Lack of inlet spill-over drag

[Bear21]

I guess the DCS aerodynamic model does not model inlet spill-over drag. Inlet spill-over drag is felt the most when flying in formation or in dogfights where it acts as an air brake when throttling back the engine at speed.

 

The physics is intakes are a tricky part of the jet design. They are too small at takeoff and low speed and to large at high speed. At high speed, they are ok when the engine swallows the air at full or almost full throttle, throttle back and the inlet chokes and start pushing air over the inlet lips to the outside. This causes air separation around the inlet lips and quite some drag. You feel it like a small air brake when going to idle in formation flying and you hear the buzz from the separation. It helps you with the stopping so you don't sail past.

 

In DCS you need to use the speed brakes more than normal as this brake effect at throttling down is missing (it seems). The thrust goes to flight idle but there is still residue thrust and the slowing down of the airframe is sloooow. This is not so in real life, you hear the inlet buzz, feel a slight buffet and the slowing down is noticeable. For most inlet/engine combinations the residue thrust+inlet drag at flight idle at normal speeds is negative.

 

It's not too complicated to complement the model with it if it's missing. I post it here as the F16 is the last ED aircraft.

Edited January 18, 2020 by Bear21

[Bouli306]

Flew quite a few times backseats in the Viper. Never noticed this and also a clean Viper is hard to slow down. No inlet buzz at all.

[Bear21]

You need to pilot the ship

 

Flew quite a few times backseats in the Viper. Never noticed this and also a clean Viper is hard to slow down. No inlet buzz at all.

 

You have to have piloted different jets in formation flying to feel the lack of flight idle drag in this model, you can't feel it as a backseater (overshoot when joining a formation is standard, the join is harder than flying in formation, this is why you initially learn to join in a curved path as you then can regulate your closing speed with the turn radius).

 

I performance model jets as my day job and know first hand you need to model spill over drag when you throttle down to flight idle at speed. Dependent on speed/altitude you are close to zero net thrust or at negative thrust from the engine/inlet combination. If you model with flight idle thrust you get the wrong total thrust-drag balance on the jet. You see it in your payload-range numbers and feel it in formation flying.

 

I now fly with a little bit of speed brake when in formation, this improves the feel.

Edited January 28, 2020 by Bear21

[Airhunter]

Unless you have piloted a GE F110 F-16 yourself you have no real reference to base your opinion on. Every jet is different, every jet engine has different characteristics in spool up/down and total flight idle drag. It isn't as simple as Jet X does this so jet Y and Z should do the same as well.

[Bouli306]

You have to have piloted different jets in formation flying to feel the lack of flight idle drag in this model, you can't feel it as a backseater (overshoot when joining a formation is standard, the join is harder than flying in formation, this is why you initially learn to join in a curved path as you then can regulate your closing speed with the turn radius).

 

I performance model jets as my day job and know first hand you need to model spill over drag when you throttle down to flight idle at speed. Dependent on speed/altitude you are close to zero net thrust or at negative thrust from the engine/inlet combination. If you model with flight idle thrust you get the wrong total thrust-drag balance on the jet. You see it in your payload-range numbers and feel it in formation flying.

 

I now fly with a little bit of speed brake when in formation, this improves the feel.

 

When i flew backseats i was actually flying. I got about 350 hours of real time flying on the Viper, because i am a flight sim instructor on the F-16 (with a PW-220(e) by the way) i needed to fly backseats, i flew formations, IFR patterns and more.

 

No noticable inlet drag. There probably is some but to my experience i feel formation flying quite the same as in real life. Although i do not have experience with the GE engine

[Bear21]

Unless you have piloted a GE F110 F-16 yourself you have no real reference to base your opinion on. Every jet is different, every jet engine has different characteristics in spool up/down and total flight idle drag. It isn't as simple as Jet X does this so jet Y and Z should do the same as well.

 

As I stated I have not flown the F16, either with PW or GE engine. Re your statement on inlet drag, are you saying this from detailed knowledge (I do, I work with engine modeling including inlets) or from an overall assumption?

Edited January 28, 2020 by Bear21

[Bear21]

When i flew backseats i was actually flying. I got about 350 hours of real time flying on the Viper, because i am a flight sim instructor on the F-16 (with a PW-220(e) by the way) i needed to fly backseats, i flew formations, IFR patterns and more.

 

No noticable inlet drag. There probably is some but to my experience i feel formation flying quite the same as in real life. Although i do not have experience with the GE engine

 

Fair enough. If the feel is the same then it's ok. Thanks.

[Bear21]

Here some data re the F110-129 installed in an F16 from a professional engine design tool. ED should have something similar for their engine modeling:

 

20kft, M0.8

Max Military thrust ~34kN/7,600lbf. Engine airflow at fan face 77kg/s, intake capture flow (Big gulp as 0.65m2, speed 252m/s, air density 0.65kg/m3) 107kg/s. Flight idle at 64% N1 (from cockpit RPM gauge), thrust -2kN/-440lbf, engine fan face flow 22kg/s.

 

35kft, M0.8

Max Military thrust ~23kN/5,200lbf. Engine airflow at fan face 48kg/s, intake capture flow (Big gulp as 0.65m2, speed 237m/s, air density 0.38kg/m3) 59kg/s. Flight idle at 70% N1 (from cockpit RPM gauge) 0kN/0lbf, engine fan face flow 17kg/s.

 

This illustrates a number of things:

 

1. A fixed intake is a BIG compromise. You have to size it big enough for takeoff, then it's too large at high speed, better at lower speed. Worse at dense air (low altitude), better at thinner air = high altitude.

 

2. It's optimized for max military thrust. Less thrust has more spill down to a max spill at flight idle. Afterburner has the same max airflow as max military so it doesn't change this for the same speed/altitude (it burns the remaining oxygen in the bypass/core flow to increase specific thrust with the same mass flow = increased thrust).

 

3. The negative thrust at Flight idle is not due to intake spill drag (this drag is not included in these tools, it's counted as an airframe drag). The effect seen is the inlet momentum drag also called ramdrag. It comes from a momentum loss when the diffusor part of the inlet slows the air to a fan face airspeed of ~M0.5 (engines work at a semi-fixed Mach, most parts at around M0.5, with the slowest part the burner at M0.2).

 

4. So on top of the net thrust, we shall add two additional inlet drags. A, the virtual increase of the inlet frontal area from the spill that results in additional pressure drag and B, any interference drag from any separation around the lips. This drags increases with increased spill such as at Flight idle when at speed.

 

I don't put it here to prove something, I do this stuff for a living so might give something back as DCS gives me a lot.

Edited January 29, 2020 by Bear21