Jet Aircraft Engine Idle Behavior

[agnitfom2000]

One of the things I see in FSX is that jet aircraft tend to accelerate on the runway even with the throttle at idle.  The example I tested went from 0 to 54 knots on 13,000 feet of runway.  Even more extreme than my old car with the idle screw set to the max! Everything in the model set-up was as realistic as possible.  Drag scalars and the jet engine thrust scalar were all set to 1.

 

My question for any jet pilots in this forum is whether this behavior is realistic, or is the jet engine performance model in FSX producing too much thrust at idle?  If it is realistic, I'll leave things alone.  If not, I'll do some experimenting to try and minimize the effect.

 

Thanks

David Allan

[mrzippy]

What jet aircraft are you talking about?

[PhrogPhlyer]

1 hour ago, agnitfom2000 said:

... is ... this behavior ... realistic

It depends on the aircraft.

A B-777 produces about 3.6% thrust at idle. Not enough to get it rolling, but once moving, enough to taxi.

A Cessna Citation can accelerate at idle, but not enough to rotate T/O. 

An F-15 produces a flight idle thrust is about 1200 lbs, with ground idle at 900 lbs. With an empty weight of 18,900 lb thrust is 4.7% of the aircraft weight. 

I would suggest you read up on the specific RL aircraft type, and then see if this is realistic or not.

Here is an article about the various engine ratings for a jet. https://en.wikibooks.org/wiki/Jet_Propulsion/Engine_ratings

 

[g5flyer]

Well. First thing to note is that jet aircraft behavior all depends on various variables to include thrust to weight ratios. There is also the ground physics behavior that becomes in an issue in aircraft. One thing that is missing in most sims is the tire behavior. When an aircraft is at rest, there is some power required to get it moving. You have the pressure from the tire to over come along with regular ground fiction to over come. The first time you move a jet, you will have to overcome the tire and the friction. A tire will tend to keep that flat spot that it has been resting on. Even when I flew DC10s, coming out of parking spots, you will get that square tire feeling once you get her moving. You will feel that clunk, clunk feeling until those flat spots work their way out. The ground fiction will always be there, especially when you come to a stop and have to get moving again. That initial movement power setting is called break away thrust. You apply it and it will take a moment before she starts moving. Even when you come to a stop, you will have to add power to get going and require less once moving. Here is a good example of the jet I fly now.

 

The G550 has great thrust to weight ratio. Coming out of parking, you have to use a little power to get going. Once she starts moving, she will accelerate at idle thrust. So we use thrust reversers during taxi. So, with no reversers, she will accel. Once you reach the speed you want, you pop one reverser to idle to maintain your current speed. When you want to slow, such as coming up on a turn, you put both reversers to idle and she will slow down. Once at turning speed, you stow the reversers and put one back out once back to taxi speed. Done right, and you will never have to touch the brakes unless coming to a full stop. Most jets will accelerate when light, but require a little power when heavy. 

[agnitfom2000]

Unfortunately, the aircraft I'm trying to model (the XB-68, essentially a 3 engine version of the B-58) is a historical paper study so it has very minimal information.  The saving grace is that it does have a fairly common engine, the J-79.  I'll try searching the flight manuals of some aircraft with this engine installed to try and find the flight idle thrust.  I just got hold of the F-4J NATOPS so I will start there.

 

From the experience with the G550, this behavior on the aircraft I'm modeling is at least plausible.  My industry background is materials, structures, making high temperature components out of exotic alloys, and program management.  I have much less experience on overall aircraft performance, so I really appreciate the assist from both of you.

[agnitfom2000]

Actually meant ground idle thrust.  Found some plots for the J-79 that at sea level showed about 490 lb.  At 3 engines around 1,500 lb.; aircraft has a typical take-off weight of 80,000 lb. so at about 1.9%.