Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Increasing jet aircraft range.

Tech Log The very best in practical technical discussion on the web

Increasing jet aircraft range.

Old 24th May 2013, 08:02
  #21 (permalink)  
 
Join Date: Jul 2009
Location: France - mostly
Age: 80
Posts: 1,689
Originally Posted by framer
any idea where I could read up about Mach effect?
Sorry, I can't help you there. Mach effects are due to the compressibility of air. They become important when the local velocity in the airflow around the airplane approaches the speed of sound. I was looking at the aerodynamic data of a wide-body transport.
HazelNuts39 is offline  
Old 24th May 2013, 08:07
  #22 (permalink)  
 
Join Date: Mar 2006
Location: AUSTRALIA - CHINA STHN
Age: 55
Posts: 257
Fourth root etc

So is that where 1.32 comes from as Vimd x1.32 = Vlrc?

I also learnt optimum altitude was best the best product of l/d x Mach for the 707 ... Which sort of lines up with the previous info.
On another fuel matter, when you plug a hold( racetrack) into an FMS it bugs up a few knots over green dot... Now this makes sense as the idea is to fly slightly faster to allow the speed to decay in the turns due to the drag increase and speed up on the straights staying above Vimd... But the idea used to be to leave the thrust constant , avoiding accelerating the engine thus using more fuel...
In modern jets FMS says ' set hold speed' and obviously the autothrust/ throttle then powers up and down as needed to hold constant target speed... Is this correct or counterintuitive to the whole reason hold speed is slightly increased in the first place?? Or am I missing something ... Should I technically be disconnecting A/T if I really wanted max endurance? No one has ever been able to,answer this question.. But some of the folks here seem pretty aero- knowledgable? WJA
woodja51 is offline  
Old 24th May 2013, 09:10
  #23 (permalink)  
 
Join Date: Jan 2012
Location: Not far from the edge of the Milky Way Galaxy in the Orion Arm.
Posts: 510
I would have thought you can get around the hold no problem and that taking autothrust out of . . auto - using manual thrust would stop the excursions of thrust - providing you have enough thrust to get round the corners, in your race track pattern, based on holding at altitude. `Some` ask for a certain speed in the hold like 230 kts.

Also, needless to say, if you "fix" your thrust at a constant setting during your max/long range cruise - you will go faster and faster, or (higher and higher)

Also, needless to mention, the level where you are getting max tailwind is more favourable than into the teeth of a jet(stream) say.

Also, don`t forget the hurricane thing - when flying the outer edges or the affected air of the tropical revolving storm its best to go around with the wind covering more ground and yet a lot faster ground speed.

What I didn't understand was the post by toffeez. What did you find so scary about my post 6, which, after all, was only another question? Was it that I asked a stupid question?
Nothing wrong with your post - you said nothing stupid, toffeez meant that he had to get the graphs out and work for a minute.

Love your thread Deefer.

Last edited by Natstrackalpha; 24th May 2013 at 09:24.
Natstrackalpha is offline  
Old 24th May 2013, 09:15
  #24 (permalink)  
 
Join Date: Jul 2009
Location: France - mostly
Age: 80
Posts: 1,689
Originally Posted by woodja51
Should I technically be disconnecting A/T if I really wanted max endurance? No one has ever been able to,answer this question..
I would think the idea is to keep the airplane on the stable part of the drag curve to avoid the engine constantly spooling up and down. Disconnecting A/T doesn't help because you would have to work the engines even harder.
HazelNuts39 is offline  
Old 24th May 2013, 09:17
  #25 (permalink)  
 
Join Date: Oct 2001
Location: England
Posts: 624
Best Range

Deefer Dog,

Despite the theory above all being sound the technique for each aircraft will vary slightly in practice.

Based on many years of experience and in flight trials you will need to record some figures for your own aircraft in flight and come up with a plan. As a guide I have used the following as starting point:

Best range in a jet is often achieved at about 1000' above the max height for weight shown in the AFM. But be careful as you are into the 1.3G margin to the buffet in 3 directions. This point will give you approx 30 AOB to the buffet in a turn.

For head or tail winds I use one third of the wind value, I would expect the 40% mentioned above is equally valid.

To decide whether to to go up or down to loose a headwind component I use a required gain of 7 to 9 kts in G/S for every 1000'. i.e. In a strong headwind if you gain 9kts of G/S by descenting 1000' you will probably be in profit.

But what you need to do is use your FMS to display the specific range i.e. the number of miles you are achieving per unit of fuel. When this is at the greatest then you will be at the best range speed. This speed will decrease with weight. The technique to work this out is to set the parameters on a long leg where wind and temp are constant then set a power for a speed and let it become stable, record the figures and then set a different speed and record the figures. Over the period of a few flights you will get a feel for your aircraft.

If you wish to discuss then please feel free to PM me.

Regards

MM
Miles Magister is offline  
Old 24th May 2013, 13:12
  #26 (permalink)  
 
Join Date: Sep 2008
Location: 41S174E
Age: 52
Posts: 2,767
Can anyone shed some light on some speeds that I noted from the FMS on a flight I did this morning.
The holding speed ( at FL390) was 220kts. The s/e drift down speed was 231kts, and the Vref 40 speed +100 was 236kts.
From what I have read in the 737 manual I would have thought that the VMD could be obtained from the s/e drift down page or approximated from Vref 40 by adding 100kts.
Bfisk stated that VMD could be obtained from the hold page but I wonder if this is true for our update 10.8 FMC because it seems too slow to me.From experience I imagine that 220kts would have been on the back of the curve and quite "speed unstable".
Any ideas about how those speeds are derived and if any of them are VMD would be appreciated.

Last edited by framer; 24th May 2013 at 13:13.
framer is offline  
Old 24th May 2013, 13:22
  #27 (permalink)  
 
Join Date: May 2003
Posts: 398
Framer,

If single engine drift down speed is for minimum rate of descent, then it should be at V minimum drag (max endurance speed). If single engine drift down speed is for max range, then it will be greater than minimum drag, ie to maximize the ratio of TAS/fuel flow. All this assumes thrust is proportional to fuel flow, which is fairly true.
hawk37 is offline  
Old 24th May 2013, 14:06
  #28 (permalink)  
 
Join Date: Jul 2009
Location: France - mostly
Age: 80
Posts: 1,689
framer,

While I don't know how and why Boeing scheduled those speeds, I would expect the s.e. driftdown speed to be the same as the s.e. climb speed, optimized for best s.e. gradient of climb/descent, without regard to fuel consumption, i.e. close to VMD. Why the holding speed is lower I don't know, but it could be due to the different engine configuration, i.e. both engines operating for level flight vs. one engine out, remaining engine at max. continuous thrust (e.g. different trim drag, different specific fuel consumption).

P.S. I was hoping to find some answers in Boeing's Jet Transport Performance Methods, but unfortunately there is nothing on holding and driftdown speeds, and chapters "Performance and FMCS" and "Speed Stability" are still "to be supplied". Perhaps you should ask Boeing?

P.S.2
Oops, holding is discussed in the "Cruise" chapter. There it says:
In the holding pattern, however, while the emphasis is still on fuel efficiency in this case we want to minimize the rate of fuel flow. Minimizing the fuel flow rate means minimizing the amount of fuel consumed while holding.
“All right,” you say, “we’ll just calculate the fuel flow at the speed for minimum drag. Minimum drag means minimum fuel flow, right?”
Sorry, no. It’s a little more complicated than that. Actually, the minimum flow will occur at a speed slightly slower than the speed for minimum drag. Let’s see why that’s true.
The explanation that follows is that the engine TSFC (Thrust-Specific Fuel Consumption) increases with airspeed.

Last edited by HazelNuts39; 25th May 2013 at 12:17. Reason: P.S.1 and P.S.2
HazelNuts39 is offline  
Old 24th May 2013, 22:28
  #29 (permalink)  
 
Join Date: Sep 2008
Location: 41S174E
Age: 52
Posts: 2,767
That's great information Hazelnuts, thanks.
It explains why Boeing suggest to fly Vref 40 +100 in the hold even though this is invariably faster than the speed on the hold page. Flying slower than VMD at FL390 is a bad idea in the -800. I'm going flying again tonight so I'll jot down the speeds again then to see if the 1.32 factor works.
Cheers.
framer is offline  
Old 24th May 2013, 23:49
  #30 (permalink)  
 
Join Date: Jun 2006
Location: Europe
Posts: 716
A couple of things to note here: the 1,32 and 1,37 constants may be completely arbitary. The speed/drag curve I posted was something I found on a google search in order to illustrate the principles. It may or not be measurable or to scale.

Secondly, I said the hold speed on the FMC will be close to Vmd, not that is Vmd pr definition.According to the FCTM: " Holding speeds in the FMC provide an optimum holding speed based upon fuel burn and speed capability; but are never lower than flaps up maneuver speed".

Last edited by bfisk; 25th May 2013 at 00:03.
bfisk is offline  
Old 25th May 2013, 01:28
  #31 (permalink)  
 
Join Date: Sep 2008
Location: 41S174E
Age: 52
Posts: 2,767
Thanks Bfisk,
I understand what you're saying. I am finding this interesting from a practical point of view for day to day flying and appreciate the info that is coming out from you guys. At this stage I am thinking that VMR is simple, just select CI 0.
VMD is looking more and more like s/e drift down speed and the speed from the hold page is slightly lower than VMD and not a speed I want to use in day to day operations when in the flight levels.
Cheers.
framer is offline  
Old 25th May 2013, 01:40
  #32 (permalink)  
Thread Starter
 
Join Date: May 2010
Location: europe
Age: 63
Posts: 641
Thank you all. Making sense now, and just got an offer from a test pilot who promised to fly with me on a trip to show me how they better book figures. Will let you know in three weeks when we need to do it, and hopefully how its achieved.
deefer dog is offline  
Old 25th May 2013, 01:58
  #33 (permalink)  
Thread Starter
 
Join Date: May 2010
Location: europe
Age: 63
Posts: 641
Just to add to the mix, here is an explanation I found:


Professional Pilot - Range Jet

Too complicated for me though, but some of it rings bells after reading posts from all who have been kind enough to respond (without being "scared" at my lack of knowledge).

Last edited by deefer dog; 25th May 2013 at 02:15.
deefer dog is offline  
Old 25th May 2013, 03:08
  #34 (permalink)  
 
Join Date: Apr 2013
Location: Earth
Age: 45
Posts: 152
Well that's what I do too...when it's late at night running into headwinds and bad weather, all the FBOs are closing down, I don't look at my FMS or pull out the flight manual for long range fuel, I start calculating TAS X 1.32 divided by PI, the Earth's ecliptic squared, subtract the cups of coffee left in the pot behind me, divided by a calculated ounces of bagged cubed, multiplied by the amount of bug splats on my windshield.


Last edited by Teldorserious; 25th May 2013 at 03:08.
Teldorserious is offline  
Old 25th May 2013, 09:13
  #35 (permalink)  
 
Join Date: Jul 2009
Location: France - mostly
Age: 80
Posts: 1,689
Why are pilots often so dogmatic?

From the "Professional Pilot" site:
Thrust Specific Fuel Consumption (TSFC)

Thrust Specific Fuel Consumption (TSFC) is defined as the ratio of fuel flow to thrust produced by the engine:

(...)

For JET engines TSFC is very close to being constant over a wide range of airspeeds. In other words the same amount of fuel flow produces the same amount of thrust at any speed from zero to close to the speed of sound.

We must emphasize the above point: TSFC is very close to being constant for a JET engine. This is a critical point. It means that to determine the fuel flow we simply use the formula: FF = TSFC x Ta. Given that TSFC is a constant this is a very useful and powerful equation. Let's emphasize it again:
From Boeing Jet Transport Performance Methods:



(...)


Last edited by HazelNuts39; 25th May 2013 at 09:30.
HazelNuts39 is offline  
Old 25th May 2013, 10:31
  #36 (permalink)  
 
Join Date: Jun 2011
Location: Devonshire
Age: 92
Posts: 300
Smile Confirmation ?

HN39

Please thank Boeing for the last paragraph, that " TSFC rises..."

This helps to explain why I had so much fuel left in the Port fuel tank of my Rapide after flying around Blackpool for more than a couple of hours, in the dark ( and ignorance ) in 1951 !

(See the thread " Fuel Economy a Pilot's Reward" #51 9th May )

It worked on twin engined biplanes of the 1930s, too.

LT

Last edited by Linktrained; 25th May 2013 at 10:34. Reason: Typo
Linktrained is offline  
Old 26th May 2013, 05:42
  #37 (permalink)  
 
Join Date: Sep 2008
Location: 41S174E
Age: 52
Posts: 2,767
Went for another fly today and jotted down some speeds.
At 69.2T and FL 370 we we had a s/e drift down speed of 242 and a hold speed of 229. We were indicating 246kts at ci 10.
229 would have been about 10kts above the amber bar (which gives 40degrees bank to the stick shaker).
My assessment of that is that VMD was 13kts above the speed on the hold page which is the minimum fuel burn speed and not a speed I would have wanted to slow to.
Thoughts?
framer is offline  
Old 26th May 2013, 08:51
  #38 (permalink)  
 
Join Date: Jul 2009
Location: France - mostly
Age: 80
Posts: 1,689
Not much to add. Your observation is not too different from the JTPM example showing holding speed at FL350 of M.72/242 kCAS for VMD of M.75/253 kCAS.
HazelNuts39 is offline  
Old 26th May 2013, 09:35
  #39 (permalink)  
 
Join Date: Sep 2008
Location: 41S174E
Age: 52
Posts: 2,767
Cool, I think I'm done here. Thank you very much to the OP and everyone else who contributed solid info and links/ examples etc. I have learnt/ confirmed a lot.
Cheers
framer is offline  
Old 27th May 2013, 17:52
  #40 (permalink)  
 
Join Date: May 2003
Posts: 398
My assessment of that is that VMD was 13kts above the speed on the hold page which is the minimum fuel burn speed and not a speed I would have wanted to slow to.
Thoughts?
Framer, before you make that conclusion, can you first be sure that single engine drift down speed, 242 kts, is indeed for least ANGLE of descent. Because if it is instead for best RANGE speed single engine, then your assessment may be incorrect.
hawk37 is offline  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.