witwas Posted December 5, 2017 Posted December 5, 2017 I watched some you tube documentaries on the harrier and noticed that the harrier can only hover for 90 seconds due to overheating engines (water runs out). Even with almost empty tanks and no payload. When i fly in dcs i can hover over the entire airfield without any issues. is this realistic?
robmlufc Posted December 5, 2017 Posted December 5, 2017 Certainly with the older less powerful engines, water was always required for hovering, even with pylons removed. I can't speak so much for the AV8B but on the Gr7/9 with the latest mk107 engine, the water was only really there as ballast and for use in hot high altitude conditions.
Rlaxoxo Posted December 5, 2017 Posted December 5, 2017 (edited) If you're really heavy with full fuel and some stores you will probably need to use the water to stay in hover. If you're empty with 80% ish fuel you can hover for a long periods of time in a hover. Keep in mind to keep the JPT (Jet Pipe Temp) below certain limits Water injection is used to cool the engine down allowing it to reach higher RPM with lower temperatures. Hence why you would need it if you were heavy You will damage the engine if you stay above ~700 degrees for longer then ~10-15 minutes I'm sure there's a thread here that specify the engine limitations so you can check that out https://forums.eagle.ru/showthread.php?t=196980&page=4 Edited December 5, 2017 by Rlaxoxo [sIGPIC][/sIGPIC] Youtube Reddit
WAR Posted December 5, 2017 Posted December 5, 2017 In addition to the replies above, all versions of the Harrier, old and modern were able to hover dry depending on all up weight, outside air temperature, atmospheric pressure and the pilot's ability/technique. The 90 sec hover figure which is sometimes mentioned is just the maximum time you have water flowing. A warm low pressure day, heavy with stores would require the water to hover, but remember that whilst you are using the water you are also reducing your all up weight by approx 500lbs of water in 90secs and approx 400lbs in fuel. If you were dependant on the water then you would check the limits during the decel and invoke the water so that you knew it would flow, if not then you still had wingborne lift to accelerate away before you ended up stuck in the jetborne regime in a continuous descent. Of course you could and should trip the engine limiters in this case to remove the JPT limit, high counts on the engine but better than crashing. Harriers have been lost due to pilots forgetting this switch! Even if water was not required to hover, it was best to use it otherwise it was just an additional weight which wasn't needed. Generally if you were coming back for a single VL landing then 90secs was sufficient, if you wanted to practice your VTOLs etc then you'd use the water up and then practice various short take-offs and landings until able to dry hover.
Flamin_Squirrel Posted December 5, 2017 Posted December 5, 2017 I'm guessing water flow rate is constant? If it was variable depending on JPT that might explain, but I've not read anything to indicate that it is.
robmlufc Posted December 5, 2017 Posted December 5, 2017 Yeah water flow is constant at about 225psi pressure. WAR, are you ex JFH?
WAR Posted December 5, 2017 Posted December 5, 2017 Yes and a little bit before! But it's been a long time now and the memory isn't what it was :ermm:
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