AoA vane Mig-23

[Timothy Quinn]

Will some kind person enlighten me please re an AoA vane placed under the forward section of a Mig 23 which I saw last Friday in Finow Airfield open air museum.Is it's purpose to determine crab angle and/or wind direction whilst descending along a radio navigational aid.I only ever saw such vanes placed at the side of an aircraft and used for AoA infromation.

[Clandestino]

It's sideslip vane. I suppose you've seen it on BM, strike version, as interceptor versions have it on the upper portion of the nose. It can't be used to determine crab angle, as crab angle is relative to ground and even as you're crabbing, you're probably flying straight through airmass and your sideslip angle is likely to be zero.

Sorry, I can't help with what was its exact purpose on MiG-23 i.e. which system used it. AFAIK, apart from slip ball there is no other sideslip indication in Flogger's cockpit. Hopefully, some MiG 23 pilot will shed some light on it.

[Timothy Quinn]

Thank you,it was the BN version only which has that vane.

[Clandestino]

Sorry about typo, it's BN, not BM. Some versions don't have it at all, but it can be seen on this ML (http://www.airliners.net/photo/Poland---Air/Mikoyan-Gurevich-MiG-23ML/1921520/L/&sid=b82623cc9c75aeada50c6263129d9703) and this UB. (http://www.airliners.net/photo/Hungary---Air/Mikoyan-Gurevich-Mig-23UB/1970621/L/&sid=b82623cc9c75aeada50c6263129d9703) IIRC, UB displayed at Finow has box-shaped cover over it.

[Bevo]

I believe you will find that the “AoA vane” is actually an antenna for the RSBN (Radiosistema Blizhney Navigatsii) ILS which was the Soviet TACAN analogue.

[Timothy Quinn]

Bevo
If you can tell me how to post an image here,you will certainly not see an antenna!
I have two images here which clearly show what seems a conventional bronze type movable vane mounted on the horizontal plane.

[Brian Abraham]

Here's your photo Tim. No idea what it's for either.

http://www.b-domke.de/AviationImages/Flogger/Images/MiG-23BN_20+55_24957.jpg

Yaw vane, static pressure port, dynamic pressure probe, 'Odd Rod' IFF aerial, second static pressure port, TAT (total air temperature) probe, and ventral doppler antenna (grey panel)

Whether this patent hints at the reason I know not. Aircraft control system - Safe Flight Instrument Corporation (http://www.freepatentsonline.com/5590853.html)

Side mounting of the angle of attack vane may lead to a problem when the aircraft is flown in yawed flight. As used herein, the term "yaw" refers to the condition of an aircraft when the longitudinal axis of the aircraft is angled to the left or right of the direction of flight. Thus, the yaw angle or side slip angle refers to the angular difference between the aircraft's heading and the aircraft's direction offlight. Under such circumstances, i.e., when an aircraft is flown in a yawed condition, the position of the angle of attack sensing vane will be determined by the local air flow passing the vane. The angle of attack vanes are usually located so that the changes in the local air flow at the vane cause the vane to pivot in accordance with the angle of attack. The local air flow at these fuselage locations are determined to a lesser extent by the yaw of the aircraft so that the indicated angle ofattack is actually a function of both the angle of attack and the yaw.

In order to overcome the problem associated with yaw, a yaw vane has been used to supply a correction factor. A yaw vane is a movable vane, similar to an angle of attack vane, but which is typically mounted on the outside of the fuselage along the aircraft's centerline. The vane is mounted for rotation about the aircraft's vertical axis so as to align with the airflow past the aircraft. Thus, the change in position of the vane when the aircraft enters a yaw condition is proportional to the sideslip angle.

[Bevo]

Timothy Quinn
I have two images here which clearly show what seems a conventional bronze type movable vane mounted on the horizontal plane. Yes you are correct – looks like a yaw vane. I have never seen one on any for the MiG-23/27 aircraft I was familiar with. No idea what its use would be unless Brian is correct and it is part of some experimental flight control system. Given the departure characteristics in the MiG-23/27 series it would not seem that it would feed a cockpit instrument since during moderate/heavy maneuvering flight, by the time you see the beta angle increase the aircraft would probably have already departed. I wonder if this is a test aircraft.

[Brian Abraham]

Not a test vehicle Bevo. The yaw vane is a common feature on the 23 (may depend on model, not checked) and some models have it located on the upper surface just in front of the windsceen.

[Bevo]

Interesting. Unless it was added starting in the early 90s it was not on the MiG-23MF (Flogger-B), MiG-23MS (Flogger-E), or the MiG-23MLA (Flogger-G). It may have been on the original MiG-23MLD (Flogger-K) which was by far the best of the series.

[Brian Abraham]

Have dug up photos of the MF, BN, ML, MLA, MLD, U and UB which all feature the yaw vane. All except the BN have it on the upper surface just aft of the radar dielectric. Other models can not find info.

[Timothy Quinn]

Everybody,I am not an aviator but I do understand the basics though!That vane has to be for a fire control and or targetting mechanism.No way is it required for flight and I thank all of you for the information.And Bevo,that is almost exactly my image too from Finow!Great minds etc....

[Bevo]

Found this information:
Early SAU-23 (autopilot) version feed two signals into the yaw channel - damping and stability. The second was feed in by a side slip sensor (DUS-3) right ahead of the cockpit. This was used to wash out side slip in horizontal flight only. It was disengaged at more than 6? roll angle automatically.

The ARI I was talking about was added with later SAU-23A versions. Beside the damping and stability (this time not yaw angle but yaw acceleration) the ARI input was fed into the yaw channel. This third input/signal was formed by pilot's stick motion and angular rate and angular acceleration of all axes (and dependent from wing sweep angle). It was automatically engaged above 24? AoA.
May have been removed on some of the aircraft I am familiar with as the the auto-pilot was not used.

[Brian Abraham]

Tim, I asked your question on the Military thread and the answer was
That is the yaw sensor for the auto stabilization system for the BN. Without auto stabilization, it was flyable but barely.

In Steve Davies "Red Eagles" all the USAF pilots hated the Mig-23BN and thought it was trying to kill them. That may be the result of the auto stabilization not working to spec!

Even with the stabilization on, at high IAS it would tend to dutch roll, but could go to a very high speed at low level (I remember 800 - 1000 kts being bandied about) while banging your helmet on the canopy from side to side. It could also sustain 7G in a turn all day long!

The Mig-23BN was a bastard child of the Mig-23MS and the Mig-27 - and those didn't have the same issues (just different ones).
You'll also find the yaw vanes on the SAAB Gripen, SU-25 and Mig-21 (not an exhaustive list, just ones I know of).

[Timothy Quinn]

Thanks Brian.