Snap Rolls

[Nexus]

Could someone please explain the aerodynamic principals of the snap roll.

I'd like to know what induces them.

I fly F4U-D and find my self on occasion flipping over while either tracking for a shot or evading a tracking shot.

Thanks,

Nexus

[Lephturn]

As I understand it, a snap roll is a condition where you stall only one wing of the aircraft, or at least stall that wing first.  This results in a quick roll, since only one wing is generating lift.  The good part is that the roll is very fast, and can usually be recovered on demand.  A snap roll can be used to your advantage.

To initiate a snap roll, you use a fairly low speed and climb abruptly, combining it with a bit of rudder to one side.  Generally, the side that the prop wash wants to roll you on anyway is the best one to choose.  In most planes, this means left rudder and maybe a bit of left roll.  A sharp pull on the stick and left rudder at the right airspeed will stall the left wing and whip you into a snap roll.  Basically you want to be going slow enough that you could pull into a full accelerated stall if you yank hard enough.  All we're doing here is arranging things so that the port wing stalls first and whips you into a fast roll.  You run into this when turnfighting in a plane, when you are banking hard in one direction.  Because the plane is turning, say to your left, the outer wing is going a bit faster in the airstream, so the inner wing will tend to stall first.  That's why you tend to snap roll out in the direction of your turn.  I'm sure that's a very simplistic explanation, but you get the idea.

I think this is right.  I do it all the time by accident in the F6F-5 hellcat, which seems to have a particularly nasty snap-roll stall.  You'll find that any plane where you pull to hard and get an accelerated stall will tend to snap roll if you are turning when you stall it.  If you catch it quick enough, opposite rudder and relaxing your back pressure on the stick will normally recover it almost instantly.

Anybody who knows better please feel free to elaborate or correct me.  

------------------
Sean "Lephturn" Conrad - Aces High Chief Trainer

A proud member of the mighty Flying Pigs http://www.flyingpigs.com

Check out Lephturn's Aerodrome for AH articles and training info!

[This message has been edited by Lephturn (edited 05-22-2001).]

[Nexus]

Thanks Lephturn

I was guessing along the same lines as your explaination - one wing stalling before the other.

And you're right it is usually directly related to an abrupt pitch movement - (usually when making that final pull for the shot), recovery is simply to release back pressure on the stick and roll.

However, I'm somewhat confused inlight of your explaination, I find it seems to always be a left hand snap roll, and even while performing a right hand bank.

Is this just torque over coming the lift on the left wing?

In other words, both wings still have lift, (the stall horn has not sounded), however, the lift on the left wing is not sufficient to offset engine torque?

I'm quessing, once the target's bank is matched, the aileron deflection is back to neutral, however the sudden pitch back changes the angle of attack thus reduces lift and allows the engine torque to roll the plane.

Could rudder deflection be an issue here also? i.e. Right hand bank with left rudder?

Curious to know.

Nexus

[Tyro48]

right hand bank with left rudder is called a dutch roll when the aircraft is operating under normal flight parameters. As Leph pointed out the aircraft is snapping to the left do to eng torque,this happens with all eng's that crank clockwise, on aircraft where u see eng crank counterclk wise it will want to snap to the right, so even under normal condition you'll find your aircraft will roll faster left if eng cranks clk wise, since fighters are engineered to be unstable you can use this to adv as Leph has already pointed out.

[Lephturn]

Well, there are lots of factors at play here.

First, lets be clear about torque.  Once the engine is up to speed and not changing RPM rapidly, there is no "engine torque".  Ever driven a race car, or a something with a really big engine?  I have.  When you rev it quickly, it will try to roll the vehicle opposite to the acceleration of the rotating parts.  However, if you rev the engine to 3,000 RPM and hold it there, there is no rolling force applied.  The only things that affect you are the speed of the propeller... so however much resistance there is to spin the propeller one direction, tends to rotate the aircraft the other.  The more important effect is the spiral flow of air from the spining propeller which tends to roll the plane in one direction.  We'll leave gyroscopic procession out if it for now.  

So you are cranked into a right bank, stabilized, and pulling hard for a shot on the bogey.  You pull too hard into an accelerated stall.  It could be that spiral airflow, propeller torque, whatever, is enough force that either it changes things so the left wing still stalls sooner, or, both wings stall at the same time and there is enough force there to roll you left.

I'm betting that the forces at work here tend to make you stall the left wing first in an accelerated stall.  You might be able to add a bit of right rudder at the right time to dead-stall you level, at least at first.  I think the rolling force imparted by the whirling prop and the spiral airflow caused by it is enough to stall the left (or right depending on engine rotation) wing first pretty consistently in an accelerated stall, regardless of the direction of bank you start with in most conditions.

------------------
Sean "Lephturn" Conrad - Aces High Chief Trainer

A proud member of the mighty Flying Pigs http://www.flyingpigs.com

Check out Lephturn's Aerodrome for AH articles and training info!

[This message has been edited by Lephturn (edited 05-22-2001).]

[Nexus]

Cools thanks for the info.

Nexus

[Zigrat]

quote:

First, lets be clear about torque. Once the engine is up to speed and not changing RPM rapidly, there is no "engine torque".


uhmm you are incorrect.

[Jekyll]

Ditto Zigrat.  Think of it this way Leph.  The engine is cranking that big prop around in an anticlockwise direction (when viewed from the cockpit).

The clockwise reaction on the airframe is torque.  The EFFECTS of torque are felt more strongly when rapidly throttling up or down, but only because that causes a trim imbalance on the roll axis of the aircraft.

But torque is ALWAYS present whilst the engine is driving that prop around.

[Tyro48]

I beleive the effects that are in question are called precession which is a product of the torque.

[Lephturn]

Well, if you want to get technical, there is always torque, but it's not going to ammount to much without acceleration... the resulting force of the friction of the rotating parts would be the extent of it.  When you rate an engine, "torque" is the force it takes to accellerate, while "horsepower" is the force used to keep the engine at that speed under load.  My point is that there are forces at work here, but they are not really "torque".  Whatever that force is called, once the engine is running at a given speed, it's quite minimal.  A 700 HP engine won't even lift itself on one side once it's running at a constant speed.  Move the throttle enough to accellerate a bit and if it's not bolted to something solid it will flip instantly though.  The important part here is the opposite force to the drag of the prop as it spins, but that is not engine torque.  I think Tyro is likely correct, and that force opposite to the prop drag is called precession  (procession?).

What I'm trying to do here is dispel the notion that an engine running at a constant speed generates huge amounts of rolling forces on the airframe.  That's just not true.  Take the propeller off the airplane, rev the engine to 2,500 RPM or whatever and leave it.  The amount of rolling force applied while that engine runs is minimal as long as it's not accelerating.  There is a force applied that is opposite to that used to whirl the propeller around.  There are gyroscopic forces at work due to the rotating parts.  There is uneven airflow over the airplane because of the whirling prop.  Those things are what we feel when flying a plane, it's not engine torque.

------------------
Sean "Lephturn" Conrad - Aces High Chief Trainer

A proud member of the mighty Flying Pigs
http://www.flyingpigs.com

Check out Lephturn's Aerodrome for AH articles and training info!

[hitech]

Leph: Sorry but you are incorect about torque.

Torque is the force that causes the acceleration, not acceleration causing torque.

Your visulization of the eng fliping on the side is not do to the change of speed but is do to the change of torque, i.e. pushing on the gas. Since this increase in torque is not absorbed by any load it ends up speeding up the shaft of the eng , and do to newtons nice law, each action has an = and opisit reactaion so there is also a torque on the eng frame = to the torque on the drive shaft.

HP = Torque * RPM.

Puting a transmition between the eng and prop simply has the effect of keeping HP constant and changing the Torque/rpm ratio.

This force is tramsited to the plane body and hence makes the plane roll in the opisit direction of the prop shaft.

In a plane when you advance the throttle the torque increases just like you are visulizing , only with a constant speed prop the prop takes a bigger bite of the air instead of the rpm increasing. Either way the torque on the shaft is the same.


The thing to rember is that even thow we state that an eng has so much HP, what we realy meen with that statement is that it has XX HP at a giving RPM. Most gas engs have a farly flat line torque curve, meening at full throttle they produce the same torque threw a given RPM range, as the eng passes some rpm the HP will continue to increase, do to the RPM increase, but the torque does start droping off. Diesel engs tend to have even a wider flat torque curvs than gas engs.

When you apply throttle i.e. manifold pressure you are increasing the torque at the prop shaft,and hence the torque on the plane. This torque reamains the same weather the prop is increasing speed or not.


HiTech

[Lephturn]

Ok... I see what you mean.  Because the transmission keeps the RPM constant by having the prop take bigger or smaller bites of air, that implies the opposite reaction means that torque is always fed back as a rolling force to the plane.

Where I was seeing the distinction was in "engine" torque vs. the opposite force of the turning propeller.  What I missed is that the whirling propeller and transmission is just the means by which the engine's torque is fed back into the airframe.  I see where I went wrong in my logic.

Thanks HT.



------------------
Sean "Lephturn" Conrad - Aces High Chief Trainer

A proud member of the mighty Flying Pigs
http://www.flyingpigs.com

Check out Lephturn's Aerodrome for AH articles and training info!

[Terror]

Made this thread....

Might help some....
 http://bbs.hitechcreations.com/smf/Smileys/default/Forum2/HTML/000832.html

Terror

[Tuomio]

If helicopters wouldnt have the rear prop, the copter would roll violently after takeoff, since theres nothing holding the plane except the surrounding air. The prop would ofcourse still be the fastest rolling chunck, but thats just becouse the helicopter has more drag on itself.

What makes the prop roll instead of the plane when the plane isnt on the ground? Thats the wings and control surfaces which make the drag to hold the plane still. Now if you have 2 blades rolling to opposite directions, like in p38, theres no torque inflicted to plane, becouse both props eliminate each others torque effect. Thats why its so steady in zooms when the opposite would be found etc. in n1k, which has huge engine which _would_ (in RL)  twist the plane baaadly in low speeds without loads of trim.