Engine performance without afterburner

[Lynnux]

Dear Heatblur,

 

is this the realistic behaviour of the Viggen ?

1) Take-off without afterburner

The Viggen can barely take-off with one external tank and 4 RB-74s. The landing gear has to be retracted immediately and the plane has to fly straight in a very, very shallow climb for some time to gain speed

=> Possible with light ordnance but not really feasible, afterburner needed.

 

2) Low-level flight with gear down

Ordnance: 2 RB-15s, one external tank, 2 RB-74

Level flight on autopilot 390 m above sealevel

Switched afterburner off after accelerating 700..800 kph. The plane loses speed and will finally crash.

Repeated after firing the two RB-15s and emptying the external fueltank (without jettisoning it): same behaviour.

=> No level flight possible at low altitude without afterburner and with extended landing gear and some ordnance.

 

3) On some Persian Gulf maps, low level flight (below 500 m)

Same as 2) but with landing gear up: plane slows down without afterburner

=> Basically the Viggen cannot fly low level without afterburner on some Persian Gulf maps, this is only possible at altitudes above roughly 1000m

Update: Tried 3 Persian Gulf maps, didn't have this anymore.

 

Military power of the RM8A is 64 kN.

It's clear to me that delta wing aircraft need a lot of power because of the drag at high AoA but the Viggen has large wings and AoA at low speeds is still less than 10°. Seems to me that the landing gear causes a huge amount of drag but that seems to be not the only component (see Persian Gulf maps).

Edited June 5, 2021 by Lynnux

[TOViper]

I was always wondering about the low performance with gear down @MIL power.

At least, I am not alone with my thoughts ...

 

What was the temperature, altitude and QNH during your tests?

I would like to test this too, and maybe you can post the mission file ...
 

 

Edited June 4, 2021 by TOViper

[Lynnux]

Temperature: +15°C

QNH: 755 / 29.72

Altitude: 390 m (1280 feet) in case of 2)

[frosen]

SMEs (IRL Viggen pilots, I remember Hilmersby and another on Youtube but probably more) has said several times that the DCS Viggen engine is too weak on mil power. They raised specific situations where they never needed to use the AB but it's necessary in DCS for example. 

[Lynnux]

Ok, thanks for the answer. But that means the Viggen will be in great trouble if the afterburner fails at take-off and also touch and go without AB may be hard, depending on the load. But AB shouldn't be so prone to failure, at least not in a sim.

 

Concerning the Persian Gulf maps: I tried this again and it seems that is fixed. I remember that on maps with this problem the engine RPM was going up and down on idle RPM, obviously engine control had some problems on those maps. Not anymore...

Edited June 5, 2021 by Lynnux

[TOViper]

On 6/5/2021 at 2:38 AM, Lynnux said:

Temperature: +15°C

QNH: 755 / 29.72

Altitude: 390 m (1280 feet) in case of 2)

I did a quick test, I managed to fly not below 330km/h with a heavier configuration than yours (after firing the RB-15), see track file (OB 2.7.1.71390).

There must have been something wrong with your configuration beeing not able to keep the aircraft in the air, or it is too long ago.

Maybe a (or a few) DCS Update(s) already fixed that?

 

2021-06-06_Viggen_GearDown_Rb-05_Dispenser_U22A_Rb24.trk

Edited June 6, 2021 by TOViper

[Sideburns]

14 hours ago, TOViper said:

I did a quick test, I managed to fly not below 330km/h with a heavier configuration than yours (after firing the RB-15), see track file (OB 2.7.1.71390).

There must have been something wrong with your configuration beeing not able to keep the aircraft in the air, or it is too long ago.

Maybe a (or a few) DCS Update(s) already fixed that?

 

2021-06-06_Viggen_GearDown_Rb-05_Dispenser_U22A_Rb24.trk 36.04 kB · 1 download

 

 

There also seems to be a bug atm with negative drag values after weapon release, so making such measurements after a weapons release could be a bugged result atm. (This is certainly true of Rb24/24j/74 and tanks, if you test the top speed with a clean Viggen vs a post Sidewinder launch / tank jettison you will see). Or alternatively there is information in this thread.

 

Long story short it does seem like the Viggen thrust / airframe drag / weapon drag situation could do with a review.

 

 

[gachatar]

Yeah, it's pretty crazy that if I want to do some corrections during landing with the gears down I often need to go at least stage 2 afterburner to not fall out of the sky.

[outbaxx]

Ok, thanks for the answer. But that means the Viggen will be in great trouble if the afterburner fails at take-off and also touch and go without AB may be hard, depending on the load. But AB shouldn't be so prone to failure, at least not in a sim.

Yes, the charts I’ve seen show a max weight of 17 tons for Mil power take offs. Clean AC with x-tank weighs 16433kg, that doesn’t give much room for weapons.
It show that on a day with 25C air temp, if your AB fails under 300kmph you will not have enough power for level flight.

The aerodynamic compendiums and charts also say/show that if your approach speed is low compared to your landing weight you have very little room for corrections before the drag will overcome the thrust and you will have to use AB to recover.
If I read it correct, the max weight for a 15.5 aoa landing is 14000kg when air temp is -30C, at +20 the max weight for 15.5 is 12.5-13000kg depending on air pressure, to have enough power to accelerate with full Mil power.
12 aoa, +20C max weight about 14kkg to be able to accelerate with full Mil.

[Lynnux]

On 6/6/2021 at 1:26 AM, TOViper said:

Maybe a (or a few) DCS Update(s) already fixed that?

1) and 2) I've tested it shortly before I created this thread, gulf maps I retested later. And tested again now on the MP server with "your" loadout (2 RB-05, 2x RB-15, X-tank): Still not possible to fly on 100% mil with gear down.

Strangely, the speed stayed at 300 kph for some time while AoA went up from 10° to 12° then it decreased further.

After firing the 2 RB-15s I flew home relatively high (540 m) and the speed stayed at ca. 370 kph. So at least this time it was better after getting rid of some ordnance and fuel.

 

But I assume also from outbaxx' reply that this is a somewhat normal behaviour then.

Edited June 8, 2021 by Lynnux

[IronMike]

Tracked for further investigation. Thank you.