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Typical Cruise Speed

Postby Hamra » Sun Mar 27, 2011 1:05 am

when i read somewhere, that the typical cruise speed for a certain aircraft is X knots, is that ground speed or airspeed?
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Re: Typical Cruise Speed

Postby someguy » Sun Mar 27, 2011 3:35 am

Airspeed. Ground speed is irrelevant, since a flying aircraft doesn't interact physically with the ground. At least, it had better not!
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Re: Typical Cruise Speed

Postby Hamra » Sun Mar 27, 2011 3:55 am

so here's my second question then. the 777-200ER is supposed to have a typical cruise speed of 490 knots at 35000 feet. but when i get my aircraft to that altitude, it barely reaches 350 knots airspeed, on full throttle. so either the aircraft is not modelled correctly... or i'm not understanding "cruise speed".

PS: ground speed would be very high, way over 550 knots (hence why i'm asking if it's what is intended)
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Re: Typical Cruise Speed  

Postby i4dnf » Sun Mar 27, 2011 5:28 am

That sounds like indicated airspeed (IAS), not true airspeed (TAS). That would make about 580 kt TAS (which is way above the cruise speed). For the cruise speed you'd need to be at about 265 kt (IAS).
True airspeed on wikipedia here.
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Re: Typical Cruise Speed

Postby Hamra » Sun Mar 27, 2011 7:58 pm

thanks! i thought i did my homework about the various velocities... guess i missed some, in particular that IAS uses air density, and thus varies with pressure, and goes very off at such high altitudes.
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Re: Typical Cruise Speed

Postby MAKG » Mon Mar 28, 2011 10:09 pm

Well, "off" is in the eye of the beholder.

Most of the aerodynamics of the aircraft varies with altitude in the same way that IAS does. For instance, the stall speed of a clean straight-and-level Cessna 172N is 44 KIAS, whether you're at 10,000 feet or at sea level. That's much faster TAS at altitude than on the ground. True airspeed is almost completely irrelevant except as a stepping-stone to ground speed. And GS is only relevant for navigation, not performance.
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Re: Typical Cruise Speed

Postby Sealbhach » Mon Mar 28, 2011 11:36 pm

The way I understand it, at higher altitudes, the air is much thinner, so it cannot apply so much pressure on the pitot tube, so an IAS of 340 knots can translate to a groundspeed of 500 knots or more. Something like that.

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Re: Typical Cruise Speed

Postby someguy » Tue Mar 29, 2011 2:32 am

That's correct. The thinner air also cannot provide as much lift, thus the stall speed (in TAS) increases with altitude along with the pitot effect. However, as altitude increases, temperature decreases, which lowers the critical Mach speed, the speed at which the wing goes supersonic and loses its boundary flow. Every plane has an altitude where the stall and supersonic speeds coincide; it's called "coffin corner" because the plane can't go either faster or slower without losing lift dramatically and falling out of the sky. At that altitude, a turn could put one wing into stall and the other above critical Mach. Needless to say, most pilots of non-supersonic aircraft won't want to go there!
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Re: Typical Cruise Speed

Postby redneck » Tue Mar 29, 2011 3:13 am

I don't think FG models critical mach :(
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Re: Typical Cruise Speed

Postby skyop » Tue Mar 29, 2011 3:57 am

redneck wrote in Tue Mar 29, 2011 3:13 am:I don't think FG models critical mach :(


I'm sure JSBSim could do it. :) Not sure about YASim...
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Re: Typical Cruise Speed

Postby redneck » Tue Mar 29, 2011 1:28 pm

I'm sure it could be done in YASim with a little Nasal.
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Re: Typical Cruise Speed

Postby MAKG » Tue Mar 29, 2011 2:56 pm

FG certainly doesn't model buffetting. Though a stick shaker would be cool....

The thing I really wish it had was elevator stall or wing wash, as that's a critical part of stall recovery (or lack thereof). A stalled 747 doesn't recover from a stall very easily because of loss of elevator authority (you can't get the nose down under some circumstances). A c172 very nearly recovers itself (still too easy in FG -- it doesn't nose over).
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Re: Typical Cruise Speed

Postby someguy » Wed Mar 30, 2011 4:13 am

Dave Culp uses a sound to indicate buffeting. Pretty hard to do stick shake when FG doesn't support force feedback sticks.
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Re: Typical Cruise Speed

Postby i4dnf » Wed Mar 30, 2011 6:44 am

On the subject of buffeting, with the IAR-80 I've simulated this using nasal, by shaking the viewpoint at near stall aoa.
(as a bonus you'll find that with the same script I shake the view on taxi/take-off above a certain speed ;) or in flight at speeds nearing Vne)
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Re: Typical Cruise Speed

Postby redneck » Wed Mar 30, 2011 1:55 pm

Something tells me that won't work too well with face tracking set up. Dynamic cockpit view stops working when it's turned on, so I imagine it will just stop working. Best thing is to at least add a sound for it.
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