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Posted (edited)

I first noticed this while doing a speed test on the AO server, but the attached track and .acmi files confirm its existence in SP.

 

At high mach numbers the Su-27 has an insane roll rate.

 

Test conditions: standard DCS day, no wind, no precipitation. I'm running the latest version of DCS as of 14/05/2018 with no mods installed.

 

See the attached track: I was at 12,500m altitude (as per HUD, not F2 view) and roughly 2.52M. I instigated a full left-stick roll. Seconds later at the 11:53 minute mark I was at 227.3 degrees per second roll rate, at 12,495m altitude though by this point I'd slowed to 2.34M.

 

The extreme roll rate dies down below about 2.2M though it's much higher than it probably ought to be at that sort of speed.

 

The manual says that the Su-27 is supposed to achieve a maximum roll rate of 180 degrees / second at approximately 700 Km/h for a clean configuration, not at 2,490 Km/h!

 

Edited to add: please excuse my poor control, I think my 12 year old joystick is finally wearing out :(

Su-27 Roll rate test ACMI.zip

Su-27 Roll rate test track.zip

Edited by DarkFire

System Spec: Cooler Master Cosmos C700P Black Edition case. | AMD 5950X CPU | MSI RTX-3090 GPU | 32GB HyperX Predator PC4000 RAM | | TM Warthog stick & throttle | TrackIR 5 | Samsung 980 Pro NVMe 4 SSD 1TB (boot) | Samsung 870 QVO SSD 4TB (games) | Windows 10 Pro 64-bit.

 

Personal wish list: DCS: Su-27SM & DCS: Avro Vulcan.

Posted
The first thing jumping out at me is an Su-27 shouldn’t be able to reach Mach 2.52. Unless I’m misinformed.

 

No, you're entirely correct. IIRC the Su-27 should top out at about M2.35. The situation was worse in the days of DCS 2.2.x when the Su-27 would go all the way up to M2.62, or 2,770 Km/h indicated TAS.

 

That being said, even if our Su-27 was properly modelled to max out at M2.35 it would still exhibit the extreme roll rate, which was really the point of my bug report as the roll rate behaviour was not observed in previous versions of DCS (before 2.5).

System Spec: Cooler Master Cosmos C700P Black Edition case. | AMD 5950X CPU | MSI RTX-3090 GPU | 32GB HyperX Predator PC4000 RAM | | TM Warthog stick & throttle | TrackIR 5 | Samsung 980 Pro NVMe 4 SSD 1TB (boot) | Samsung 870 QVO SSD 4TB (games) | Windows 10 Pro 64-bit.

 

Personal wish list: DCS: Su-27SM & DCS: Avro Vulcan.

Posted

The speed considered should be TAS speed, not GS so roll rate should increase with air density. Also factors to be considered regarding roll rate in respect to TAS are inertial matrix or weight distribution meaning that roll rate is lower if wing tanks are full.

Posted
The speed considered should be TAS speed, not GS so roll rate should increase with air density. Also factors to be considered regarding roll rate in respect to TAS are inertial matrix or weight distribution meaning that roll rate is lower if wing tanks are full.

 

For a standard DCS day M2.52 at 12,500m altitude is roughly 2,700 Km/h true air speed.

 

My fuel was pretty low by that point (under 3,000Kg I believe) indicating that the wing tanks probably were empty.

System Spec: Cooler Master Cosmos C700P Black Edition case. | AMD 5950X CPU | MSI RTX-3090 GPU | 32GB HyperX Predator PC4000 RAM | | TM Warthog stick & throttle | TrackIR 5 | Samsung 980 Pro NVMe 4 SSD 1TB (boot) | Samsung 870 QVO SSD 4TB (games) | Windows 10 Pro 64-bit.

 

Personal wish list: DCS: Su-27SM & DCS: Avro Vulcan.

Posted (edited)

Maximum roll rate is dependent on TAS and helix angle the ailerons (and other control surfaces) balance out at. Aileron movement causes the wingtip to be at different aoa in regards to local airflow and that aoa will generate rolling force. Roll rate causes shift in local aoa due to roll and the roll acceleration stops when the local aoa is zeroed. Essentially helix angle is function of aileron deflection. The faster you fly through air, the faster the wingtip will move as it follows along the helix. Inertia and air density only affect roll acceleration (along with EAS), ie. at high altitude or slow speed the plane seems to have more inertia as the aerodynamic force is less.

 

Non-hydraulically boosted planes have a roll rate maximum somewhere below max speed as the control force grows too great for the pilot to move the ailerons and achieve a helix angle that would maintain the roll rate. Hydraulically assisted planes also tend to have limitations due to control forces. Modern fighters have roll rate limiters that limit the maximum roll rate to keep it at controllable levels and also prevent excess aerodynamic force on the ailerons. At high altitude you can have very high speed with relatively low aerodynamic forces acting on aileron so you can have a relatively steep helix angle in conjunction with high speed and hence stupendous roll rates if it's not limited somehow.

Edited by Bushmanni

DCS Finland: Suomalainen DCS yhteisö -- Finnish DCS community

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SF Squadron

Posted (edited)

Respectfully, I'm aware of the theoretical considerations. The attached is from page 114 of the Su-27 flight manual, describing the function of the ACS and roll rate. The ACS does artificially limit roll rate at higher speeds to limit stress on the rear portion of the airframe. At no speed / altitude should it reach 227 degrees / second.

1593419571_Page114.jpg.4532413765869bd5c724b5c0c01c86fb.jpg

Edited by DarkFire

System Spec: Cooler Master Cosmos C700P Black Edition case. | AMD 5950X CPU | MSI RTX-3090 GPU | 32GB HyperX Predator PC4000 RAM | | TM Warthog stick & throttle | TrackIR 5 | Samsung 980 Pro NVMe 4 SSD 1TB (boot) | Samsung 870 QVO SSD 4TB (games) | Windows 10 Pro 64-bit.

 

Personal wish list: DCS: Su-27SM & DCS: Avro Vulcan.

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