the speed above a certain point;
In other words, even though you still have 2000 horsepower, as you go faster and faster above a certain point, the propeller thrust decreases. So drag not only increases the amount of power we need to offset the thrust we We are losing to compensate for that resistance, but it increases even more. This is hard to understand and I'll go over it briefly, but I want to say that Adam The Engineer has a great video on this topic, so I'll put a link in the description in case my basic description doesn't help you. You can go see his things.
Well, you should go see them anyway. There is a good channel. anyway, take a look at this formula, it shows that the power required is equal to the thrust required multiplied by the speed divided by 325. now the thrust required is equal to drag those for the purposes of this discussion, they are the same thing as use the number 325. With speed in knots, if you were using miles per hour, it would be 374. Now let's do some simple calculations, suppose we have two identical airplanes, except that one has a 2000 horsepower engine attached to a 100 by 1 propeller. one hundred efficient and the other has a jet. engine with two thousand pounds of thrust somehow all other factors of the plane are equal.
This is a fantasy scenario. Note that the required drag and thrust are the same number, so in this case two thousand pounds of thrust offsets the two thousand pounds of drag and with that much thrust, our fictional jet plane can make 325 knots, the pr in the formula is for the power required, we need that for a propeller plane, not the plane, we do the math and see that at 325 knots, our 2000 horsepower will exactly offset the 2000 pounds of drag it will do perfectly so far, 2000 horsepower prop plane equals 2000 pound thrust jet, which means at that specific speed 2000 pounds of thrust is effectively the same as 2000 horsepower, now you'll get a little more complicated, let's try accelerate the plane up to 450 knots and see what it would take, so we are a design team and we have to do this.
We can't change anything on the plane, but we have to make it work. except the power and we have to make it go well at 450 knots, if we calculate the drag it will increase to 3834 pounds of drag, generally speaking, I didn't do this exactly, I just assume it's all parasitic drag, which is mostly so now Our airplane needs three thousand eight hundred and thirty-four pounds of thrust, that's a lot more than two thousand pounds, but how does it compare to the propeller airplane? The propeller plane now needs 5,309 horsepower. Consider what it would take to get 2000 horsepower from 2800 up to 5309 horsepower, I don't think it's possible while maintaining any kind of reliability, even the much larger Wasp Major never reached that level in a normal racing application. , although it is possible that some racing aircraft such as the Dreadnought have such high numbers.
Either way, going from 2000 horsepower to 5309 without increasing drag was going to be a problem, on the other hand the plane only needs to go from 2000 pounds of thrust to 38-34, that's still quite a bit, but it's no small feat. realistic for For example, the Rolls-Royce turbojets on the British Meteor started with 2,000 pounds of thrust each and will eventually go up to 3,600 pounds of thrust each. Consider February 1946, just six months after World War II ended. The Republic F-84 flew with an engine that gave it over 5,500 pounds of thrust, soon that same engine would increase to 7,500 pounds with water injection, so if you consider the thrust requirements for speed in terms of power and thrust levels available in different engine types, it is very clear that higher speeds greatly favor jet aircraft and that is not to mention the propeller tip speed limitations.
I'll talk about propellers in another video, so it's no mystery why Republic went ahead with their new fighter jet. which became the f-84, first flew in early 1946 and soon they were flying over 600 miles per hour in this aircraft, even then the f-84 became obsolete a year after its first flight due to the new swept-wing aircraft, progress was advancing. It advanced at an incredible rate, to the point that the XP-72 and many other late-war aircraft became obsolete before their first flight, even post-war aircraft were often obsolete after just a year in service, that That's all I have for today.
I want to thank everyone. my patreon followers as this video goes online i am sending links to the patreon page which has some updates. I don't have a manual for the xp 72. I think it's pretty obvious, but I put a bell p63 manual in the patreon section. It has an excellent textual description of the variable supercharger unit, unfortunately there are no images, but the text is useful so some may find it interesting. I also added a p51h manual and more manuals related to the latest airplane video. I put Corsair manuals. 109 manuals and more. a great day bye oh and uh just one more thing when you travel internationally you usually start the trip expecting certain things to be different from country to country maybe people speak another language driving on the other side of the road you might see Los Street Vendors selling chocolate covered crickets not only will sometimes the food be a little different, but when you find the same food, it will usually have a different name.
We all know the Royale with cheese. What I find most interesting are the unexpected differences, which we will talk about. Elevators We've all been in elevators and they all work the same way, right? Aren't you coming? You press a button telling it you want to go up or down. Eventually a door opens. You enter. You press a button for the desired floor. and voila, elevators work the same way in the US, Germany, Australia, Asia, the Middle East and Africa almost everywhere, but in Sicily, at least in some places, they have another way. It confuses people at first, but I think it's actually superior to everyone else's way. we're doing it, we're in our hotel in Sicily heading to the elevator complex and there are four elevator doors here, they're labeled b, c and d and there are no up or down arrows to indicate where we want to go, just a keyboard that we put on. at our destination, which is floor zero, it tells us to go to door d, we cannot guess which door will open because the keyboard tells you, at door d we enter the elevator and once you are here, you continue the journey There are no, there are no buttons to change your destination, so the system works quite well, I think it's kind of interesting and another trivia is that in Europe the ground floor is almost always floor zero, not floor one, they have a floor different. way of telling I guess that's all anyway, bye
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