High thrust required to taxi

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Driver170
Posts: 2052
Joined: 29 Feb 2016, 02:22
Location: Edinburgh

High thrust required to taxi

Post by Driver170 » 14 Jul 2019, 09:47

Not saying its the flight model but more like P3D taxi friction.

ZFW 56t
GW 62t

I’m needing atleast 30-35% N1 to keep the plane rolling if not the taxi speed will decline fast.

So during taxi (short or long) i’m need Atleast 300-400kg of taxi fuel because of ground friction and high thrust.

Is there any kind of mod out there?
Vernon

Flyengravity MIP Cpflight ICS V2
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Lapi
Posts: 2665
Joined: 26 Nov 2011, 02:00

Re: High thrust required to taxi

Post by Lapi » 14 Jul 2019, 10:24

Driver170 wrote:
14 Jul 2019, 09:47
Not saying its the flight model but more like P3D taxi friction.
ZFW 56t
GW 62t
I’m needing at least 30-35% N1 to keep the plane rolling if not the taxi speed will decline fast.
A 60T plane (summer & dry because friction also depends on those factors) should start rolling by adding approx. 32-34 % N1.
Then to maintain a 15-18kts speed you need 26-29% N1.

If you plane was 62T GW (don't mention ZFW for this purpose pls.) and you needed 30-35% then something was wrong on your end.
It can be several things, so please test it with exact 60T, balanced loads and tell me your result.

regards
Lapi

Driver170
Posts: 2052
Joined: 29 Feb 2016, 02:22
Location: Edinburgh

Re: High thrust required to taxi

Post by Driver170 » 14 Jul 2019, 12:24

With approximately 60t it does start to roll with 30-35% N1. Soon as i place the throttles to idle the speed will decrease. I’m not sure if this Flight model and P3D take into consideration ground friction forces or surface contaminates? Dry, Wet or slippery!

I have noticed something strange with the flight model. During taxi out a few days ago i only needed about 26-29% N1 roughly as you said. But after takeoff then done a quick arrival approach back to the same airport i needed higher thrust to get the aircraft moving and keeping the speed steady!

I will test again with a GW of 60t and let you know the figures.
Vernon

Flyengravity MIP Cpflight ICS V2
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Lapi
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Joined: 26 Nov 2011, 02:00

Re: High thrust required to taxi

Post by Lapi » 14 Jul 2019, 15:36

Driver170 wrote:
14 Jul 2019, 12:24
With approximately 60t it does start to roll with 30-35% N1. Soon as i place the throttles to idle the speed will decrease. I’m not sure if this Flight model and P3D take into consideration ground friction forces or surface contaminates? Dry, Wet or slippery!
When you put the throttles to idle, the GS will obviously decrease.
It's not an Airbus, so usually one cannot taxi on idle power with a 738, apart from some winter or very low-weight situations.
Just as I told you, initially apply around 33% N1 thrust to get rolling, then decrease the thrust but not to idle. It should be around 26-29 %N1 to keep it rolling on a constant speed.

Under ISA conditions your idle N1 should be exactly 21 %, so different weather may give different numbers, always compare those to the ISA 21%.
This numbers stand for a 60T GW, so more or less weight needs proportionally more or less thrust while taxiing.
I have noticed something strange with the flight model. During taxi out a few days ago i only needed about 26-29% N1 roughly as you said. But after takeoff then done a quick arrival approach back to the same airport i needed higher thrust to get the aircraft moving and keeping the speed steady!
Leave the flight-model alone please :twisted: specially if you experience the problem you've just described.
Check if you DO NOT have any hidden or unwanted braking action and mind it that this is the summer, when by the changing humidity and temperature pots usually need a serious re-calibration.
But you are in Edinburgh where the weather is always humid and relatively cold (not relatively, very cold... I worked there for a week) so perhaps you need no calibration, unless the sun comes for more than 3 days a year...).
It is highly probable that you have a braking action on ground that you do not notice. It also happened to me several times in the past.
Before all, check you brakes.

regards
Lapi

Driver170
Posts: 2052
Joined: 29 Feb 2016, 02:22
Location: Edinburgh

Re: High thrust required to taxi

Post by Driver170 » 14 Jul 2019, 17:18

Lapi wrote:
14 Jul 2019, 15:36
Driver170 wrote:
14 Jul 2019, 12:24
With approximately 60t it does start to roll with 30-35% N1. Soon as i place the throttles to idle the speed will decrease. I’m not sure if this Flight model and P3D take into consideration ground friction forces or surface contaminates? Dry, Wet or slippery!
When you put the throttles to idle, the GS will obviously decrease.
It's not an Airbus, so usually one cannot taxi on idle power with a 738, apart from some winter or very low-weight situations.
Just as I told you, initially apply around 33% N1 thrust to get rolling, then decrease the thrust but not to idle. It should be around 26-29 %N1 to keep it rolling on a constant speed.

Under ISA conditions your idle N1 should be exactly 21 %, so different weather may give different numbers, always compare those to the ISA 21%.
This numbers stand for a 60T GW, so more or less weight needs proportionally more or less thrust while taxiing.
I have noticed something strange with the flight model. During taxi out a few days ago i only needed about 26-29% N1 roughly as you said. But after takeoff then done a quick arrival approach back to the same airport i needed higher thrust to get the aircraft moving and keeping the speed steady!
Leave the flight-model alone please :twisted: specially if you experience the problem you've just described.
Check if you DO NOT have any hidden or unwanted braking action and mind it that this is the summer, when by the changing humidity and temperature pots usually need a serious re-calibration.
But you are in Edinburgh where the weather is always humid and relatively cold (not relatively, very cold... I worked there for a week) so perhaps you need no calibration, unless the sun comes for more than 3 days a year...).
It is highly probable that you have a braking action on ground that you do not notice. It also happened to me several times in the past.
Before all, check you brakes.

regards
Lapi
I will indeed test under those conditions lapi as you mentioned and also use those N1 figures.

No braking action as i checked the Red BRAKES at the bottom left and nothing was showing.

Surprisingly were getting some good weather just now but probably will snow next week hahahha
Vernon

Flyengravity MIP Cpflight ICS V2
Cpflight Pedestal
Flybycockpit yoke, Rudder pedals, Floor structure and shell.

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Driver170
Posts: 2052
Joined: 29 Feb 2016, 02:22
Location: Edinburgh

Re: High thrust required to taxi

Post by Driver170 » 14 Jul 2019, 23:26

Seems to be matching your figures correctly at 60t

But with one engine taxi it stops dead?
Vernon

Flyengravity MIP Cpflight ICS V2
Cpflight Pedestal
Flybycockpit yoke, Rudder pedals, Floor structure and shell.

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