Simviation Forum - High altitude performance

Aug 22^nd, 2004 at 10:23am

edzmen Offline
Colonel
United Kingdom

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Posts: 537

Hi there!

Im trying to alter the CFG file settings for an add-on MD Phantom jet that im using.
The problem is it seems to loose power and thrust at high altitude, which isnt accurate to 'the reall thing'.

I have noted its current performance characteristics below:

The jet seems to perform best at low altitude like 10-17th ft. At these heights it maintains around 650-700kts fine.

However, if i want to take it up to 35th ft and above - [max service alt is around 50th ft-] the power in the engine drops off rapidly and im lucky to get 430kts out of it in level flight!!!

Now, the real Phantom is quoted to cruise at around 600kts at 45th ft - mine wont even do that at 20th ft!!

What do i have to change in the .air or CFG file to alter this?

Also, will it effect the handling performance at low level?

Hope you can help, thanks!
Ed

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Reply #1 - Aug 22^nd, 2004 at 2:56pm

JerryH Offline
Colonel
Bellevue, WA USA

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Posts: 155

edzmen,

I know little about the Phantom, but I think it may be operating correctly. It looks to me more like a question of Indicated Air Speed (IAS) versus True Air Speed (TAS). The pilot flies the aircraft using IAS from the Airspeed Indicator, but the performance specs are quoted as TAS.

Your first example of 430 knots IAS at 35,000 feet would result in a TAS of about 680 knots.

The next example of 600 knots cruise speed (TAS) at 45,000 feet would show a cockpit reading of about 300 knots IAS.

The Airspeed Indicator measures the pressure of the air molecules being rammed into the pitot tube. As the aircraft climbs, it's actual airspeed (TAS) must increase in order to compensate for the thinner air. Some FS2002 panels display TAS (often incorrectly). The GPS unit seems to be correct with its ground speed value, which is the same as TAS in a zero-wind condition.

This subject gets discussed periodically in the forums, but it is often misunderstood. If any of the above is new to you, I suggest a Google search to get some good technical references. If all of this is well-known to you, then the Phantom has a problem and others may be able to help you with the .cfg file.

Regards,
JerryH

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Reply #2 - Aug 23^rd, 2004 at 8:54am

edzmen Offline
Colonel
United Kingdom

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Posts: 537

Thanks for your help Jerry, that's cleared things up for me!

The IAS gauge in the cockpit must be wrong. It seems a problem though, as most FS2002 aircraft ive flow have IAS that decrease rapidly at height.

If anyone knows of an accurate IAS gauge that could be really usefull for loads of FS2002 jets!

On final question:-
In the cockpit of my Phantom there is a light gauge that glows bright indicating full thrust engine power, and gradually gets fainter as the power decreases.

At full power the light remains brightest between 1,000 and 19,000 feet. But anything above that and the light dims or goes out - the engine pitch also drops even though full power is on.
Is this because of the thinner air causing a drop in engine performace- or is it a engine probelm in the Phantom?

Regards,
Ed

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Reply #3 - Aug 23^rd, 2004 at 9:42am

IanK Offline
Colonel
Honey, where'd you park
my Harrier?

Posts: 124

Hello Ed,
to calibrate your ASI have alook at my ISA tables which allow you to look up airspeeds and MNs at all speeds and altitudes.
It is posted at:
http://perso.wanadoo.fr/hsors/FS_Soft/fsdocs.html

If the meters are correct then the performance is wrong and the FD author should have to fix it. You or they should edit the .air file with AirEd or Karl's tool.

It is usually due to wrong thrusts and drags, but playing with drags will alter glide/landing performance.

Ian

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Reply #4 - Aug 23^rd, 2004 at 2:00pm

JerryH Offline
Colonel
Bellevue, WA USA

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Posts: 155

Ed,

I'm getting more curious now about the flight characteristics of your Phantom.

If you will give me the filename and/or website where you downloaded the Phantom, I'll install it and check it out.

This bird will add some zing to my hangar full of DC-3s and B707s.

Regards,
JerryH

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Reply #5 - Aug 23^rd, 2004 at 5:07pm

OTTOL Offline
Colonel
Fintas, Kuwait (OKBK)

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Posts: 918

Quote:

Thanks for your help Jerry, that's cleared things up for me!

The IAS gauge in the cockpit must be wrong. It seems a problem though, as most FS2002 aircraft ive flow have IAS that decrease rapidly at height.

If anyone knows of an accurate IAS gauge that could be really usefull for loads of FS2002 jets!

You guys sound like an aviation version of "who's on first!" JerryH analyzed your problem correctly in the first post but seemed to miss it entirely in the second.

In actual conditions, INDICATED airspeed DOES decrease with altitude and a PROPERLY indicating A/S indicator will show IAS not TAS.
You do not use IAS at high altitude, you use percentage of Mach.
In real world flying conditions the only thing that you would use IAS for is performance planning and ATC planning and operations.
The only TRUE indication at these altitudes will come from your GPS(TAS +/- head or tailwind(which is groundspeed)), or if your aircraft has an ADC(air data computer).

« Last Edit: Aug 23^rd, 2004 at 10:55pm by OTTOL »

.....so I loaded up the plane and moved to Middle-EEEE..........OIL..that is......

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