Aerodynamics behind roll reversal and adverse yaw

[MRaza]

Two questions:

 

1. Why is adverse yaw more pronounced in the Tomcat vs the F-15, 16, etc? Is it the absence of fly-by-wire?

 

2. Why does roll reversal occur? Is it because at high AoA, adverse yaw is so strong that it causes a full on flatspin?

[Victory205]

Two questions:

 

1. Why is adverse yaw more pronounced in the Tomcat vs the F-15, 16, etc? Is it the absence of fly-by-wire?

 

2. Why does roll reversal occur? Is it because at high AoA, adverse yaw is so strong that it causes a full on flatspin?

 

It occurs aerodynamically in those airplanes as well, but it biased out (in the F16, not as familiar with the F15) by the FBW computers taking control inputs and tailoring surface responses to produce the desired result.

 

So if a pilot moves the stick laterally, the appropriate flight controls move to produce a roll. A gross oversimplification, but by way of example, you get aileron at low alpha, rudder at high alpha, if that makes sense.

 

Boring!

[Sergeant_Hamlet]

Not an expert so I speak under correction, but this is how I understand it.

Aerodynamically, adverse and proverse yaw and both caused by one wing experiencing higher drag than the other. In the Tomcat's case i assume this is because the spoilers that deploy on the up-spin wing and the maneuvering flaps that deploy on the down-spin wing have different surface areas.

[Victory205]

Oops, forgot to answer that part. The roll is caused by sideslip that is caused by control inputs.

 

Because of dihedral effect, swept wings mostly, the outer wing panels actually display anhedral, the aircraft will roll away from the sideslip...

[Sergeant_Hamlet]

According to NATOPS (11.5.2) "the F-14 exhibits positive dihedral effect throughout the positive AOA envelope" So at high AOA, the shape of the outside wing causes enough drag to produce side slip in the opposite direction to the roll, and eventually the dihedral roll overpowers the commanded roll input. Does that sound about right?

[Victory205]

No, visualize an aircraft with swept wings. Now rotate it about it's vertical axis so the airflow comes say, twenty degrees from the left of the nose (you would do this by pushing on the right rudder pedal). The flow will be more perpendicular to the left wing and at a much higher angle to the right. Therefore, the left wing creates more lift than the right, so the aircraft rolls right.

 

Right rudder, right roll. The effect is amplified at higher lift conditions/AOA. To a point.

[Sergeant_Hamlet]

So I'm trying to picture what happens at high AOA to produce roll reversal. Let's assume about 25 units with unswept wings.

 

1. Right lateral stick is commanded, maneuvering flaps deploy on left wing, spoilers deploy on right wing.

2. Aircraft rolls right, but yaws left due to higher left wing drag, as the right wing spoilers are "hidden" from the airflow due to high AOA.

3. Left yaw exposes right wing to perpendicular airflow, and generates higher lift on the inside wing.

4. Aircraft rolls left.

 

That sequence feels like I'm missing a component. It's also a huge oversimplification.

I share OP's desire to understand. The manual talks a lot about WHAT will happen during lateral control reversal, but doesn't talk at all about WHY it happens.

Edited February 13, 2019 by Sergeant_Hamlet

[aaron886]

Sergeant_Hamlet: not really. (Edit: I took so long to respond it looks like you more or less answered your own question. Here's this anyway.) The dihedral effect imparts lateral stability, meaning the jet will stop rolling when you stop commanding it to, or at least more than if it didn't exhibit some degree of positive dihedral effect. Note that this is not to say the outer wing panels did not have geometric anhedral as Victory205 says. In this case "dihedral effect" implies the combined effects of all sources of rolling stability, including the high-mounted wing and low-mounted engines. I'll try to summarize it below. Note how most fly-by-wire jets feature mid-line engines and wings because these effects can be produced by a computer.

 

For the original post:

 

1. Yes, fly-by-wire can help cover up aerodynamic flaws. Of course there's more to it than that, if you stacked the F-14A up against the F-15A I'm confident you'd find the latter displays far fewer roll-yaw coupling effects. NATOPS will probably tell you the jet exhibits proverse yaw (yaw in the direction of roll,) at most airspeeds due to the way roll rate is generated: spoilers and differential stabs.

 

The differential stabs probably don't contribute as much, being close to the center of mass, but they're the only effective roll controls when the wings are completely swept. Even then, in a left roll, the left stab (deflecting nose-down) is producing negative lift, with a corresponding increase in induced drag. The opposite is happening on the right. That will produce a proverse yaw component. The spoilers are simple. They create drag on the down-going wing, unlike ailerons (or the differential stabs) which produce drag on the up-going wing. You can imagine the result.

 

Short story long, the Tomcat has a lot of aerodynamic features that produce roll-yaw coupling, and was designed before fly-by-wire could fix the problem. It highlights the ways in which aeronautical engineering is an exercise in compromise.

 

2. This gets a little complicated but I'll keep it shorter. The Tomcat NATOPS flight characteristics section (should be chapter 11 per the Navy's standard) will have some amplifying information, albeit probably in testpilotese. The gist is simple, at very high AOA, due to a number of factors like fuselage blanking and airflow separation, a conventional jet will display reduced directional stability. (Yaw stability... it won't attempt to realign itself with the wind in yaw, and may do the opposite.) When this happens to an airplane with high dihedral effect like the Tomcat, the result of a lateral input is effectively adverse yaw now, the nose yawing opposite the direction of stick deflection. This brings one wing forward, increasing its effective span, reducing spanwise flow, and increasing its lift production. On the other side, the wing experiences blanking and increased spanwise flow. The result is a rolling moment opposite the direction of lateral stick input.

 

The solution is to recognize reduced roll performance and use rudder to roll as AOA increases. The mechanism of roll is just the same, except it occurs in the direction expected. Fly-by-wire jets (like the F/A-18) do this without the pilot's knowledge.

Edited February 13, 2019 by aaron886

[Victory205]

OK, I understand your question.

 

The maneuvering flaps extend symmetrically on the F14, so with right stick, you only get four spoilers up on the right wing, TE up on the right stabilator, and TE down on the left stabilator. This, because of vortices, stab TE blanking and rudder blanking, causes adverse yaw from the stabs to the left, resulting in right side slip. Dihedral effect under this condition creates a left roll. The horizontal stabs are far apart (engine cores are 9.5 feet apart) compared to single engine jets and centerline thrust F15's and 18's, so the yaw effects are more pronounced as well.

 

And editing after noticing the simultaneous post above, NATOPS Fight Characteristics is Section IV in the old manuals, and again, the examples you see online reflecting ARI and DFGS that aren't relevant. I don't know where or how those ended up online.

 

The spoiler control mechanism by the way, is fly by wire on the F14. So we aren't a complete relic from the canvass surfaces and wire control cable days.

Edited February 13, 2019 by Victory205

[Sergeant_Hamlet]

OK, I understand your question.

 

The maneuvering flaps extend symmetrically on the F14, so with right stick, you only get four spoilers up on the right wing, TE up on the right stabilator, and TE down on the left stabilator. This, because of vortices, stab TE blanking and rudder washout, causes adverse yaw to the left, resulting in right side slip, with dihedral effect creating a left roll.

 

Thank you. That makes complete sense. I hadn’t considered that the flaps weren’t differential. Sounds like learning the jet is gonna be an absolute blast.

Edited February 13, 2019 by Sergeant_Hamlet

[MRaza]

What exactly is sideslip? Is it the same thing as adverse yaw?

[Bungle_uk]

Sideslip is the difference between where the aircraft is pointing and where it is heading in the horizontal axis.

 

Its the same as AOA (Angle of attack) but in the Yaw axis instead of pitch.

 

https://www.google.com/url?sa=i&source=images&cd=&cad=rja&uact=8&ved=2ahUKEwjSqq3XgLzgAhXv0eAKHT8aDL8QjRx6BAgBEAU&url=https%3A%2F%2Fwww.theairlinepilots.com%2Fforum%2Fviewtopic.php%3Ft%3D396&psig=AOvVaw0_F0C3st_CtecPkmkPs4kF&ust=1550260534829402

[MRaza]

Maneuvering flaps deploys symmetrically and do not play a part in rolling the aircraft correct?

[Sergeant_Hamlet]

Maneuvering flaps deploys symmetrically and do not play a part in rolling the aircraft correct?

 

Yes. I was incorrect in my assumption that the maneuver flaps were used to roll the plane. After reviewing the NATOPS, it says: Lateral control is effected by differential displacement of horizontal stabilizers and augmented by wing spoilers at wing-sweep position of less than 62 percent (2.22.3). The flaps aren't mentioned in the flight controls section at all.

 

It appears the flaps are used for high-lift augmentation and not for attitude control. NATOPS again says that they are commanded symmetrically by the CADC, and purely as a function of AOA or Mach number.

Edited February 23, 2019 by Sergeant_Hamlet

[MRaza]

Another question: Since the spoilers help with rolling the aircraft, they do deploy asymmetrically correct?

[Sergeant_Hamlet]

Yes that's correct. The spoilers deploy on the wing that is inside the roll

[Looney]

This thread would've been better with pictures but alas, my drawing skills really suck :D

[MRaza]

It's interesting to me how the drag on the outside wing of a roll, caused by lift, is greater than the drag on the inside wing caused by increased surface area of the spoiler.