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Power / speed issues

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snave View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snave Quote  Post ReplyReply Direct Link To This Post Posted: 08 Sep 2017 at 5:43pm
`100% fuel` is NOT take off weight. Please understand the terms you use are meaningless unless you also include the context...

Here is what is needed to look at take-off performance:

1: Take-off weight - this can be summarised as the combined weight of aircraft, fluids, passengers and baggage. If you give ANY weight or load, give them all or you are wasting everyones time. Especially your own. Its simple enough to read off the figures from the load screen.

2: The weather: Temperature and pressure affect density altitude. These early t/p's had a great power:weight ratio compared to vintage piston props, but when compared to modern turboprop airliners, not so much. Density altitude can cripple performance. Tailwinds and headwinds can boost or limit take-off and climb performance.

3: Runway altitude and direction: For the same reason as above, but also as part of the density altitude calculation.

4: CG - if your Centre of Gravity is outside the correct parameters, the aircraft will struggle to rotate or will fly in an exaggerated posture that increases drag.

At max take-off the 748 was NOT a STOL aircraft. Most flight operations were conducted UNDER max take-off weight to get acceptable performance. In fact in the Fifties and Sixties it would have been rare to the point of `not happening` to have an airliner operate regularly at MTO. Look at the climb rate of the 748 at MTO in ISA conditions for a clue - under 500 ft/min. 

So in addition to calibrating the aircraft for comparison please also recalibrate your expectations. It's a vintage aircraft, brilliant in its day but by the standards of today? Asthmatic, portly and sedate...

A good example I've seen in any number of videos is the exaggerated angle on take off. The 748 did not take off like that. It just didn't have the power or the aerodynamics to climb like a homesick angel, UNLESS the weather was kind and the weight reduced. If you are looking at airshow videos please understand the aircraft was specially lightened, with minimal fuel to to do those parlour tricks.

Compare the weight, engine power and performance stats with a Dash 8-300 for what 20 years did in aircraft development.
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almerejo View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote almerejo Quote  Post ReplyReply Direct Link To This Post Posted: 08 Sep 2017 at 6:30pm
I knew answering to this thread was going to be pointless.

I take it all those points you with your expertise have listed here bullet-fashion are all things that are available with this aircraft to standard of one of the high-end that has an FMC where fuel, load, CG, etc,etc to take into account?

Tell you, what.............................I am out of here. Guess, I may as well send this aircraft back and ask for a refund. After all, 'the terms I use are meaningless', so I may as well give up flightsimming now and leave the field to the 'experts' !!
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almerejo View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote almerejo Quote  Post ReplyReply Direct Link To This Post Posted: 08 Sep 2017 at 6:32pm
To the Moderators, please DELETE this account forthwith!
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snave View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snave Quote  Post ReplyReply Direct Link To This Post Posted: 10 Sep 2017 at 11:33am
Originally posted by almerejo almerejo wrote:

I knew answering to this thread was going to be pointless.

I take it all those points you with your expertise have listed here bullet-fashion are all things that are available with this aircraft to standard of one of the high-end that has an FMC where fuel, load, CG, etc,etc to take into account?

Tell you, what.............................I am out of here. Guess, I may as well send this aircraft back and ask for a refund. After all, 'the terms I use are meaningless', so I may as well give up flightsimming now and leave the field to the 'experts' !!

Or use the information imparted to become one. ALL the information asked for can be supplied by simply reading your sims menus and indeed, are within the capability of the simmer to specify. I have no idea why you even mention an FMC? 

This aircraft didn't have one.

If you cannot or will not supply that information we cannot help you. Ouch

YOU are the one seeking help, yet apparently getting catty when it's offered, based on what you need to supply to enable advice to be proffered..? Angry

Yes, you may as well give up, as you only gain knowledge through `education`. You, apparently, don't want to be educated.

No one here can help you with that. 


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tdumontelle View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tdumontelle Quote  Post ReplyReply Direct Link To This Post Posted: 11 Sep 2017 at 3:37am
OK. I'll try to get the thread back on track.

I did another test the other day. This time I used 50percent fuel load and default passenger load. I believe the TOW was 31k. I'm not sure if I missed the gust lock in the previous flights, but I for sure did it this time. For take off I rotated right around 120-130 knots with no problem. So maybe the sluggish takeoff was user error (although I didn't see the step written into the learning flight).

Other problems were still there. Mainly that engine power settings by the numbers do not result in the expected flight profile. Recommended climb is at 14200 RPMs with an airspeed of 140-150kias. Setting the RPM's to 14200 I end up with a 15 degree or higher pitch to maintain the airspeed. It seems to want to settle in more like 180 on the climb with a 7-9 degree pitch.

Cruise also has problems. The aircraft overspeeds well before the recommended 220 KIAS. This was at FL19. Looking at the previous freeware manual it recommended leaving RPMS at 14200 but that results in an overspeed as well.

I got so frustrated that I didn't even try to descend and land.
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tdumontelle View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tdumontelle Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2018 at 1:35am
More tests this week (I really want to use this aircraft). At full load, definitely no rotation at 120-130 (more like 150). Climb out when set to the recommended 14200 rpms and speed of 140-150 or 160 yields an 18 degree pitch. At cruise, the aircraft overspeeds at around 213 or so even though cruise is supposed to be 220 with overspeed I think at 248 or so. At 18000 feet I can't get it to 14200 RPMs but it seems to want to either go 207 or 160. As I raise and lower the the throttle and RPM's it seems to settle into these speeds.

Again, all tests on FSX/SP2, null weather this time.
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snave View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snave Quote  Post ReplyReply Direct Link To This Post Posted: 09 Mar 2018 at 3:56pm
Take off speed is FAR too high. Normal t.o speed was 105 Kts.

Max cruise for the Series 2A was 241 knots. Normal Cruise was 220Kts. Both at 15,000 ft, 

SL take off distance at GW, ISA was 4,180 ft to 35ft.
Landing distance ISA, SL, Max Landing weight was 3,300 ft. Landing speed 94 kts.

expect 1,450 ft/min initial climb rate.

Climb chart available here:
https://doc8643.com/aircraft/A748

Note how fast climb rate falls off with altitude.

Sounds like the thing is still overpowered but if you set the sim up for ISA and SL, and then fly it correctly at Gross of 44,495 lb, using the numbers above we can get a suitable yardstick.



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tdumontelle View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tdumontelle Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jul 2018 at 12:31am
I finally got a chance to run some more tests on the flight model.  Tests were done with clear weather, gross weight at 44,496 at TNCM which is basically sea level.  Same system as before running FSX on windows 7 64bit, 400 quad processor, no real config tweaks except the highmemfix or whatever that one is. .

Take off - tried to rotate at 110 and nothing through like 125, then only slightly with maybe breaking the ground at 140.

Climb - as usual the climb was crazy overpower.  Manual says 90 percent power, but I don't know how to set that given there is no percent power gauge.  At 14200 rpms, to maintain 160 I'm looking at 15 degree pitch up and still edging to 180 plus.  At 7 degree pitch up as in the manual, I couldn't really keep it below 190 with very little throttle.

Cruise was different this time.  No overspeed.  I'm not sure if the speed gauge is off or I was reading it wrong, but I was able to get 220 KIAS without overspeeding.  I used the shift z red aircraft info to see where the speed actually fell.  It looks like 220 is 1 notch above 200 on the outer dial rather than 2 notches behind the 240 on the inner dial.  So, no overspeed and fuel burn was about 800 pounds per hour per engine at 15,000 feet running at 11800 rpms.

Not sure about the decent profile.  I was able to get down following the basic procedures in the manual and land.  Full flaps, 110 or so decreasing crossing the threshold.  Wasn't pretty but I got her down.  No judgement on the sim.  All landing a descent could be pilot stuff.  Weird that I couldn't steer after I landed.  Not sure if it was my yoke or the aircraft. 

So mainly, still has significant problems under power at take off, and over on the climb.

I'm thinking about doing another ticket on this one.  The last response to my previous one was that they were sending it to the designers.
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Simon73 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Simon73 Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jul 2018 at 12:29pm
Originally posted by tdumontelle tdumontelle wrote:

I'm thinking about doing another ticket on this one.  The last response to my previous one was that they were sending it to the designers.


Feel free to as we're happy to follow up for you.

https://support.justflight.com/support/tickets/new
Just Flight / Just Trains Customer Service Staff
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