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

Extra margins with assume thrust

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

Extra margins with assume thrust

Old 1st Nov 2014, 23:53
  #21 (permalink)  
 
Join Date: Sep 1998
Location: wherever
Age: 51
Posts: 1,619
@ Meikleour

The fadec corrects for the full difference. The N1 is corrected for the difference between ambient and assumed.

@Gysbreght
If for example the perf at assumed temp was WAT limited second segment and we use that perf at a lower actual temperature the TAS difference will give us a higher gradient.
The actual gradient we will achieve is higher than the gradient we would make at assumed.
FE Hoppy is offline  
Old 2nd Nov 2014, 04:20
  #22 (permalink)  
 
Join Date: Jun 2007
Location: Wanderlust
Posts: 2,478
Life on top
You are not clear about this topic. Google this document it is from Boeing.


PDF]B737 Reduced Thrust Considerations - SmartCockpit
www.smartcockpit.com/download.php?path...Reduced_Thrust...pdf
vilas is offline  
Old 2nd Nov 2014, 07:50
  #23 (permalink)  
 
Join Date: May 2014
Location: In thin air
Posts: 186
Originally Posted by FE Hoppy
the TAS difference will give us a higher gradient.
No, that is not correct. The lower temperature will give you a lower TAS and a lower rate-of-climb. The gradient is not changed by TAS, it depends on the thrust-to-weight ratio and that is equal for the assumed and the actual temperature because you set the same thrust.

Likewise, the acceleration in terms of kts TAS per second does not change, the number of seconds changes and therefore the distance.
Gysbreght is offline  
Old 2nd Nov 2014, 08:50
  #24 (permalink)  
 
Join Date: Sep 1998
Location: wherever
Age: 51
Posts: 1,619
@ Gysbreght

Take a look at the climb gradients for your aircraft at rated thrust below flat rated temperature.

Same thrust to weight ratio but the gradient is higher as temperature reduces.
FE Hoppy is offline  
Old 2nd Nov 2014, 09:43
  #25 (permalink)  
 
Join Date: May 2014
Location: In thin air
Posts: 186
@ FE Hoppy,

sorry, I don't fly an aircraft and don't have access to an AFM.
The B777 Ops manual doesn't support your assertion.

Can you give an example?

P.S.
Have just looked at the Boeing presentation for the B737-800 linked in Vilas' post #22 above.

Page 18 shows that for that airplane there is an increased gradient at the lower actual temperature, but that is due to a higher thrust, not the difference in TAS.

Last edited by Gysbreght; 2nd Nov 2014 at 10:22. Reason: P.S.
Gysbreght is offline  
Old 2nd Nov 2014, 10:26
  #26 (permalink)  
 
Join Date: Jan 2014
Location: N5109.2W10.5
Posts: 647
Hi Gysbreght,
Page 18 shows that for that airplane there is an increased gradient at the lower actual temperature, but that is due to a higher thrust, not the difference in TAS.
Try reading a bit further: e.g. Slide 21

"Climb Gradient Margin Due to the True Airspeed Effect Increases With Higher Assumed Temperature"
Goldenrivett is offline  
Old 2nd Nov 2014, 10:43
  #27 (permalink)  
 
Join Date: Jun 2007
Location: Wanderlust
Posts: 2,478
Gysbreght
If you go through the entire document carefully you will get all the answers you are looking for.
vilas is offline  
Old 2nd Nov 2014, 10:49
  #28 (permalink)  
 
Join Date: May 2014
Location: In thin air
Posts: 186
Originally Posted by Goldenrivett
Try reading a bit further: e.g. Slide 21
I did. Can you explain that graph to me?
Gysbreght is offline  
Old 2nd Nov 2014, 11:12
  #29 (permalink)  
 
Join Date: Jul 2007
Location: uk
Posts: 673
FE Hoppy:Because the thrust scheduled at actual and assumed is the same and therefore the N1 required to achieve that thrust is the same.

Sorry to bang on about this however - reference to Villas's link pages 7 & 10
suggest that the density issue is not fully factored out by the FADEC. Boeing say that there is in fact more thrust produced at the assumed temp. than would be at the actual higher temp. This I think is what the OP was after.
So, the "density" issue with respect to the engine thrust would be from the actual lower ambient temp rather than the top of the flat rate?

PS Keith Williams or Old Smokie please feel free to intervene!!
Meikleour is offline  
Old 2nd Nov 2014, 11:15
  #30 (permalink)  
 
Join Date: Jun 2007
Location: Wanderlust
Posts: 2,478
Gysbreght
gradient= change in height/distance travelled. If ass. temp is 40 degrees at actual temp of 15 degrees TAS at 15 is less than what would be at actual 40 degrees. So distance travelled is less and higher gradient.
vilas is offline  
Old 2nd Nov 2014, 12:21
  #31 (permalink)  
 
Join Date: May 2014
Location: In thin air
Posts: 186
Vilas,

Read my post #23 again. TAS and V/S are both speeds, and change at the same rate with ambient temperature.

For constant weight, CAS and thrust, gradient and acceleration do not change with temperature.

I honestly don't understand what figure 21 is trying to show. In figure 18 the weight is 71000 kg, i.e. the climb-limited weight at 35 deg C. In figure 20 (titled "Lower Takeoff Weight ...") the climb-limit weight at 45 C is 65100 kg. What is the weight in figure 21? Why is the gradient constant above 45 C?

Last edited by Gysbreght; 2nd Nov 2014 at 12:34.
Gysbreght is offline  
Old 2nd Nov 2014, 12:27
  #32 (permalink)  
 
Join Date: Feb 2009
Location: Virginia
Posts: 332
How about this: An aircraft has the same lift, weight, and drag at a given IAS regardless of the density and TAS. This means that the rate of climb (in feet per minute) is also the same.

But at a higher density, the TAS will be lower, and so will the ground speed. Since the aircraft is gaining the same amount of altitude in the same time, but covering less ground, the climb gradient is higher.
Chu Chu is offline  
Old 2nd Nov 2014, 12:57
  #33 (permalink)  
 
Join Date: May 2014
Location: In thin air
Posts: 186
Originally Posted by Chu Chu
This means that the rate of climb (in feet per minute) is also the same.

(...)Since the aircraft is gaining the same amount of altitude in the same time, but covering less ground, the climb gradient is higher.
Wrong statements .....
Gysbreght is offline  
Old 2nd Nov 2014, 16:11
  #34 (permalink)  
 
Join Date: Sep 1998
Location: wherever
Age: 51
Posts: 1,619
@Gysbreght

e190 single engine climb gradients. Sea Level 40 tonnes

temp-----------gradient
0---------------6.03
2---------------6.02
4---------------6.01
.....
20-------------5.88
22-------------5.86
24-------------5.84
.......
30-------------5.79---Flat rated temp
32-------------5.42
34-------------5.05
36-------------4.71
38------------4.42



Note the gradient loss with increase in temperature up to flat rated temp.
Same thrust, same weight, same IAS.

The difference is TAS. (well it's rho really but that's reflected in TAS)

Last edited by FE Hoppy; 2nd Nov 2014 at 17:03.
FE Hoppy is offline  
Old 2nd Nov 2014, 20:44
  #35 (permalink)  
 
Join Date: May 2014
Location: In thin air
Posts: 186
@FE Hoppy,

Since the change in gradient for flat-rated temperatures is not caused by TAS, the explanation must be found in the flat-rated thrust not being truly constant for this airplane.
Gysbreght is offline  
Old 2nd Nov 2014, 22:57
  #36 (permalink)  
 
Join Date: Jan 2011
Location: England
Posts: 643
This is certainly a curious one.

If we look at a couple of equations for % Gradient.

Climb gradient = 100% x ( (Thrust – Drag) / Weight)………Equation 1.

Climb Gradient = Approximately 100% x ( ROC / TAS)……Equation 2.

Strictly speaking it should be (ROC/Ground Speed) in the equation 2, but for small angles in still air the Ground Speed is approximately equal to TAS.

Both of the above equations are equally valid, so it must be possible to use both equations to explain the reason for the reduction in gradient shown by FE Hoppies post.

Looking at equation 2, if the TAS increases due to increased OAT, while the ROC remains constant, then the % gradient will indeed decrease.

Looking at equation 1, the increasing OAT will not change the weight, so if the flat rating keeps the thrust constant, the % gradient will decrease only if the drag increases.

One possible explanation might be that the increasing OAT causes the KIAS value of V2 to decrease. V2 is less than Vmd, so this decrease will increase the drag. This increased drag at constant thrust would cause a direct reduction is the % gradient in equation 1. It would also reduce the ROC, which would in turn cause a reduction in % gradient in equation 2.

I’m not stating that the above explanation is a fact. I am simply musing over the matter.
keith williams is offline  
Old 3rd Nov 2014, 01:16
  #37 (permalink)  
 
Join Date: Jun 2007
Location: Wanderlust
Posts: 2,478
Gysbreght
One thing is certain it is an official document so it's veracity is not in question. Some of the assumptions in your argument may be incorrect.
vilas is offline  
Old 3rd Nov 2014, 02:57
  #38 (permalink)  
 
Join Date: May 2001
Location: A few degrees South
Posts: 813
Fact is that the aircraft operates in a lower density altitude then calculated for.
Actual performance will thus be better.
latetonite is offline  
Old 3rd Nov 2014, 06:39
  #39 (permalink)  
 
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 84
Posts: 474
@FE Hoppy

I'm with Gysbrecht. If you plot out the gradients listed you will find they vary directly with temperature. TAS however varies with the square root of temperature.
There is nothing in Keith Williams equations to link gradient with TAS - on the contrary, Equation 1 excludes any such relationship.
Checking the net for data on the E190 speeds I found that V2 at a given TOW is constant EAS over the flat rating temperature range.
So if it isn't TAS and it can't be drag at a constant EAS then it must be something in the way the engine is flat rated.

Last edited by Owain Glyndwr; 3rd Nov 2014 at 06:41. Reason: spelling
Owain Glyndwr is offline  
Old 3rd Nov 2014, 10:33
  #40 (permalink)  
Moderator
 
Join Date: Apr 2001
Location: various places .....
Posts: 6,567
One thing is certain it is an official document so it's veracity is not in question.

I admire your faith in the OEMs of the world.

Your comment doesn't necessarily follow as day follows night ... over the years I have referred a variety of OEM Manual errors back to the relevant OEM for subsequent correction. Indeed, I have picked up a number of Regulator errors in Flight Manuals and referred them back to the source for correction - the fact is that none of us is incapable of error ... doesn't matter for whom one works.

Some of the assumptions in your argument may be incorrect.

While errors are always a possibility for any of us, Gysbreght has a great many runs on the board .. I'd err on the side of presuming him to be correct until proven wrong. On this point, Owain Glyndwr (another with many runs to his credit) agrees.
john_tullamarine is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information

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