Blackhawk_FR Posted March 27, 2018 Posted March 27, 2018 (edited) Hello, Well, almost all is in the title. In the game, the Yak can fly inverted for 10 seconds, with the MP105 and its carburetor system. The 109, with injection system, can fly inverted for 10sec, also. What I know: - Aerobatics aircraft equipped with injection engine can fly inverted for few minutes. I don't know about the 109 with DB600 and 605, but I bet it should be more than 10 seconds. - I know someone who fly a Yak9 in Switzerland (https://www.46aviation.com/warbirds/). He clearly said the Yak's engine can't support negative G for more than very few seconds. Is there any reasons of why the Yak's engine (and others equipped with carburetors, except I16 which only hold inverted flight for ~3 seconds) keeps working for 10 seconds under negative Gs? Thank you ! Edited March 27, 2018 by F/JG300_Faucon
JtD Posted March 27, 2018 Posted March 27, 2018 Because only some float type carburettors immediately flood and cut out the engine under negative g. Other float type carburettors are more resistant, and injections carburettors have no real problem with negative g. Injection carburettors were fitted to most carburettor equipped fighter aircraft in WW2. The reason both the Yak and the Bf109 cut out after about the same 10 seconds is probably that in both cases the fuel pump fed from the fuel tanks sucks in air, with the fuel being on the wrong side of the tank. (The same thing will also cause oil pumps to suck air, leading to lubrication problems quickly, which can easily kill an engine. You shouldn't fly inverted for long in these birds.) 2
Blackhawk_FR Posted March 27, 2018 Author Posted March 27, 2018 But that still doesn't explain why a real Yak9 can't hold negative G more than few seconds? 1
ZachariasX Posted March 27, 2018 Posted March 27, 2018 2 minutes ago, F/JG300_Faucon said: But that still doesn't explain why a real Yak9 can't hold negative G more than few seconds? As JtD sayd, none of these WW2 aircraft is made to be constantly operated under negative g's. They are nothing like an Extra 300. Both tank and oil system are fed at the bottom of the tank. They hold a reservoir for momentary negative feed under negative g. So you are very lucky if the engine runs out of fuel before it runs out of oil while flying upside down. It is a lot of fuel flowing at higher power ratings. If you want longer engine time than a couple of seconds, then you have to make that little tank larger, using up space you might not have. Plus, there is no such tank for the oil system. The longer the engine runs inverted, the higher the chance that it is not sufficiantly lubricated anymore and *bang* there go your pistons. 1
JtD Posted March 27, 2018 Posted March 27, 2018 (edited) 17 minutes ago, F/JG300_Faucon said: But that still doesn't explain why a real Yak9 can't hold negative G more than few seconds? That Yak-9 is powered by an Allison engine and will therefore have a fuel system different from the standard Yak or an Allison powered aircraft. You would have to ask the owner about the specifics of that aircraft/fuel system/carburettor. A 1475hp Allison should have an injection type carburettor, so it's probably not the carb that gives up. Edited March 27, 2018 by JtD 1
unreasonable Posted March 27, 2018 Posted March 27, 2018 109 crashed a few years ago after doing 10 seconds of inverted flight. See accident report. 76d4dfdb8440aa5b39111a089cbcc567_HCLJ510-2013-242.pdf 1 3
Inkophile Posted March 29, 2018 Posted March 29, 2018 On 2018-03-27 at 5:57 PM, JtD said: The reason both the Yak and the Bf109 cut out after about the same 10 seconds is probably that in both cases the fuel pump fed from the fuel tanks sucks in air, with the fuel being on the wrong side of the tank. This. Most WW2 fighters have a 10-15 second buffer tank (shorter and longer durations exist) for inverted flight at combat powers, but that's it.
303_Kwiatek Posted March 29, 2018 Posted March 29, 2018 For prolonged inverted flight there is need fuel delivery but also oil pressure special system. Some planes got fuel injection and have no problem with fuel delivery but all these planes didnt have special oil system to keep oil pressure under zero or negative G force. Thats why it was forbiden prolonged inverted flight.
Inkophile Posted March 29, 2018 Posted March 29, 2018 (edited) 3 hours ago, 303_Kwiatek said: For prolonged inverted flight there is need fuel delivery but also oil pressure special system. Some planes got fuel injection and have no problem with fuel delivery but all these planes didnt have special oil system to keep oil pressure under zero or negative G force. Thats why it was forbiden prolonged inverted flight. Having fuel injection or not doesn't matter at all for inverted flight. There are plenty of carburettor-fed engines that can work just fine when inverted. As has been mentioned in several posts above it is shuffling fuel from the fuel tanks that's the issue; not to portion it appropriately into each combustion chamber. No fuel injection system in the world can keep the engine from starving if there's no fuel to inject to the engine, and WW2 airplanes didn't have a fuel system could feed from the tanks when inverted. That's why there is the (also mentioned in several posts above) buffer/header fuel tank of just a few litres, which is situated between the fuel tanks and the engine, from which fuel can be pumped when being inverted, because it doesn't just have a simple bottom-suction pump. I can promise that the engine will seize from fuel starvation WAAAAAY before you suffer any lubrication-issues. Edited March 29, 2018 by Inkoslav
19//Moach Posted April 2, 2018 Posted April 2, 2018 (edited) the difference is: here's a float carburetor, the kind that cuts off when you nose over: now, here's a pressure carburetor which allows inverted flight (for longer, mind you, not indefinitely) as you can see - there's one major difference between the two - that being, the float carburetor relies on gravity to keep fuel level constant. If you start messing with gravity, such as by changing the way it makes things go, then it won't work properly and you'll earn yourself some free glider lessons. Mind that no engine that I'm aware of is actually able to sustain unlimited time in inverted flight - Even the fuel injected DB-605's have a 10 second restriction due to the oil scavenger pump which starts sucking in air if the oil doesn't trickle down into the sump. Some engines are simply capable of holding that kinda attitude longer than others this is why oil flow becomes the limiting factor for negative G, notice the position of the oil sump pick-up Edited April 2, 2018 by 19//Moach 1
Asgar Posted April 3, 2018 Posted April 3, 2018 (edited) On 27.3.2018 at 5:15 PM, F/JG300_Faucon said: Hello, Well, almost all is in the title. In the game, the Yak can fly inverted for 10 seconds, with the MP105 and its carburetor system. The 109, with injection system, can fly inverted for 10sec, also. What I know: - Aerobatics aircraft equipped with injection engine can fly inverted for few minutes. I don't know about the 109 with DB600 and 605, but I bet it should be more than 10 seconds. - I know someone who fly a Yak9 in Switzerland (https://www.46aviation.com/warbirds/). He clearly said the Yak's engine can't support negative G for more than very few seconds. Is there any reasons of why the Yak's engine (and others equipped with carburetors, except I16 which only hold inverted flight for ~3 seconds) keeps working for 10 seconds under negative Gs? Thank you ! You are using a very simplified view on things, the carburetor is not the only thing affecting how long you can fly inverted, modern stunt planes as in your example use modern fuel pumps, if you invert a 109 or pretty much any other WW2 aircraft for that matter (as far as i'm aware) the fuel pump will not be able to deliver fuel to the engine Edited April 3, 2018 by 6./ZG26_Asgar
LLv34_Temuri Posted April 11, 2018 Posted April 11, 2018 On 3/27/2018 at 7:37 PM, unreasonable said: 109 crashed a few years ago after doing 10 seconds of inverted flight. See accident report. 76d4dfdb8440aa5b39111a089cbcc567_HCLJ510-2013-242.pdf Was this the incident where they were not using an originally desgined fuel tank (which can handle inverted flight), but some more modern "better" one, that caused the problem?
unreasonable Posted April 11, 2018 Posted April 11, 2018 The report says that the fuel tank configuration was not as per the manual, so that may have been relevant, you would have to have a detailed knowledge of the systems to tell. But it also notes that the G-6 engine manual states that the engine is not suitable for inverted flight. Again, whether this document is of the WW2 era I cannot say. What is clear is that fuel injection by itself is not enough to allow prolonged inverted flight - the whole fuel (and oil) delivery system has to be designed to cope with it, and AFAIK WW2 aircraft did not have this. (Why would they?) I think the specific origin of this meme is the problems the early Merlins had with cutting out even after momentary negative Gs in a bunt - which Miss Shilling's Orifice ameliorated in 1941 and was later fixed by changes in the carburettor design. Prolonged inverted flight is a different animal.
ZachariasX Posted April 11, 2018 Posted April 11, 2018 It seems most people are not aware that the injector pump (in an "injector engine") does only that, inject fuel into the cylinder. But what it doesn't do is fetching the fuel from the tank. This is why for inctance even GA prop planes of today featuring injector engines (such as a Cessna 182) have a dedicated fuel pump as well. That one is used for instance for take-off- But in flight, gravity flow is sufficient to provide enough fuel for the injector system and you turn that pump off. Then it is also a design choice. You drain your tank at the bottom, so you can literally use up the last drop. But if you want to fly inverted, then you have to splice in a tank that is always kept full and can emptied in inverted flight. The size of that tank decides your time of inverted flight. You are basically using space you don't have for unneeded functionality for a WW2 fighter aircraft. If you made an arrangement as seen in R/C planes, where you have a flexible tube with a weight on the end entering the tank at the middle position (and keeping tank center line at the level of the carb) then you don't need the second tank. but you will hardly be able to empty your tank completely. Depending on the aircrafts attitude, the engine might quit while there is still something in the tank. Thus it is also a wasteful arrangement.
PainGod85 Posted May 13, 2018 Posted May 13, 2018 On 27.3.2018 at 5:15 PM, F/JG300_Faucon said: Hello, Well, almost all is in the title. In the game, the Yak can fly inverted for 10 seconds, with the MP105 and its carburetor system. The 109, with injection system, can fly inverted for 10sec, also. What I know: - Aerobatics aircraft equipped with injection engine can fly inverted for few minutes. I don't know about the 109 with DB600 and 605, but I bet it should be more than 10 seconds. - I know someone who fly a Yak9 in Switzerland (https://www.46aviation.com/warbirds/). He clearly said the Yak's engine can't support negative G for more than very few seconds. Is there any reasons of why the Yak's engine (and others equipped with carburetors, except I16 which only hold inverted flight for ~3 seconds) keeps working for 10 seconds under negative Gs? Thank you ! The 109 cannot fly at negative G loads for more than a few seconds at at time, see the linked crash report from 2013: http://havarikommissionen.dk/index.php?option=com_contentbuilder&view=list&Itemid=161&lang=da&contentbuilder_download_file=c43f2f96ca5292e717bcfe1f71c67b2ddaef0539
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