Engine questions help

[MiTasol]

Additional questions.

1. Why were motor driven cannons not explored during the war?

Who knows but electric motors and electric generators/alternators were VERY heavy and very inefficient. Same to a lesser extent with hydraulics and engines did not have drive pads for large hydraulic pumps. From memory, motorised Gatling guns were tested on aircraft in ww1 and discarded as impractical at that time.

 

[BiffF15]

Who knows but electric motors and electric generators/alternators were VERY heavy and very inefficient. Same to a lesser extent with hydraulics and engines did not have drive pads for large hydraulic pumps. From memory, motorised Gatling guns were tested on aircraft in ww1 and discarded as impractical at that time.

The hydraulics point of view is interesting as I believe the Fw-190 had electric gear and flaps.

 

[z42]

The hydraulics point of view is interesting as I believe the Fw-190 had electric gear and flaps.

Using hydraulics was AFAIU the 'standard' solution at the time. FW 190 and B-29 were pioneers in going more in the direction of electrically powered things. From https://www.aopa.org/news-and-media/all-news/2011/september/pilot/the-lady-has-a-history about the B-29:

Almost everything aboard the Superfortress was operated electrically, including the landing gear, flaps, propellers, and cowl flaps. Electrical systems were considered less vulnerable to enemy fire than hydraulic systems. The airplane had six engine-driven generators plus a small auxiliary power unit (APU), a 15-horsepower, two-cylinder engine (called the "putt-putt") that drove a seventh generator. Only the brakes were hydraulic. The APU provided electrical power for starting those huge engines. It also was used while taxiing to power the electric hydraulic pump for the brakes.

The trend towards more and more electrically powered auxiliary systems has continued post-war. The latest poster child in the airliner space being the B787, but also in military aircraft. See e.g. https://arxiv.org/pdf/2510.03887

 

[GrauGeist]

In addition, WWII fighters didn't have very powerful electrical systems because there weren't many systems that drew a lot of power. The spark plugs would have been drawing much of the power!

The vast majority of WWII aircraft had two electrical systems.

The ignition system was powered by way of a magneto and the radio equipment plus any form of radar (like IFF early on) was on a separate, generator powered system which included a battery.

The onboard electrical systems were typically high-amp, 24 VDC.

 

[don4331]

I'm going to start with the bonus question - why were motor driven cannons not explored during the war?

The reason engine* driven guns were explored in WWI was 2 fold:​

1. Machine guns were relatively new and not overly reliable - hence the gun within reach of the pilot to clear jams. By using the power of the engine, the mechanism of the gun could be guaranteed to cycle.​

2. Synchronization - As the propellers were fixed pitch, the engine speed varied greatly from climb to cruise to dive. Again, by driving the mechanism by the engine, you guarantee the gun only fires when the propeller is out of the way.​

​

Solution 1: Machine guns very quickly because more reliable (and the power of the engine often made the jam worse)​

Solution 2a: Largely the electrical engineers, either by solenoid or electric primer, resolved the synchronization problem.​

2b: Propellers changed to constant speed, greatly simpilying the problem - with the engine only turning a set speed designers have more leeway in firing.​

​

*Note 1: I changed the answer slightly from motor to engine.

​

As for sleeve valves, everyone so far has missed that they smooth out engine vibrations (Going into a little extra depth for your engineers)

On a single cylinder engine**, you want counter weight to balance the weight of the crank throw (where the rod is attached).​

But you also need balance weight of the big end of the rod, so you put the rod on a scale and weigh the big end and add that to the crank throw​

Then you do a bunch of math to determine how much of the weight of the small end of the rod/piston you want to balance.​

But you can't win - you will either have vertical (same direction piston is moving in) or horizontal imbalance. In most cases, you pick the lowest weight because extra material causes all sorts of knock on issues.​

​

With sleeve valves, you have the sleeve going up and down in the cylinder bore. By properly adjusting the ports in the sleeve, you can "time" the sleeve, so its motion is opposite the piston.​

​

The result is a very smooth running engine = quiet.​

​

**Note 2: Inline 6 cylinder engine (and V-12, arrow 18, X-24 are all multiples of Inline 6) is statically balanced without any counter weights!

Now, while it is statically balanced, there are torsional forces as the cylinders go through the firing cycles - so you will need to analyze where better to make a crank with larger bearings (to resist the torsion) or some counter weighs which allow smaller bearings.​

​

This is where engineer earn their pay as there is no "right" answer <but there are lots of "wrong" ones>.​

​

As for connecting rods.

When you use 2 large bolts versus 4 smaller bolts:​

a. You concentrate the load​

b. The larger bolt is more apt to have material imperfections, so you need to 'de-rate' the strength.​

Which is why you see the articulated rods which S Shortround6 has pictured.

But they have the issues with balance - the articulated rod doesn't follow the same centerline as the crankshaft. Again, the issue is minimized in a V-12, but its still there demanding a heavier crankshaft and better bearings.​

​

As for the blade and fork. As has been the theme, if you center the load on the bearing journal, you can minimize the stress on the crankshaft = lighter crank = lighter engine = lighter airplane.

There is also a secondary advantage - the blade rod holds the fork rod big end round. At mid stroke, there is significant sideway force on the rod, which results in the bearing becoming less than perfect circle - which can result in pinching out all lubrication... Which is why the forked rod in the RR V-12s is of 3 piece construction***. The significantly larger diameter of the blade rod doesn't have the same issue - but if RR made the crank with rod journals the size of the forked rod, it would have been very heavy.​

​

Note 3: Junkers did a more elegant solution of a simple cut in the rod to minimize the lateral force - which could be applied to both the blade and fork rods, allowing them to get away with just 2 piece rods (main rod and cap) The same Junkers solution is used to today's F1 engines (per Callum).

 

[BiffF15]

Using hydraulics was AFAIU the 'standard' solution at the time. FW 190 and B-29 were pioneers in going more in the direction of electrically powered things. From https://www.aopa.org/news-and-media/all-news/2011/september/pilot/the-lady-has-a-history about the B-29:



The trend towards more and more electrically powered auxiliary systems has continued post-war. The latest poster child in the airliner space being the B787, but also in military aircraft. See e.g. https://arxiv.org/pdf/2510.03887

z42,
I didn't realize the B-29 was so electric biased. I actually fly the 787 and will agree to its heavy electrical engineering to include its brakes (and its very reliable).
Cheers,
Biff

 

[MIflyer]

Somebody hung an electric motor, a couple of pullies and a v belt on a Gatling gun before 1900 (?).

It was the USN who evaluated it. Reportedly they rejected it on the basis that it fired too fast.

 

[ThomasP]

re the amount of electrical and/or electro-hydraulic power needed by power driven gatling type guns

re possible power sources for gatling.

During the 1960s and 1970s there were several gun pods developed that used independent power sources for the 20mm M61 Vulcan gun and linkless feed system.

One pod (the SUU-16/A) used a ram-air driven generator (located in the pod) to generate electricity for the gun and feed drives. ROF was the same as for the standard M61A1 gun at 4000 or 6000 rpm (or shorter bursts of 3, 15, 30, or 60 rounds). However, a minimum aircraft speed of 300 knots was necessary for the ram-air turbine generator to produce enough power for full ROF. Ammo load was 1200 rounds and AUW was 1650-1719 lbs.

Another pod (the SUU-23/A) used a modified gun (the GAU-4/A 6-barrel) capable of self powering. A relatively small electric motor is used to start the rotation and feed the first round(s). After the first round is fired the continued operation is via the propellant exhaust gases. The exhaust gases drive unit only adds 25 lbs to the basic 252 lb weight of the standard 6-barrel gun. ROF is same as the SUU-16/A. Ammo load was 1200 rounds and AUW was 1730 lbs.

And another pod (the GPU-2/A) used a rechargeable NiCad battery pack. This pod carried a modified gun (the M197) with only 3-barrels, along with 300 rounds. The batteries could fire 3 complete loads before needing recharge or replacement. ROF was either 700 or 1500 rpm. AUW was 586 lbs.

If we assume that one or more of the above systems could be integrated into a WWII airframe, the volume and weight of the SUU-16/A and SUU-23/A set-ups would seem to me to prevent the practical installation in anything smaller than a P-70, Mosquito (it fit the 57mm Molins gun so...), or similar size aircraft (ie a twin engine as tomo pauk suggested up-thread). Maybe the late-war F7F?

The weight of the GPU-2/A system with the M197 3-barrel gun - at 586 lbs - is actually lighter than the gun load of the 4x 20mm gun Spitfire VC (~880 lbs) or Hurricane IIC (~750 lbs), so weight is not the issue with a single-engine airframe. The problem is where to put the gun? You cannot put the gun in the wing, as both the volume required, weight, and off-center recoil forces would make that unworkable. Also, again as mentioned up-thread, the 20mm Vulcan gun is a large weapon (6' 2" long just for the gun and electro-hydraulic drive) so it would even be a problem to fit it in the nose of the P-39/P-63 (I think, although it might be possible to rearrange some weighty items like radios and armour? ) - maybe extend the nose a bit to allow the installation. Maybe in the nose of the P-38? Although that is a twin-engine airframe.

Dropping down to a .50 cal sized gatling gun (the GAU-19/A 3-barrel) opens up a considerable number of options for installation. The larger single-engine fighters (P-47, F6F, F4U, etc) of the later war period could probably fit one in each wing, and fitting one in the nose of the P-39/P-63 with a useful ammo load should not be a problem. The P-38 could probably fit at least 2 (with a useful ammo load) in its nose. (Note that the GAU-19/A .50 cal gun fits inside an Ø18" gun pod.)

Dropping down still further in the size range, the GAU-2 .30 cal minigun could be made to fit in the about same volume as a 20mm cannon of any type (or in the place of a .50 cal Browning with a bit of a wing bulge?). ROF is from 2000-6000 rpm depending on the drive motor and settings. Weight of the basic GAU-2 gun with a full 3000-round magazine is only about 250 lbs. AUW of a SUU-11B/A gun pod variant (which includes 1500 rounds and a NiCad battery for power) is 325 lbs. (Note that the GAU-2 .30 cal minigun fits inside the Ø12" SUU-11B/A pod.)

Another significant problem is ammo load vs ROF/firing time vs accuracy.
1. Even the comparatively large and heavy modern jet aircraft carry a limited amount of 20mm ammo - with the F-15 (940) carrying the most, followed by the F-14 (675), F-18 (520), F-16 (510) - among the US fighter airframes.
2. At max ROF that is only about 10 sec of fire for the F-15, 7 sec for the F-14, and a little over 5 sec for the F-16 and F-18.
3. When the Vulcan project was begun (in 1946?47?) it was realized that the very high rates of fire would only be acceptable if a commensurate increase in hit probability was achievable. I do not know if even the late-war GGS gunsights would allow effective use of such high ROFs. In the early-war with reflector gunsights . . .? Its not going to do any good firing a 1 second 100-round burst of 20mm/.50 cal/.30 cal if you cannot hit the target, although if you are thinking of shooting at large and relatively unmaneuverable heavy bombers?

Just some thoughts.

 

[MIflyer]

A guy I went to college with in the early 1970's had been a USAF enlisted man. He said his F-4E unit deployed to Vietnam and almost immediately after arriving were tasked with a ground support mission. He said that when they fired their M-61's you could count how many bursts they fired because the gun skipped firing one or two rounds between bursts. After that first combat mission they found there were NO unused rounds.

I recall reading that they tested a 5.56MM minigun that fired 11,000 rounds/min.

And on supersonic jet fighters getting rid of the empty shells was a concern. On the F-104 they ran the shells over inside the wings to keep from tossing them outside where they might damage something. The CF-104's were built without M-61's installed because their original role was low altitude recon and nuclear strike. When Canada went non-nuclear M-61's had to be installed in their 104's.

 

[z42]

And on supersonic jet fighters getting rid of the empty shells was a concern. On the F-104 they ran the shells over inside the wings to keep from tossing them outside where they might damage something.

Also, at high speed ejection ports cause a lot of drag. At some point most decided it wasn't worth it and cycled spent shell casings back into the magazine.

 

[z42]

The trend towards more and more electrically powered auxiliary systems has continued post-war. The latest poster child in the airliner space being the B787, but also in military aircraft. See e.g. https://arxiv.org/pdf/2510.03887

More interesting(?) reading on the topic: https://www.researchgate.net/public...dubG9hZCIsInByZXZpb3VzUGFnZSI6Il9kaXJlY3QifX0

 

[MiTasol]

The hydraulics point of view is interesting as I believe the Fw-190 had electric gear and flaps.

As Z z42 said most aircraft of the period had hydraulics for flaps and landing gears. On a significant percentage the hydraulic pump was manually operated. A few, like the Curtoss P-36/40 series, had a hydraulic system powered by an electric motor. Most with engine driven pumps had a hand pump for emergencies and a smaller number had a separate emergency hydraulic reservoir as well.

 

[Juha3]

In fact there were engine-driven machine guns in use during the WWII. Namely 7.92 mm and 12.7 mm Hungarian Gebauer GKM Machine Guns. GKM = Gebauer Kényszermeghajtású Motorgéppuska.

 

[Shortround6]

Hungarian Weapons Gebauer Machine Guns, Hungary 1918-45

Manowar's Hungarian Weapons Gebauer Machine Gun, Hungary 1918-45

www.hungariae.com

Some context is needed for the Schwarzlose machinegun. While fairly reliable it was one of the slowest firing machine guns of any army adopted anywhere. The .303 Browning gun could fire about 3 times faster than Schwarzlose so needing a connectection to the petrol engine and two barrels to fire only 33% faster in the early/mid 30s was not exactly putting itself in the for front of gun design.

"This gun is remarkable for being the only machine gun based on the retarded blowback system which met with any degree of success prior to 1950"

 

[PStickney]

Also, at high speed ejection ports cause a lot of drag. At some point most decided it wasn't worth it and cycled spent shell casings back into the magazine.

From my readings of weapons development at the time, and working on some 1950s jets, I think that it was less an issue with drag, and had more to do with, among other things, reducing the possibility of damage to the firing airplane due to getting hit by ejected cartridge cases and belt links (The Hunter, for example, dumped the cases, but was getting damaged by the links - which is why the "Sabrinas" - the streamlined fairings under the gun bay were scabbed on to store the links rather than dropping them out.
The M39 guns used on the F-100 and F-5 dump the cases
With Gatlings like the Vulcan and the GAU-8, mounted internally, with their linkless feeds feed the cases back into the drums - This keeps the CG shift from moving all that weight around to a minimum, and makes the problem of what to do with the rounds that feed, but don't get fired, when the gun spools down, or that misfire and get extracted as part of the normal operation of the gun. The gun pods save the links, but drop the cases.