Inertia or drag?

[Spirale]

Hello,

In a mission, long transit, my Viggen is set to fly level @15k ( 4500m). I fly "buster" 100% dry, no pc at all.

My payload: 8 xM71HD + U22A-KB + 2 x RB74 + Fuel Tank

I noticed this: I leveled A15, speed was mach 0.6. Then my Viggen begun to regain a bit a speed but was unable to go higher than mach 0.72 and it was very very very slow and it took a bunch amount of time to reach mach 0.72.

Then this: i cranked stage 1 PC to reach the exact mach 0.80 then i cut the PC and return "Buster" 100% dry. If this speed is reached my Viggen doesn' t loose any speed/ energy and STAY at mach 0.80 with no PC, and even "accelerate" to reach mach 0.81 and keep that speed (again no PC) .

I assume my title is perhaps dubious but is that behavior accurate? The Viggen being heavy, is its inertia the cause of this phenomenon? Or is everything is normal?

Thx

Edited May 8, 2023 by Spirale

[renhanxue]

I don't know how well modeled it is in DCS (probably not very, tbh), but on the real aircraft there's a weird phenomenon with max dry thrust; it decreases slowly with increasing airspeed, but between M0.7 and M.08 the trend reverses and it very slightly increases again (then it's more or less constant up to M0.9 and then it falls off a cliff in the transonic region). The effect is quite subtle though, and I'm kind of doubtful it's sufficient to explain what you're seeing (note how much the drag increases in the same region, for example). It's hard to make out in the charts in the AJ 37 SFI, but you can see it in the JA 37 SFI (note chart is for zero altitude):

 

Edited May 8, 2023 by renhanxue

[Spirale]

Awesome Renhanxue

Can i conclude that what we have in DCS for the Viggen is credible?

I think it's very ( too exagerrated) , a little bit weird difference between mach 0.72 stopped and after a PC1 thrust to reach mach 0.80 then 100% dry which no longer decrease.

Thx again for this very interesting chart.

Edited May 8, 2023 by Spirale

[TOViper]

A little help here:
Look at the thrust equation: Thrust equals Mass flow times (exhaust gas velocity minus true airspeed)
The problem is: the faster you go, the less thrust you have (actually forgetting about RAM effect).

The RAM effect gives you a certain amount of re-compression of the air when it hits the inlet (IIRC goes with a quarter times (1+Machnumber)^2), thus the thrust minimum at medium Machnumbers is preventing further acceleration due to already large aircraft drag (increased by a2g loadout). Once the engine "feels" enough RAM re-compression at higher subsonic Machnumbers, the thrust might then be enough to compensate for the higher drag at higher Machnumbers. It now STRONGLY depends on how high the aircraft drag really is (very difficult to predict, must be measured during flight test) and how good the re-compression of the inlet works. Obviously, in DCS Viggen these two values (total drag versus thrust) are the same at M0.8 for that configuration which you figured out.

Edited May 8, 2023 by TOViper