I don't think that anyone went ad hominem against you. Trouble might be that you've provided next to no evidence to prove numerous points you've made, despite repeatedly being challenged to do so.
Why so few planes that fired thorugh the propeller hub?
- Thread starter Rufus123
- Start date
I don't think that anyone went ad hominem against you. Trouble might be that you've provided next to no evidence to prove numerous points you've made, despite repeatedly being challenged to do so.
Shortround6
Lieutenant General
Detroit diesel
It has four exhaust valves in the head. Now you need to get the fresh air in. The Detroit diesel used ports in the bottom of the cylinder that are uncovered and covered by the movement of the piston. The Fresh air is forced in by a supercharger.
If you want the piston to push the exhaust out and to the suck the fresh air in you need some sort of port/passage for the fresh air to come in and some sort of valve or seal to keep the combustion pressure from blowing back out through the intake. A sleeve, rotating valve, reed valve, something that opens when the piston is going down on the intake stroke and ONLY opens when the piston is going down on the intake stroke. Then it has to close and be strong enough to contain the combustion pressure, which on a diesel can be over 1500 psi for a brief period of time. With out some sort of device (sleeve, rotating valve, reed valve) to seal of the ports/perforations/slots the intake air came in through the combustion pressure is going take the easy way out and go out through the intake rather than push the piston down.
It has four exhaust valves in the head. Now you need to get the fresh air in. The Detroit diesel used ports in the bottom of the cylinder that are uncovered and covered by the movement of the piston. The Fresh air is forced in by a supercharger.
If you want the piston to push the exhaust out and to the suck the fresh air in you need some sort of port/passage for the fresh air to come in and some sort of valve or seal to keep the combustion pressure from blowing back out through the intake. A sleeve, rotating valve, reed valve, something that opens when the piston is going down on the intake stroke and ONLY opens when the piston is going down on the intake stroke. Then it has to close and be strong enough to contain the combustion pressure, which on a diesel can be over 1500 psi for a brief period of time. With out some sort of device (sleeve, rotating valve, reed valve) to seal of the ports/perforations/slots the intake air came in through the combustion pressure is going take the easy way out and go out through the intake rather than push the piston down.
And where is YOUR evidence? You are obviously not interested in whatever I might have to say, so any further involvement on my part is fruitless. I am sorry I wrote to such eminent authorities in my attempt to contribute to a dialogue. I will not try it again
Shortround6
Lieutenant General
Well, when you present a timeline that has an engine (engine A) developed from an engine (Engine B) that didn't exist and would not exist for several years after Engine A was introduced there are going to be some sceptics.
Perhaps we are have language barrier or perhaps you a working from a less than ideal translation of a foreign book/article. Some of your descriptions of how engines work are contrary to common knowledge. For example.
"And BTW, when the exhaust valves open the pressure differential between the burnt fuel air mixture and outside air also create what you would call a vacuum as the piston moves upward in the exhaust stroke. In all kinds of internal combustion engines."
We have high pressure in the cylinder when the piston reaches the bottom of the power stroke, at least higher pressure than the outside air. It is much lower pressure than when the fuel is injected and the combustion is taking place but it is still higher than the outside air. The rising piston does NOT create a vacuum. it creates even more pressure to help force the burned gases out of the cylinder. Vacuum doesn't start to occur until the piston is moving down on the intake stroke. This is true for both Diesels and gasoline engines.
This is the conventional accepted knowledge which can be found with practically any google search on a variety of web sites.
If someone wants to claim they are all wrong then I believe it is up to the challenger to provide some proof or sources, not up to the people who are on the side of the common.old and easily referenced knowledge.
In post #152 here.
Indeed, I'm not interested in whatever one might have to say. What I'm interested is that people can support their opinions with sources and facts. So far we you have not provided any source or a fact that will support your statements that:
- V-2 was a version of AM-38
- that either AM-34 or AM-38 were diesel engines
- that all 4 valves of the Mikulin engines and of the V-2 were exhaust-only.
Have a good day.
swampyankee
Chief Master Sergeant
- 4,306
- Jun 25, 2013
This has gotten rather far afield. To go back to the OP's question, I think we need to see some data (not anecdotes or little quips from fighter pilots) about the actual value of through-hub vs synchronized vs wing mounted weapons. I suspect that it doesn't exist, and almost certainly doesn't exist in any meaningful form.
The questions these data should answer are:
Why do I think the data don't exist? Simple: the air forces that used through-hub weapons had already determined they were superior and were not going to revisit that assumption., and those that did not had already determined that the through-hub weapons were not superior or at least not enough superior to wing-mounted guns to be worth the design compromises in engine, propeller, gearbox, and cowling to be worth the bother.
The questions these data should answer are:
- How much did through-hub weapons compromise engine, propeller, gearbox, and cowling design? An extreme case is the P-39. How much loss of engine and aircraft performance resulted from, for example, the location of the supercharger and inlet and exhaust plumbing for the engine? How did they effect c/g travel as ammunition was expended? Did they impair forward visibility by pushing cockpit location back or increasing cowling bulk?
- Were through-hub weapons significantly more accurate at normal combat ranges? If so, how do you quantify that? Would it take 20 rounds fired for a through-hub cannon to destroy an aircraft vs 40 with wing mounted guns? Could that difference be reduced or eliminated by training and sight design?
Why do I think the data don't exist? Simple: the air forces that used through-hub weapons had already determined they were superior and were not going to revisit that assumption., and those that did not had already determined that the through-hub weapons were not superior or at least not enough superior to wing-mounted guns to be worth the design compromises in engine, propeller, gearbox, and cowling to be worth the bother.
I'm not sure that there were any 'bad' compromises with regard to the engine, prop, or gearbox. Cowling and exhaust plumbing were same as with 'classic' V12 engines, bar the P-39 as specific case.
Appeal of the hub weapon was ability of, otherwise weak aircraft with weak engine, to have a shell-firing gun installed and usable. You can have one such gun, and go out and kill other aircraft. Trying to install such two guns (they being much heavier and bulkier than LMGs most people used from ww1 on) in the wings might involve unacceptable loss of performance and maneuverability. So our Spad XIII will be able to have one 37mm cannon installed in the Vee, but not two such cannons in/on the wings. This is before we can attest that Spad's wings can actually carry such two cannons in the 1st place. Engine warmth keeps the weapon warm by default, mount is as rigid as it will ever be, ammo feed is usually reliable.
Once we move in 1930s, the equation still holds - one 700-800 HP engine can propel the fighter with single big cannon (talk Hispano or the big Oerlikon) and it's amo well enough, but less well when it has two big cannons, no need to provide heating either. Granted, we can install two smaller and lighter cannons (tipically a version of the Oerlikon FFF), and accept trade-off between shell MV and total RoF. Main purpose of many air forces' fighters being killing the bombers that are about to bring doom via bombing and gas.
Once at 1100-1300 HP, the big 30-40mm cannon is a possibility for the fighter that has a suitable engine in the 1st place. Some cannons being better than the others, some engines being better then the others.
Once engine power is high, talk 1500-2000 HP and fighters can carry up to 4 big cannons, the appeal of hub weapon seems moot - the excess power should be able to cater for increase of weapon weight. Hub weapon still has appeal if your fighters are expected to kill enemy bombers and/or tanks on daily basis, so we have Ta-152 series and latest Bf 109 with the modified MK 103 firing through the prop as option. MK 103 being less than ideal as wing weapon, and probably impossible for the 109 to carry at all.
Granted, Germans and Soviets were thinkering about even bigger cannons, 45 to 55 mm, as hub weapons.
Sorry if this lengthy piece does not answer your questions
Shortround6
Lieutenant General
I agree somewhat with that but the way that some air forces "determined" that through the hub was better seems to be pretty sketchy. The French seem to have started in WW I but I am not sure any tests were done, aside from seeing if it work at all.
The attraction of through the hub was than a heavy gun could be carried with the engines of the time (late teens/20s/early 30s) that were not powerful enough to provide good performance with more than one heavy gun. Wing guns were a rarity until the 1930s for much the same reason. More than 2 machine guns (or up to four) was too much of a performance penalty for the engines and propellers of the time.
An exception that proves the rule was the Sopwith Snark with two Vickers guns in the fuselage and four Lewis guns under the lower wing.
It was a triplane powered by the infamous ABC Dragonfly engine of 360hp (rather good for 1918-19 but the engine could have lost the air war for the allies in 1919 it was so bad).
It was about the only fighter to use wing mounted guns (aside from over the pilot ) until the 1930s. However the guns (with their 97 round drums) could not be reloaded nor could the pilot even beat on the gun with a gloved hand to try and clear a jam. Wing mounted guns would require another generation of guns of higher reliability.
Hispano got on the through the Hub band wagon and designed their 1920s engines with pretty much a clear area between the cylinder banks even though Hispane had no gun to put there at the time. This carried on through the 1930s and the licence built Russian M-100 through M-105 series.
The 1-0 was designed for either two cowl mounted machine guns OR one hub mounted cannon. The hub mount does, as you know, do away with the need to synchronize the gun.
The First German cannon was a real beast firing the same cartridge as the Flak 30 AA gun and weighed 64kg (as much as five 7.9mm MG 17s) so carrying multiples of this gun was pretty much out of the question with a 600-700hp engine. The 20mm MG FFs in the wings of a 109 were under 30kg each.
So the Hub mounted advocates had picked their path and started designing guns and engines to suit without any real combat experience (2-5 aircraft shot down in WW I by hub mounted cannon and most/all? by one pilot?) or even testing. The wing mounted machine gun/cannon crowd also fell into position almost by accident. You can't use a hub mounted gin on a radial, the Merlin wasn't designed for hub mounting (neither is the Allison but it is late to the game). and most of the 20mm and larger in 30s could not be synchronized so fitting such a gun inside the prop disc area to simulate a through the hub gun (at a lower rate of fire was out.
The US and Japanese didn't fit wing guns to much of anything until 1939 or after. Italians didn't fit wing guns to fighters very often until after the war started. They did use wing guns on a few attack planes.
The French, Germans and Russians were pretty much the proponents of the hub mounted gun.
swampyankee
Chief Master Sergeant
- 4,306
- Jun 25, 2013
I think the way they determined hub guns were better was more "gee, if we do this, we don't need to synchronize" than the result of actual testing of wing vs through-hub guns.
Shortround6
Lieutenant General
A lot of the automatic cannon of the 20s and 30s could NOT be synchronized which rather ruled out any sort of single fuselage mount that fired through the propeller disc.
You either figured out a way to shoot through the prop hub or resorted to something like this.
Great if the Germans kept building Zeppelins, otherwise............................
The Germans had this in WW I but were trying to figure out how to use it in a single seat, single engine fighter.
I know it says tank but it was used as a flexible gun on some Zeppelins and two/multi seat aircraft. It was tested in an Albatross firing down at an angle to clear the propeller.
However it weighed 30Kg and the magazines were only 10 and 15 rounds. Loaded 15 round one went 5kg (?) rate of fire was about 300rpm
MV was only 490M/S so as a whole it doesn't show up that well against a multiple machine gun battery.
Some people could see the potential but is would take a lot development to turn it into the Oerlikon gun/s we are familiar with.
You either figured out a way to shoot through the prop hub or resorted to something like this.
Great if the Germans kept building Zeppelins, otherwise............................
The Germans had this in WW I but were trying to figure out how to use it in a single seat, single engine fighter.
I know it says tank but it was used as a flexible gun on some Zeppelins and two/multi seat aircraft. It was tested in an Albatross firing down at an angle to clear the propeller.
However it weighed 30Kg and the magazines were only 10 and 15 rounds. Loaded 15 round one went 5kg (?) rate of fire was about 300rpm
MV was only 490M/S so as a whole it doesn't show up that well against a multiple machine gun battery.
Some people could see the potential but is would take a lot development to turn it into the Oerlikon gun/s we are familiar with.
Deleted member 68059
Staff Sergeant
- 1,056
- Dec 28, 2015
The main disadvantage of putting the blower on the side to allow for the prop-cannon, was not envisaged by Germany when the RLM drew up the guidelines in
something like 1930 (sadly I have never found the original document, only refereces to it). This was that if one wishes later to have a two-stage
supercharger, the packaging suddenly gets a lot more unpleasant than something like a Merlin-60. This is because you either double-stack
the blowers which is VERY difficult to then fit into something like a 109 without it poking right out into the airstream, OR you fit one
blower on each side of the engine, which then leaves you with a real birdsnest of ducts to put in. The idea of a 2-stage supercharger being
a likely service requirement in 1930 was probably not on the minds of most people - although it was not far fetched either.
Later, Germany certainly placed a great deal of store in considering gun "weave" in firing, and at least during the war produced a very lengthy report on
improving aerodynamic stability of fighters purely for the purpose of keeping the guns on target. I have this report in English as it was translated by
the R.A.E. in 1946. Its mostly full of maths, but has some practical conclusions scattered about. Directional stability of the 109 seems to have been
a bit less than pilots found comfortable so it says.
Very impressive work you are doing on your book.
ThomasP
Chief Master Sergeant
Hey guys,
For anyone who is interested, modelwiz is correct in his statement:
"And BTW, when the exhaust valves open the pressure differential between the burnt fuel air mixture and outside air also create what you would call a vacuum as the piston moves upward in the exhaust stroke. In all kinds of internal combustion engines."
modelwiz used the words "as the piston moves upward in the exhaust stroke" instead of 'during the exhaust stroke' which may have contributed to a misunderstanding, but what he meant is correct.
In the Merlin engine at +18 lbs of boost the piston rising only pushes about 5% of the combustion gases out of the cylinder, the exhausting high pressure/high speed gases draw the other ~95% out of the cylinder. In a naturally aspirated diesel with a 22:1 compression ratio the piston would only push about 3% of the combustion gases out, with the other 97% being drawn out by the exhaust gases.
The reason one can conceptually know this applies to an internal combustion engine is due to the piston velocity vs the exhaust gas velocity. The piston velocity in the Merlin is only 50 ft/sec at 3000 rpm. If the exhaust gases were being pushed out by the piston the maximum velocity of the gases would be about 570 ft/sec and there would be a loud harsh hiss (for lack of a better descriptive) instead of the bangs that we hear. The reason we hear the bangs (of course) is that the gases are expanding/traveling at more than the speed of sound (typically around 1700-2000 ft/sec). The pulses of high velocity expanding/traveling gas in effect act as short-lived virtual pistons being thrown away from the exhaust ports, creating a low pressure area in their wake while traveling down the exhaust manifold (and to a lesser degree after they leave the manifold), drawing the gases out of the cylinder. In the Merlin, by the time the piston has traveled ~1/2 of the way up on the exhaust stroke over 90% of the exhaust gases have already left the cylinder.
This is all due to the same principle under which a wing generates lift, i.e. the Bernoulli effect.
The above principle is the majority of the reason for putting 2 exhaust ports in a cylinder, the minority of the reason being a more uniform effect within the cylinder. If it were practical to increase the single exhaust port to the same area as the cylinder area (say by using the entire top of the cylinder as an exhaust port) it would be more efficient to do so.
For anyone who is interested, modelwiz is correct in his statement:
"And BTW, when the exhaust valves open the pressure differential between the burnt fuel air mixture and outside air also create what you would call a vacuum as the piston moves upward in the exhaust stroke. In all kinds of internal combustion engines."
modelwiz used the words "as the piston moves upward in the exhaust stroke" instead of 'during the exhaust stroke' which may have contributed to a misunderstanding, but what he meant is correct.
In the Merlin engine at +18 lbs of boost the piston rising only pushes about 5% of the combustion gases out of the cylinder, the exhausting high pressure/high speed gases draw the other ~95% out of the cylinder. In a naturally aspirated diesel with a 22:1 compression ratio the piston would only push about 3% of the combustion gases out, with the other 97% being drawn out by the exhaust gases.
The reason one can conceptually know this applies to an internal combustion engine is due to the piston velocity vs the exhaust gas velocity. The piston velocity in the Merlin is only 50 ft/sec at 3000 rpm. If the exhaust gases were being pushed out by the piston the maximum velocity of the gases would be about 570 ft/sec and there would be a loud harsh hiss (for lack of a better descriptive) instead of the bangs that we hear. The reason we hear the bangs (of course) is that the gases are expanding/traveling at more than the speed of sound (typically around 1700-2000 ft/sec). The pulses of high velocity expanding/traveling gas in effect act as short-lived virtual pistons being thrown away from the exhaust ports, creating a low pressure area in their wake while traveling down the exhaust manifold (and to a lesser degree after they leave the manifold), drawing the gases out of the cylinder. In the Merlin, by the time the piston has traveled ~1/2 of the way up on the exhaust stroke over 90% of the exhaust gases have already left the cylinder.
This is all due to the same principle under which a wing generates lift, i.e. the Bernoulli effect.
The above principle is the majority of the reason for putting 2 exhaust ports in a cylinder, the minority of the reason being a more uniform effect within the cylinder. If it were practical to increase the single exhaust port to the same area as the cylinder area (say by using the entire top of the cylinder as an exhaust port) it would be more efficient to do so.
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Jumo 213E had three-speed, two-stage supercharger, poking nastily to the right hand side airstream.
I think I have read pretty much all the German and Finnish 109 pilots bios there are. There was much to complain about the Bf109. But directional stability was not among them.
Possibly they did not read the RAE math.
Deleted member 68059
Staff Sergeant
- 1,056
- Dec 28, 2015
213E was never fitted to a 109 - but you know this so I dont really understand what your comment is trying to imply.
The RAE report was TRANSLATED from German, the original report and its conclusions about directional stability of the 109 were all written by Eisenmann,
after many test flights at Rechlin by their test pilots.
"Flight tests were made on Me108, Me109F, Me109G, Me210, Me410, Me262. The aim of these tests was to find a means of improving the objectionable
yawing oscillation behaviour of the aircraft as quickly and as simply as possible without very large alterations to the type in production.
Systematic scientific measurements were largely abandoned and the observations were confined to the influence of the particular remedies proposed...."
Shortround6
Lieutenant General
He is incorrect when he states " also create what you would call a vacuum as the piston moves upward"
We already know there is a pressure differential between the gases in the cylinder and the outside air. Now if the out rushing gases did create a vacuum as they left the cylinder they would actually be sucking the piston up (slightly) due the higher pressure in the crankcase.
You don't get a 'vacuum' until the piston starts down on the intake stroke.
A better diagram
Bottom is the ideal Otto cycle which never happens in a real engine.
Or laid out in a line
Now we obviously have some very large pressure variations occurring in a very short space of time but getting lower than atmospheric pressure in a cylinder without the piston going down is going to be a real trick.
We can twist and pull these diagrams to cover different engines but there are reasons for valve overlap (both intake and exhaust valves open at the same time) in the real world.
One is that valves do not open and close instantly (on most engines, lets leave modern formula I engines and their kin out of this) so the valves have open early in order to be fully open at the right time. Another reason is better scavenging. The incoming "fresh" air helps displace the remaining exhaust gases allowing for more fuel/air mixture (or more fresh air in the case of the diesel/direct injection engine).
BTW - will the DB 601C/D get any coverage in your book?
ThomasP
Chief Master Sergeant
Hey Milosh and Shortround6,
We would have to ask modelwiz to be sure why he used the term vacuum, but most likely he was trying to express the 'vacuuming' (verb in the english language) effect of having a lower pressure area next to a higher pressure area, similar in effect to using a vacuum cleaner. The term 'drawing vacuum' is commonly used in some (many?) countries to describe this effect, usually specifically at the air intake to a naturally aspirated engine describing a lowering of the MAP, but sometimes elsewhere in the system also. If you look in an English language automotive maintenance book you will see the terms like "vacuum system" and "vacuum leak" used to describe the various effects in the engine, but usually not relative to the exhaust. There is no total lack of air pressure in the engine that the physics term vacuum implies.
from Shorround6's post#178,
"Now we obviously have some very large pressure variations occurring in a very short space of time but getting lower than atmospheric pressure in a cylinder without the piston going down is going to be a real trick."
Keep in mind that the effect modelwiz is referring to occurs due to the pressure differences relative to the area 'behind' the exhaust gases and the remaining combustion gas pressure in the cylinder, not relative to the incoming air pressure or ambient air pressure.
Using the idea of air foil lift created by the Bernoulli effect, if you increase the ambient air pressure to the same internal cylinder pressure as in the Merlin, the local pressure on top of the wing would still be lower than the pressure on the bottom of the wing, causing lift. In the same way, the local pressure behind the exiting exhaust gases is lower than pressure remaining in the cylinder, causing 'suction'.
If we use the bottom graph you posted above (very nice by the way) you can see that from the time the exhaust valve opens to the time the piston has traveled ~1/2 the distance on the upstroke (i.e. at about 270º) the pressure due to the remaining combustion gases in the cylinder has dropped from a value of ~100 to a value of ~20 for a drop of about 80% - not the 90% value I quoted but that is likely due to the different parameters used in the engine model of the graph. If the boost pressure/peak pressure is lower in the graph's engine model, the difference between the pressure at the point of exhaust valve opening and the pressure at the ~1/2 upstroke position would be lower (i.e. 80% rather than 90%), plus different valve timing could have some effect also.
Another example of the effect we are talking about here is a bore evacuator (aka fume extractor) - the somewhat odd looking cylinder usually located at about the midpoint on the barrel of modern tank guns. The same process modelwiz referred to occurs in the bore of the gun, but without any incoming fresh air. (File:Rauchabsauger.gif - Wikimedia Commons)
We would have to ask modelwiz to be sure why he used the term vacuum, but most likely he was trying to express the 'vacuuming' (verb in the english language) effect of having a lower pressure area next to a higher pressure area, similar in effect to using a vacuum cleaner. The term 'drawing vacuum' is commonly used in some (many?) countries to describe this effect, usually specifically at the air intake to a naturally aspirated engine describing a lowering of the MAP, but sometimes elsewhere in the system also. If you look in an English language automotive maintenance book you will see the terms like "vacuum system" and "vacuum leak" used to describe the various effects in the engine, but usually not relative to the exhaust. There is no total lack of air pressure in the engine that the physics term vacuum implies.
from Shorround6's post#178,
"Now we obviously have some very large pressure variations occurring in a very short space of time but getting lower than atmospheric pressure in a cylinder without the piston going down is going to be a real trick."
Keep in mind that the effect modelwiz is referring to occurs due to the pressure differences relative to the area 'behind' the exhaust gases and the remaining combustion gas pressure in the cylinder, not relative to the incoming air pressure or ambient air pressure.
Using the idea of air foil lift created by the Bernoulli effect, if you increase the ambient air pressure to the same internal cylinder pressure as in the Merlin, the local pressure on top of the wing would still be lower than the pressure on the bottom of the wing, causing lift. In the same way, the local pressure behind the exiting exhaust gases is lower than pressure remaining in the cylinder, causing 'suction'.
If we use the bottom graph you posted above (very nice by the way
) you can see that from the time the exhaust valve opens to the time the piston has traveled ~1/2 the distance on the upstroke (i.e. at about 270º) the pressure due to the remaining combustion gases in the cylinder has dropped from a value of ~100 to a value of ~20 for a drop of about 80% - not the 90% value I quoted but that is likely due to the different parameters used in the engine model of the graph. If the boost pressure/peak pressure is lower in the graph's engine model, the difference between the pressure at the point of exhaust valve opening and the pressure at the ~1/2 upstroke position would be lower (i.e. 80% rather than 90%), plus different valve timing could have some effect also.Another example of the effect we are talking about here is a bore evacuator (aka fume extractor) - the somewhat odd looking cylinder usually located at about the midpoint on the barrel of modern tank guns. The same process modelwiz referred to occurs in the bore of the gun, but without any incoming fresh air. (File:Rauchabsauger.gif - Wikimedia Commons)
