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Author Topic: Speeds and similar things ....  (Read 3760 times)

lonewulf47

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Speeds and similar things ....
« on: March 04, 2012, 01:48:11 pm »

Based on my "instincts" that have grown over years using various simulations I suspected something to be not so kosher with actual speed indications. I made a few measurements to verify my assumptions.

Measurement 1:
Covering 1 min latitude exactly North-South direction with the old Tatra.
- Time used 1 min 10 sec for 1 NM = 1852 m -> average speed ~95 km/h
- Speed indication on the Tatra's Speedometer -> >90 km/h which seems to be correct
- Speed indication on HUD average 27 KTS -> ~50 km/h which is far too slow

Measurement2:
Covering 1 min climb with the Cessna at indicated ROD of 500 FPM.
- Altitude gain approx. 250 - 270 ft which is only half of what is should be
- speed indication HUD vs. IAS indicator is coincindent which means that the Cessna is flying far too fast (reaching some 105 KIAS on straight flight). Furthermore the speed indication is always coincident with the IAS indicator of the A/C. I assume that the HUD deals with actual angular (ground) speed. Therefore the IAS of the A/C should become smaller with height.

Edit:
Measurement 3:
Covering 10 min. latitude inflight with the Cessna in 5:50 min
- groundspeed of approx. 102 KTS which is correct according to the HUD's indication

Measurement :
Flying at 1000 ft indicated altitude (Cessna's altitude indicator)
-  ~3250 ft altitude on the HUD

It seems that the speed indications/calculations are presently a bit at loss. The only reliable speed indication seems to exist on the old Tatra  :).
« Last Edit: March 05, 2012, 11:44:04 am by lonewulf47 »
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SilentEagle

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Re: Speeds and similar things ....
« Reply #1 on: March 04, 2012, 06:01:43 pm »

The HUD speeds are indicated air speed.  This is proven by taking the cessna up higherin altitude and watching the IAS or dropping it from space and seeing the IAS indicator read zero.
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lonewulf47

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Re: Speeds and similar things ....
« Reply #2 on: March 05, 2012, 09:48:17 am »

Possibly yes, but still far from reality. If your drive by car at sea level it should nevertheless be equal to groundspeed. You might have a bit a closer look to my measurement 1 results. My proposal would be to always indicate groundspeed at first priority in a HUD that can be used by ground vehicles also. IAS will always be available in an A/C on the ASI.
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lonewulf47

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Re: Speeds and similar things ....
« Reply #3 on: March 05, 2012, 01:43:41 pm »

Ok, I found the culprit about the speed thingy:
the speed label on the HUD should read "m/sec" rather than "KTS" when using it on cars. Calculating the speeds in m/sec reveals the proper values. Kind of the same applies for the Cessna's altimeter. It seems to indicate the altitude in METERS instead of FEET.
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SilentEagle

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Re: Speeds and similar things ....
« Reply #4 on: March 05, 2012, 02:46:32 pm »

Ah, that makes since about the truck, but I can say with 100% certainty that the cessna's hud altitude is in ft, because I have written out log files from the jsbsim flight model and it matches up with h-sl-ft parameter, which is altitude above sea level in feet.
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lonewulf47

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Re: Speeds and similar things ....
« Reply #5 on: March 05, 2012, 03:34:06 pm »

... but I can say with 100% certainty that the cessna's hud altitude is in ft, because I have written out log files from the jsbsim flight model and it matches up with h-sl-ft parameter, which is altitude above sea level in feet.
I am referring to the Cessna's altimeter, not to the HUD! The HUD is of course indicating ft, but the Cessna's altimeter (we call that thingy on the panel altimeter) which should be in feet as well is obviously indicating meters  8)
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SilentEagle

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Re: Speeds and similar things ....
« Reply #6 on: March 05, 2012, 04:43:11 pm »

Ah okay, I thought you were still referring to the HUD.  To be honest, I haven't used the panel instruments much because they are always so dark, so I can't read them.
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