What's the difference between a flick (snap) roll and an incipient spin?

[DB6]

The flying club where I do some part-time instructing recently received some ex-Navy Grob Herons (similar to Tutors but with 160 hp and fixed pitch props). They are cleared for erect and inverted spins - the latter with a recommended entry speed of upwards of 80 kts - yet 'snap' manoeuvres (their wording) are prohibited. Tutors are cleared for flicking so is this likely to be some paperwork limitation or is there some fundamental difference between flicking and spinning of which I am unaware?
And don't worry, I haven't been flicking them but I have to teach incipient spinning as part of the aeros syllabus so would like to know where the limits are.

[Milt1]

Could be related to the g limit on the aircraft.

What is max g specified?

[DB6]

+6/-3 g limits. It's cleared for virtually everything bar flicking and has inverted fuel and oil systems. I thought perhaps something to do with power i.e. spins usually power off, flicks power on but incipient spin training for aeros will usually feature at least some with power on.

[John Farley]

I have no experience of the types in question but to my mind spinning is essentially a low IAS and so low structural load exercise. Flicking on the other hand may be carried out with a much higher IAS and as we know the loads are related to V squared.

Some crankshafts may not be cleared for the gyroscopic loads that can arise during flicks. These of course depend on prop RPM rather than power.

[kebab kid]

its probably an engine related limitation if the engine/prop unit is the principal difference. Some (all?) of the serious aerobatic engines have solid crankshafts to withstand the additional stress from the prop during dynamic manoeuvres like flicking, which produce high gyroscopic loads. The tutor may have a strengthened engine whereas the heron has a more standard engine, but still with inverted systems to allow limited aerobatics.

There is also an issue with the different propeller type. During aerobatics, three bladed props cause less of a fatigue problem for the engine’s crankshaft and hub than an equivalent two bladed unit (for a given engine type), the reason being that the moment of inertia of the two bladed prop varies greatly depending where the blades are in their arc of rotation. The extremes are when the two blades point straight up/down relative to the manoeuvre’s rotational axis and the propeller has a large moment of inertia, and when the propeller is horizontal and the converse is true. This large variation of load (and stress on the propeller mounting, which causes the fatigue issue) is much reduced with a three bladed prop. As well as a greater variation in stress for the two bladed unit, I imagine that the peak stress is also quite a bit higher. Therefore it should be possible, at least in theory, for an engine to be cleared for flicking with a prop with three blades but not with two, although practical considerations may ensure that this is never the case.

That’s my complicated explanation to your straightforward question! The reason for your limit is most likely just because Lycoming or whoever thinks it'll bust the engine.

Anyone know what other mods aerobatic engines have - different bearings, crank case strengthening etc? sickbag?!:yuk:

[sycamore]

DB6, I think it is probably the fact that the inverted stall speed is higher than the normal upright stall, due to wing section, PE`s etc. Worth checking in a loose formation at different IAS`s with another aircraft as to what your inverted IAS`s are vs normal upright speeds.It may also be that there is insufficient `down` elevator at lower speeds to give a clean `break`to initiate an inverted entry.. Syc.

[DB6]

This really is an excellent forum. John, thanks, I am thinking that may have something to do with it. I had been lazy and said power as it's fixed pitch. Sycamore, that may be worth looking at if the opportunity arises, I certainly couldn't get a break at lower speeds nearer the erect stall.
Kebab kid, that's just brilliant! However my head hurts now :ok: . I had thought the Heron would have an advantage with (presumably) a solid crank but the 2-/3-blade thing is very interesting.

[Incipient Sinner]

I think I can help on this one, forgive me if I start with the basics.

In order for an aircraft to spin it must first have one of its wings in a fully stalled condition and the other at least partially flying. There are different ways of making this happen.

A flick manoeuvre is carried out by forcing a spin onto an aircraft that is otherwise flying quite happily; it is done at a relatively normal flying speed and the spin is induced by loading or rapidly slowing the wing in question. This manoeuvre puts stress on many areas of the aircraft in particular the tailboom area.

A regular spin, initiated at the stall or slow speed, is carried out by again slowing one wing more than the other, the rapid increase of drag on that wing at low speed then increases that tendency and the aircraft spins.

The other and more shocking way of spinning is when the outer wing in a turn stalls due to excess loading and AoA and the spin is initiated away from the direction of turn.

As for placards in aircraft, type certification will require some form of spinning test but that does not mean that an aircraft becomes cleared for intentional spinning (some gliders for istance where Vne could be exceeded on recovery or the recovery is difficult).

Spinning is a low stress manoeuvre (aircraft not mindframe) whereas flicking is usually high stress and less likely to be cleared.

:zzz: :zzz:

The use of the terminology Incipient Spin is slightly contentious itself and often the term autorotation or just rotation is used to emphasise the difference between a wing drop which one just picks up (rudder not aileron) and the full blown spin which may only be called fully developed when the mode settles down after a few turns. :8

[A37575]

Incipient Spinner.

You wrote: used to emphasise the difference between a wing drop which one just picks up (rudder not aileron)

"Picking up a wing with rudder" is a hoary old myth from war time days, yet amazingly is still taught by some flying schools as God's Own Truth. You appear to have fallen for that myth.

If a one wing drops at point of stall, the correct technique is to prevent further yaw caused by the downgoing wing with appropriate rudder while at the same time unstalling the wings with forward stick and levelling the wings with aileron.

[John Farley]

A37575

Thank you for tidying that up. A true service to aviation!

JF

[Incipient Sinner]

Gents,

Quite true. I'm a glider pilot and we often fly around in tight circles, close to stall speed and in highly turbulent air at the max all up weight of the aircraft (only 750kg granted). On a day with strong thermals I'll tend to drop a wing at least once in most thermals; this is usually a gust stall which is only temporary so either a quick boot of rudder or a momentary unload with elevator catches it before the sensation of movement becomes a proper wing drop and then tightening back into the turn usually does the trick without disturbing the path of the aircraft in the thermal.

I wouldn't necessarily advise this as a standard recovery technique for all aircraft types and situations :E

IS

[Gegene]

But this term would suggest dymnamic stall to me.

[Incipient Sinner]

Quite true, a gust stall would be a basic form of dynamic stall, without any of the frequency and oscillation issues of helicopters, turbines etc.

[Gegene]

I was thinking about the effect of laoding Gs on the aircraft with increased AoA and then 2destroying the airflow over the wing by puting the stick sideway to turn the roll.

If i might, this would get the "inside" wing to loose its lift quiet dramatically with the change of its curvature (aileron up) when the need is for more camber to make sure dynamic the G-load isn't passed over?

Depending on tha mount of Gs this could cause adynamic stall on one wing only and the result is a spin methink.

At very high AoA i would personnaly try the rudder to get the wings around, depending on the qualities of the airframe it works on some aircrafts and looks safer.