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TO EPR behaviour (Read 3355 times)

windplayer Senior Member Offline Posts: 568 Location: Izhevsk, Udmurtia, Russia Joined: Aug 5^th, 2011 Gender:	TO EPR behaviour Apr 16^th, 2012 at 3:12pm	Print Post
	Guys, real world manuals says that you gotta set power by EPR in speed range about 40-80 kts. Q: If i set TO EPR that way correctly, what EPR gauges will indicate as aircraft goes faster then 80 kts? will it increase or decrease? Q: Does that apply to setting power by N1? will N1 increase or decrease with speed while rolling on ground? Q: Can i set TO power by N1 while i stopped like in this vid: http://www.youtube.com/watch?v=_RKa0_Qr1s4 ; ?


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Jettrader Full Member Offline Posts: 138 Joined: Dec 14^th, 2011	Re: TO EPR behaviour Reply #1 - Apr 16^th, 2012 at 4:58pm	Print Post
	windplayer wrote on Apr 16^th, 2012 at 3:12pm: Guys, real world manuals says that you gotta set power by EPR in speed range about 40-80 kts. Q: If i set TO EPR that way correctly, what EPR gauges will indicate as aircraft goes faster then 80 kts? will it increase or decrease? Q: Does that apply to setting power by N1? will N1 increase or decrease with speed while rolling on ground? Q: Can i set TO power by N1 while i stopped like in this vid: �http://www.youtube.com/watch?v=_RKa0_Qr1s4 � ? After setting EPR it will stay where you set it, EPR is the Engine Pressure Ratio, so basically the ratio between the engine inlet and outlet pressure. It's used to set a fixed(!) power - if it changes with speed, it would be a bad indicator for engine power output. N1 is the RPM in percent of the low pressure turbine (the fan). It does not change with aircraft speed. Of course you can stop the aircraft before accelerating the engines to Take-Off Thrust! Depends on the take-off technique you're using. If you have a short runway and need all the space, it's better to wait for full power until you start rolling. A rolling take-off is less time consuming and it's doesn't use up the brakes so much.

	&&http://www.youtube.com/user/JohannAviationMedia - http://www.facebook.com/JohannAviationMedia
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windplayer Senior Member Offline Posts: 568 Location: Izhevsk, Udmurtia, Russia Joined: Aug 5^th, 2011 Gender:	Re: TO EPR behaviour Reply #2 - Apr 16^th, 2012 at 10:25pm	Print Post
	Yep, but there is some hidden details. EPR - speed dependent parameter. Lou said once that table numbers good for a range of speeds. But he did not mentioned how it changes with speed. When the Aircraft goes faster, inlet pressure changes, coz the engine isnt a straight pipe, it have some resistance, or whatever Thats why it interesting. Same for N1- its speed might depend on inlet air pressure. CoastalDriver once said something about pressure distribution in jet-engine. And off-course fan have some performance too, incoming air stream affect it, or something I have no idea how, or will it change, but there is something outa there it isnt that straight, im sure. P.S. Look - E-3 drag racing!!! (in the end of video, at 1:55) �http://www.youtube.com/watch?v=7-0zPczN0_E


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Jettrader Full Member Offline Posts: 138 Joined: Dec 14^th, 2011	Re: TO EPR behaviour Reply #3 - Apr 17^th, 2012 at 6:30am	Print Post
	windplayer wrote on Apr 16^th, 2012 at 10:25pm: Yep, but there is some hidden details. EPR - speed dependent parameter. Lou said once that table numbers good for a range of speeds. But he did not mentioned how it changes with speed. When the Aircraft goes faster, inlet pressure changes, coz the engine isnt a straight pipe, it have some resistance, or whatever Thats why it interesting. Same for N1- its speed might depend on inlet air pressure. CoastalDriver once said something about pressure distribution in jet-engine. And off-course fan have some performance too, incoming air stream affect it, or something I have no idea how, or will it change, but there is something outa there it isnt that straight, im sure. P.S. Look - E-3 drag racing!!! (in the end of video, at 1:55) �http://www.youtube.com/watch?v=7-0zPczN0_E Yes the inlet pressure might be affected by the ram air pressure as well, but I would only imagine those effects marginal. I talked to a WestJet 732 Pilot and he always says, that the EPR values are for reference only and never 100% exact. Nevertheless, the FSX EPR values are most propably derived from the N1, as FSX doesn't simulate anything like this - hence I wouldn't worry about it for this simulation. Even the original boeing documents don't tell anything regarding this topic. This is more a matter of fluid dynamics.

	&&http://www.youtube.com/user/JohannAviationMedia - http://www.facebook.com/JohannAviationMedia
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windplayer Senior Member Offline Posts: 568 Location: Izhevsk, Udmurtia, Russia Joined: Aug 5^th, 2011 Gender:	Re: TO EPR behaviour Reply #4 - Apr 17^th, 2012 at 8:44pm	Print Post
	Im asking more out of curiosity FSX really not that precise. Just want to know how it is, - to be the EPR in accelerating plane


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audiohavoc Full Member Offline Posts: 160 Joined: Sep 13^th, 2009	Re: TO EPR behaviour Reply #5 - Apr 18^th, 2012 at 4:50am	Print Post
	EPR and N1 don't seem to vary much in turbofan engines unless you change the engine power setting. As for speeds and N1/EPR, to get a desired airspeed there is usually a corresponding N1/EPR value in the performance tables of airliners. Of course, the N1/EPR values for a desired speed change with aircraft weights, winds, and other variables.

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windplayer Senior Member Offline Posts: 568 Location: Izhevsk, Udmurtia, Russia Joined: Aug 5^th, 2011 Gender:	Re: TO EPR behaviour Reply #6 - Apr 18^th, 2012 at 12:14pm	Print Post
	Ok! you win! im gonna think it not vary a lot If i understand correctly - spinning up engines to TO power on brakes takes less rwy length for takeoff?


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Jettrader Full Member Offline Posts: 138 Joined: Dec 14^th, 2011	Re: TO EPR behaviour Reply #7 - Apr 18^th, 2012 at 12:27pm	Print Post
	windplayer wrote on Apr 18^th, 2012 at 12:14pm: Ok! you win! im gonna think it not vary a lot If i understand correctly - spinning up engines to TO power on brakes takes less rwy length for takeoff? yes - it's a common practice for short field, as you apply maximum power before starting the take-off run versus engine acceleration during take-off run on a normal take-off. I will save you some runway lenght.

	&&http://www.youtube.com/user/JohannAviationMedia - http://www.facebook.com/JohannAviationMedia
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windplayer Senior Member Offline Posts: 568 Location: Izhevsk, Udmurtia, Russia Joined: Aug 5^th, 2011 Gender:	Re: TO EPR behaviour Reply #8 - Apr 18^th, 2012 at 1:30pm	Print Post
	Thanks guys! That info will be handy


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