HS748 Cruise Speed

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HS748 Cruise Speed

Postby WhisperJet » 25 Aug 2011, 13:15

Gentlemen,

I've learned that the HS748's cruise speed at say 20.000ft is about 240 knots.
Whatever I do I don't manage to get the model quicker as 180 - 190 knots (indicated airspeed), which is quite a bore on longer routes...

I follow the onboard checklist, reducing to climb thrust after T/O at about 1.500 ft. (And I've badly experienced many times what will happen if I don't)
Any ideas on what I could be doing wrong? Or is the model a bit slower in general... :dunno:

Any inputs are appreciated!
Many thanks in advance & best wishes,

Nick
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Re: HS748 Cruise Speed

Postby DaveB » 25 Aug 2011, 13:43

Hi Nick :hello:

The figure you quote of 240kts is groundspeed mate.. you'd never ever see anything close to that as IAS. I don't have my Flight Manual to hand but when I find it, I'll post the figures from the book.

In the meantime, the model is giving you pretty much bang on what you should be seeing (I thought it was faster than this too but it's not) ;)

EDIT:.. these are for the Andover CCmk1 around which the model was based (though modded to take the civilian version into account)
Vne - 260 knots
Vno - ZFW 37,000lb or less - 225 knots
Vno - ZFW between 37,000lb and 37,500lb 1. with more than 700lbs fuel in each wing tank - 225 knots
..................................................2. with less than 700lb fuel in each wing tank - 220 knots
Va - 155 knots

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Re: HS748 Cruise Speed

Postby Garry Russell » 25 Aug 2011, 14:38

Hi Nick

Turn the GPS on and see what the ground speed is in the conditions you mention :)
Garry

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Re: HS748 Cruise Speed

Postby cstorey » 26 Aug 2011, 07:36

The ASI measures the pressure differential between the air entering the pitot tube, and the static pressure. As the aircraft climbs , the air pressure diminishes - in broad terms by about 3% ( compound rather than linear ) per 1000 feet at a constant temperature - and so does the pressure differential . Thus as you climb the Indicated airspeed is always lower than the True Air Speed . However, the underread is not as great as the simple drop in pressure would suggest, because the temperature of the air drops , and thus the relative density increases, as you climb. Thus In International Standard Atmosphere conditions , with a temperature of about - 25C at 20,000 feet, an IAS of 180 knots equates to a True Airspeed of 247 knots . Your groundspeed will be that plus or minus the tailwind/headwind vector . Incidentally, for those aircraft with a machmeter, you can always get an instant approximation of TAS by multiplying the mach number by 600 . Thus .82 Mach approximates to 492 knots TAS . It's not quite accurate in all circumstances, but is near enough for these purposes
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Re: HS748 Cruise Speed

Postby WhisperJet » 26 Aug 2011, 08:19

Hi all and many thanks for your inputs so far!!

Hmm, I'm pretty aware of the difference between IAS and ground speed.

The manual reads:
Manoeuvring Speed 155kt
Never Exceed 260kt IAS
Normal Operating Speed Limit 225kt
(this must be IAS as well then...??)

Then, cruising around at 180 kts IAS would mean I'd be able to set flaps to 7 ½ ’- 15 ’ at any time of the flight... theoretically... 8) B-)
So I still kind of doubt I'm getting it right... :lol:

By the way,
here's a link to the manual online:
http://chomikuj.pl/TomiGos/GRY/Poradniki+Flight+Simulator/Hawker-Siddeley+HS.748++V1+Manual,250723337.pdf

Best wishes,

Nick
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Re: HS748 Cruise Speed

Postby DaveB » 26 Aug 2011, 09:36

Hi Nick :hello:

I think it's best to get your head around the fact that the 748 is/was no speed merchant and accept it as such ;) The figures I quoted above come straight from the 'real' manual and do not indicate IAS.. simply knots. It's also worth pointing out that the Vno.. in fact all the Vref's are 'limits' so if the manual had shown a Vno of 225kias for a given weight (and not 225 knots), your indicated cruise speed (IAS) would be a safe margin below.
I only ever fly the 748's on short, feeder, hops so am content to poodle around at an indicated 190kias at 9000ft which is pretty much what you'll see in ISA conditions at that alt at the prescribed 14,200rpm cruise setting ;)

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Re: HS748 Cruise Speed

Postby DispatchDragon » 26 Aug 2011, 11:04

Hey Gents

In the Dan Air days the 748 never operated above the mid teens -- even on "long" legs like Basle and Tarbes.


And as an aside just doing some house keeping next door -- amazing how the 748, Argosy and Vanguard are still ranked in the top fifteen downloads consdering how long they have been around, says something about the quality of Ricks work....The Argosy is almost at ten thousand d/ls!!!!

Thank you Mr Piper

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Re: HS748 Cruise Speed

Postby WhisperJet » 26 Aug 2011, 11:32

DaveB wrote:I think it's best to get your head around the fact that the 748 is/was no speed merchant and accept it as such ;)


You're probably right, Dave! :agree: :agree:
And I have no problem at all with accepting that, I just wanted to figure out if there was a certain clue I didn't see or so... ;)

DispatchDragon wrote:amazing how the 748, Argosy and Vanguard are still ranked in the top fifteen downloads consdering how long they have been around, says something about the quality of Ricks work


Ohhh yes, definitely!! :agree: :agree:
I have the 748 since years and I'm still fascinated!!

Again, thanks for your replies &
Have a nice weekend!!

Nick
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Re: HS748 Cruise Speed

Postby DaveB » 26 Aug 2011, 12:08

RGr that Nick and I second your thanks to Mr P Leif :thumbsup:

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Re: HS748 Cruise Speed

Postby cstorey » 26 Aug 2011, 16:27

Dave : I don't want to nitpick but all V speeds given in this, or any other , manual are IAS, except of course that on faster aircraft at higher levels there are also Mach no. limitations which are are given as Mmo or whatever
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