Thrust Decrease After Takeoff

[scandinavian13 5,192]

Using FSUIPC at all?

[adamant365 28]

I am, but primarily to process key commands. I have my primary flight control axes calibrated in Windows and assigned through FSX as recommended by the intro guide. The only adjustment I have made was to put a little positive curve in the elevator axis in FSUIPC because I found it to be too sensitive.

 

My throttle isn't processed in FSUIPC at all, only in FSX. I have the third (far right) throttle axis on my CH yoke assigned to reverse thrust in FSUIPC (with a generous null zone). I have a toggle switch on the CH yoke assigned to TOGA and another for flaps and the right/left fingertip buttons on the CH yoke are assigned in FSUIPC to send a keyboard command for A/T and A/P disconnect to be read by what I've defined in the PMDG key options.

[AlanB77L 0]

Hey Adam, how have you implemented the reversers via FSUIPC? I tried it long ago and can't use the LUA script I use for the PMDG twins because I can't figure out how to apply it to 4 axes.

 

Thanks!

[adamant365 28]

Hey Adam, how have you implemented the reversers via FSUIPC? I tried it long ago and can't use the LUA script I use for the PMDG twins because I can't figure out how to apply it to 4 axes.

 

Thanks!

Okay, so on the "Axis Assignment" tab, I moved the axis I wanted to assign to reverse so FSUIPC would "see" it. In my case, it's the right-most of the three levers on the CH Yoke. Then in the "Type of Action" I selected the "Send Direct to FSUIPC Calibration." Below that, I checked the first box and in the drop-down selected "Reverser." Then on the "Joystick Calibration" tab, on page 7 of 11, I assigned it as you would anything else and checked the "Rev" checkbox. I started my range above the lowest possible setting to have a "null" zone.

[AlanB77L 0]

Great thanks, I'll give that a go!

[scandinavian13 5,192]

This could be why you're seeing an issue, however. Temporarily move the FSUIPC.ini file out of your modules folder and try a takeoff to see if the issue persists.

[adamant365 28]

Thanks Kyle, I actually did that while I was looking up how I set up the reversers. No change after moving the .ini out of the modules folder.

[GusRodrigues 1]

I think if you looked at performance tables for engines, you might see that the behaviour of engines cannot be simplified to "higher TAT as a result of airspeed changes = less thrust". How efficient is an engine at 65kts compared to an engine at 185kts?

 

If target thrust is achieved by 65kts and the aircraft reaches, say 185kts at 400' what is the actual TAT increase as a result of a 120kt airspeed increase minus the 3 deg C/1000' lapse rate.

 

N1 as a thrust indicator should not be undervalued. If you saw how EPR is calculated on an RB211, you would realise that a lot of "fudging" is going on, too. You have a single sample of inlet pressure which is compared with several samples of coldstream and hotstream pressures to somehow produce an EPR value consistent for all temperatures/airflows.

Every "jet engine" is as simple as "higher TAT as a result of airspeed changes = less thrust". High TAT implies there is movement, therefore, there is ram drag.

 

The Net Thrust is calculated as Gross Thrust - Ram drag, thus if you (and your engines) are stationary (i.e. flight velocity is zero), your Net thrust will be equal to the Gross Thrust. As soon as you start to move, you start to have Ram drag. The faster, the colder and denser the air, the more drag you will have.

 

I do know how RR measures EPR (a normalized comparison). But N1 is far from being a linear output of thrust.

[Qavion2 39]

I do know how RR measures EPR (a normalized comparison).

 

 

Actually, I was referring to the less-than- rocket-science of mixing/sampling air pressures in a mechanical device known as multiple connector whose accuracy can be (sometimes) affected by the number of bugs and the amount of water in it

 

A PW engine only measures pressure exiting from the core, not from the fan. How accurate can that be?

 

All I'm saying is that, whatever parameters are used, they are all modified to give the pilots a simplified version of reality which they use to operate their aircraft.

 

Cheers

John H Watson

[kevinh 409]

It has an EEC (simpler) not FADEC. The amber limit uses TAT (total) and true altitude.

An EEC is any kind of electronic engine control, including FADEC. With the GE engine if the engine rating ends in an F, e.g. B1F, B5F, it's FADEC.

 

Back on topic, this is the problem then. The engine will not achieve the FMC calculated EPR target in TO mode if the engine control is using actual TAT. In the RR and PW versions EPR remains roughly constant until HOLD changes to THR REF. Then the EPR drops by about 0.04 and is controlled to that value. At thrust reduction to CLB the EPR lines up with the reference value.

 

The question is should this error be present?

 

If you continue to climb without retracting flaps it stays in TO mode and the error between FMC target and actual EPR remains 0.04.

 

The GE version matches the FMC target N1 very closely throughout.

[JRBarrett 3,074]

Actually, I was referring to the less-than- rocket-science of mixing/sampling air pressures in a mechanical device known as multiple connector whose accuracy can be (sometimes) affected by the number of bugs and the amount of water in it

 

A PW engine only measures pressure exiting from the core, not from the fan. How accurate can that be?

 

All I'm saying is that, whatever parameters are used, they are all modified to give the pilots a simplified version of reality which they use to operate their aircraft.

 

Cheers

John H Watson

John,

 

Where does the RB211 take its inlet pressure reference for EPR? I ask, because the RR Tay engine that powers the Gulfstream IV, there is no inlet pressure sensing as such, EPR is simply calculated against static air pressure as measured by the DADC.

 

Of course the Tay is a much smaller engine than the RB211, but the core design is a scaled down version of the core found on its "big brother", and the engineering design is about the same vintage. The Tay has no EEC as such though - but there is a stand-alone auto throttle computer.

 

I know that the Tay can experience slight EPR roll-backs on takeoff in winter ops - when climbing out of a shallow layer of very cold air near the ground. Low-level temperature inversions are common in such conditions.

[fabristunt 25]

I'm still not understanding it.

 

I select a D-TO2 with an assumed temp that gives me let's say EPR 1.54.

When taking off, after the engines have stabilised, I press the toga switch and off we go. The EPR reaches 1.54 and shortly after, when passing 65-70 knots IAS the FMA shows HOLD. After takeoff it goes to THR. Now comes the bit that I can't figure out. Why are the engines spooling down (I can hear them, and the N1 reading confirms that) as soon as THR appears? The rating in green stays  at 1.54, but the throttles move back to 1.49-1.51. Why?

 

I tried to remove the fsuipc.ini, even tried without my hardware yoke and stuff. It still happens and I don't get it.

 

Any help? 

 

Regards,

Fabrizio

[Qavion2 39]

John,

 

Where does the RB211 take its inlet pressure reference for EPR? I ask, because the RR Tay engine that powers the Gulfstream IV, there is no inlet pressure sensing as such, EPR is simply calculated against static air pressure as measured by the DADC.

 

 

Tricky question. During "normal" operations, the RB211 uses engine sensors, but they are constantly validated by comparing them to the aircraft sensors. i.e. if they agree, the engine sensors are used. What I'm not sure of is whether or not this system is always in operation and what the tolerances are (i.e. How much difference is allowed between the airplane sensors and the engine sensors). If there are differences, then various comparisons are made via different paths and using different data sources.

 

I know that the Tay can experience slight EPR roll-backs on takeoff in winter ops - when climbing out of a shallow layer of very cold air near the ground. Low-level temperature inversions are common in such conditions.

 

Makes sense, but what kind of temperature drops are we talking about here?

 

What I'd like to establish are the differences between pure theory and reality. If someone has a video of 747-400 RB211 engine parameters from 65kts to 400 feet, please put up their hands    (preferable with OAT less than 30C and relatively calm conditions)...  I've scoured YouTube without success. I wouldn't expect the TAT to change more than a couple of degrees C between these two point (and a relatively insignificant thrust change).

 

Cheers

John H Watson

[JRBarrett 3,074]

 

Makes sense, but what kind of temperature drops are we talking about here?

 

Here in upstate New York in midwinter, we can get situations where there is arctic aircin place with calm winds. During the overnight hours, temperatures at the surface can drop down to -20C. (Our airport is in the lowest point of a valley). The extremely cold layer may only be a few hundred feet deep, with temps perhaps -5 C just above the inversion layer.

 

I don't know whether the OP was using any external weather engine. If so, perhaps try a takeoff with the default "Clear weather theme?

[Andrej 1]

 

 

I don't know whether the OP was using any external weather engine.

 

As per his post (first page):
 

I didn't use any weather add ons and the OAT stayed at +15 the whole time.

 

Cheers,
Andrej Lippay