Aerodynamic cause of the Wing-rock effect?

[Voyager]

To preface, I understand that wing rock is induced by high AoA flight, from Victory 205's F-14 Practical Handling Tips, and having gotten myself into it on my first flight (apparently a 1980's jet interceptor has a higher landing speed than 1940's P-47. Who'd have thunk?*)

 

What I'm trying to understand is why it does that when its at high AoA's. My intuitive expectation when a plane starts to go over like that is that its about to enter a spin, rather than stop and go the other direction. The Tips pdf mentions there is an interaction with vorticies coming off of the intake ducts. Is this a case of the vorticies only disrupt the vertical stabilizers when they are in the plane of travel**, and when they are out, the aircraft has enough natural centering force to drive them back into it? I expect I've pureed the terminology; my background is not in aerodynamics.

 

Is the causing mechanic anything like what caused Sabre-dance? Or are the results just visually similar?

 

Thank you,

 

Harry Voyager

 

 

*That was pseudo humor, partly. I've mostly flown WWII eras aircraft, so in the landing pattern, I was looking for my normal approach rates. It did end in a successful landing, and, as far as I could tell, I didn't brake the airplane, even without knowing about the spoilers or anti-skid brakes....

 

**plane of travel in this context is being used to describe the plane defined by the Aircraft Datum Line and the Vector of Travel.

Edited April 21, 2019 by Voyager

[McTschegsn]

AFAIK:

 

Wing rocking, more to speak aircraft rocking itsself can have various reasones:

 

One is caused by temporal loss of laminar airflow over various ares of the wing basically.

Laminar airflow lost -> loss of lift and slight dropping of nose (normally)

Laminar airflow recovered due to slight nose down attitude -> lift is restored -> plane rocks

 

Other causes of rocking are: turbulences/vortices caused by the plane itself hitting tail surfaces and so on.

You also experience slight rocking / vibrating when approaching the sopund barrier for example causend from air compressions.

[ChockP51]

From what I gather so far, wing rock is a dynamic process.

 

To start with, you need a slight roll rate in one direction. This roll rate will influence the vortex location (this mechanism is similar to a flow past a rotating cylinder). The vortex will now be slightly asymmetric but it wil generate a restoring roll moment. Ideally this should damp the roll rate.

 

However due to the slight delay where the asymmetric vortex has to travel down the airframe, the asymmetric vortex is still exist on the large part of the airframe while the vortex in the forbody is symmetric. This will cause a roll to overshoot and thus reverse the process.

 

Note that if you hold a stick full aft with no lateral motion in hb f14, there will be no wing rock.

 

See the attach image below if this gibberish doesn’t make sense to you.

[BlackLion213]

To preface, I understand that wing rock is induced by high AoA flight, from Victory 205's F-14 Practical Handling Tips, and having gotten myself into it on my first flight (apparently a 1980's jet interceptor has a higher landing speed than 1940's P-47. Who'd have thunk?*)

 

What I'm trying to understand is why it does that when its at high AoA's. My intuitive expectation when a plane starts to go over like that is that its about to enter a spin, rather than stop and go the other direction. The Tips pdf mentions there is an interaction with vorticies coming off of the intake ducts. Is this a case of the vorticies only disrupt the vertical stabilizers when they are in the plane of travel**, and when they are out, the aircraft has enough natural centering force to drive them back into it? I expect I've pureed the terminology; my background is not in aerodynamics.

 

Is the causing mechanic anything like what caused Sabre-dance? Or are the results just visually similar?

 

Thank you,

 

Harry Voyager

 

 

*That was pseudo humor, partly. I've mostly flown WWII eras aircraft, so in the landing pattern, I was looking for my normal approach rates. It did end in a successful landing, and, as far as I could tell, I didn't brake the airplane, even without knowing about the spoilers or anti-skid brakes....

 

**plane of travel in this context is being used to describe the plane defined by the Aircraft Datum Line and the Vector of Travel.

 

So wing rock is a "standard" characteristics of swept wing jet aircraft that generally occurs due to the interplay between dihedral effect, decreasing lateral stability at increasing AOA, and adverse yaw. In the Tomcat, wing rock is actually a stabilizing effect due to the interaction between the yaw effects of roll and dihedral effect that allows it to extend through an AOA range of decreased yaw stability (20-28 units) to a range with improved yaw stability (30+ units).

 

There is also some debate as to the actual cause of wing rock in the F-14 vs other fighters, whether it is leading edge stall of the wing opposite the side slip or the result of yaw alone with strong dihedral effect. In either case, the wing rock arrests the sideslip effect and prevents the aircraft from developing nose slices or departures in this AOA range. The only way to depart is to deliberately sustain an input that causes considerable sideslip.

 

The vortices mentioned by Victory do not lead to less lateral stability, they lead to the onset of buffet in the F-14 and are notable for not being a sign of aerodynamic stall. Buffet is also a rather variable phenomenon that means different things in different aircraft. The Phantom video below discusses it somewhat.

 

It is also worth noting that the F-14 has several handling characteristics that are new for DCS players (like roll reversal and the need to use rudder at high AOA for roll), but these are "standard" handling features of fighter aircraft dating back to the F-86 Saber.

 

Also, this video on handling the F-4 Phantom discusses the issues of dihedral effect and adverse yaw due to the effect of high AOA on ailerons.

 

 

Dihedral effect is the development of roll moments related to yaw on a swept wing aircraft, basically because the wing seeing sideslip has "less sweep" and more lift while the other wing has less lift due to the higher effective sweep. This causes an aircraft to roll towards the direction of yaw.

 

Adverse yaw is the effect of the nose yawing away from the intended direction of intended roll, which leads to "roll reversal". It has several different causes, but for most aircraft it is the fact that the "down aileron" has more drag than the "up aileron" due to blanketing of the upper surface of the wing at high AOA. The Tomcat has a similar effect in that the differential tails cause disparate drag at high AOA (and the wing spoilers generally have little overall effect at high AOA).

 

Lastly, virtually every jet has a reduction of yaw stability at high AOA and wing rock is often an early sign of this effect.

 

You are correct that many aircraft develop wing rock shortly before the onset of aerodynamic stall. The Phantom typically develops wing rock about 3-4 units of AOA below the stall AOA.

 

However, the Tomcat does not actually stall like a traditional fighter so the wing rock is not a warning like it would be for other aircraft. Instead it does tell you that you are past optimal turn performance AOA and that the pilot is not managing sideslip very well. In the Phantom, the combination of decreasing yaw stability at high AOA and wing drop associated with aerodynamic stall was very likely to cause a true departure and high risk of spin. One of the revolutionary features of the Tomcat was that it would not depart under these conditions due to the stabilizing effect of the wing rock (in the Tomcat) and the increase in yaw stability above 28 units AOA. This allowed the aircraft to reach AOAs as high as 50 deg without a risk of departing, though sufficient control is not really there to use it beyond a snapshot.

 

It is true that you can cause the Tomcat to spin at high AOA with bad control inputs - namely lateral stick above 20 units. The difference is that the Phantom would still depart at high AOA even with proper control inputs and the Tomcat will not. This represented a significant improvement in the ability to exploit high AOA flight and made the aircraft way more forgiving than prior jet fighters.

 

-Nick

[jcdata]

Read up on the DTIC doc on the F15B wing rock to the bitburg roll..

 

 

 

Sent from my SM-N950U using Tapatalk

[SinusoidDelta]

From a paper discussing a roll stability augmentation system to suppress wing rock in the F-14