Question about Spinning...

[TO_GA]

Hi guys,

I have a question about spinning. I regularly fly aerobatics in an Alpha 160a and often like to incorporate spinning. I haven't flown aerobatics in any other aircraft type so far, but I've noticed after about 3 rotations in the Alpha the propeller stops spinning.

Can anyone explain why this happens, and if it is something that is common to other aircraft types?


Thanks.

[EDMJ]

... after about 3 rotations in the Alpha the propeller stops spinning..

Are you trying to say that the engine quits? What happens when you exit the spin?

Wild guess: Centrifugal forces negatively affecting fuel flow somewhere.

[DB6]

All sorts of aerodynamic and gyroscopic forces can occasionally conspire to stop a prop in a spin. Not a problem as long as it starts again on recovery! As far as I know it's not type specific, the Grobs I fly aeros in will sometimes stop in a spin (and in a tailslide), but start again OK. You could get the engineers to check the idle isn't set too low, or leave a bit of power on as you go into the spin, or try spinning the other way. Have fun .

[Mark1234]

The one I used to fly would sometimes stop, but not always. I put it down to the unusual forces messing up the carby operation - carburettors rely on the acceleration being substantially 'down' to work properly. It will also cease producing power at the top of a stall turn, and under negative G, but in the latter case airflow will keep the prop windmilling. As an aside, the spin does get a bit more wound up/interesting past (from memory) 4 turns or so - that was noted in the flight manual in aus.. not sure elsewhere.

Generally, the other a/c I've spun don't stop, but they're all injected.

Sure Backpacker will be along shortly..

[BackPacker]

Sure Backpacker will be along shortly..

Guess what... I was on a business trip and thinking "Mark1234 will be along shortly, so there's no rush..."

Anyway, I think there's two effects at work here.

One is the yawing motion which may unport the fuel inlet in the tank, leading to fuel starvation after, say, 10 seconds or so. (Note that an upright spin still carries positive vertical g's, so it's not exactly the same as what happens when flying inverted.)

Second is that the forward airspeed is Vs or less, and that there's actually a sideways component to the airflow. This will upset the windmilling effect.

It's the combination of these that, I think, will stop the engine.

When I started aerobatics and started getting confident in the R2160, I did a little test. Went all the way up to FL55 (the highest we can go in our airspace), set half throttle (2000 rpm or so), pulled the mixture to ICO and tried to get the prop stopped. In a full stall, with the sticks to the back stops and keeping straight with the rudder, the prop kept windmilling. I actually had to put the aircraft in a half-g humpty-like maneuver, so that I could fly the 30 knots or so that were required to stop the prop. And the subsequent dive was to 140 knots to get the prop windmilling again. (Or a quick application of the starter...)

My mentor later demonstrated the five-turn spin (over left), and indeed the prop stopped. I haven't done a five-turn spin over right.

[BackPacker]

You could get the engineers to check the idle isn't set too low, or leave a bit of power on as you go into the spin, or try spinning the other way.

Our idle is deliberately set very low (full idle actually is a very rough running engine at at little over 600 rpm) to make sure you can do a decent (competition-grade) spin entry.

And entering a spin with any power left on is a big no-no. I haven't tried it, but I've been told that it's far easier to enter into a flat spin that way, from which it's incredibly hard (or maybe impossible) to recover.

The R2160 POH only describes spins which are entered with the throttle closed, and where the throttle remains closed until after recovery. So anything else and you're a test pilot.

[DB6]

Don't think the fuel flow thing is the cause; all the aircraft I've had the prop stop in have been injected, inverted systems types.
BackPacker, I don't have the Robin handbook to hand so can't say for that type, but I sometimes give my students a full power spin from a vertical climb to demonstrate that the recovery actions will work from other than an academic entry. The point of academic spin recovery training is to prepare people for a possible inadvertent spin from manoeuvre so I like to confirm that to them. Throttle on recovery must be closed, yes, but on entry shouldn't matter if we are being realistic.

[BackPacker]

all the aircraft I've had the prop stop in have been injected, inverted systems types.

FWIW, the Robin 2160 doesn't have an inverted flight/oil system, and is not fuel injected. At the very least that would make it easier for the engine to be starved of fuel than if it had an full inverted flight system and injection.

Wouldn't mind trying the full power spin from a vertical climb though, but probably not in this aircraft...

[Mark1234]

Originally Posted by BackPacker

Guess what... I was on a business trip and thinking "Mark1234 will be along shortly, so there's no rush..."

I'm a bit nervous of 'out of the ordinary' spinning in the 2160 for two reasons:
1) The aforementioned acceleration after 4ish turns - One day I met this, applied normal recovery. Nothing happened for a full turn, perhaps a little more. Recovery is usually immediate. I was just reaching the ****.. what do I do now stage when it recovered. Now, the manual does say 'it does that', but..
2) Somebody saw fit to bolt a large 'keel' on the back end. It can't have been a deliberate aesthetic decision, so I deduce it needed some extra directional stability for some reason. Spin recovery perhaps?

I have played with flat, accelerated and even inverted rotation in the decathlon, but I'm a little leery of such things in the 2160. Incidentally, I've also (once) had the decathlon prop stop in an accelerated spin with a near vertical deck angle, and a rather high rate or rotation. Promptly re-started with no worries.

In short, I think there's a certain amount of 'sometimes it just does that' in the stopping phenomenon, whatever the reason..

[Katamarino]

Here's a question for Backpacker; why do we not wear a parachute in our 2160 for aeros? I'm sure it's been thought about, and a sensible decision made...but, what is the reasoning? It is this lack, where in all my previous aerobatics I've worn one, that makes me hesitant to spin it.

[BackPacker]

Can't give you a 100% convincing answer to that one. But...

- R2160 seats and harnasses are not really designed for parachutes
- We don't have them anyway
- I don't know if the canopy is suitable for in-flight jettisoning. There's nothing in the POH on this either. The handles you see are mostly for loosing the canopy attachments in case of a landing mishap which distorts the engine cowling, so the canopy can no longer slide forwards
- We use significant lower club margins (-1/+4 g) for aerobatics than the manufacturers limits.
- The R2160 has very docile spin characteristics. You've really got to work to make them "snappy" (competition-grade).

But having said that, if we would have parachutes and the appropriate training/procedures for them in place, I'd probably wear them. Not because of a spinning mishap but for an overstress of the airframe.

And the wow-factor obviously. Jumping into an aircraft with a flight suit and a parachute, plus the right set of shades and one of these cloth helmets to keep your headset in place, projects a totally different image than jeans and a shirt (which is my normal attire, even for aeros).

[Katamarino]

That was quick!

Thanks for the reply

[Mark1234]

Originally Posted by Katamarino

Here's a question for Backpacker; why do we not wear a parachute in our 2160 for aeros? I'm sure it's been thought about, and a sensible decision made...but, what is the reasoning? It is this lack, where in all my previous aerobatics I've worn one, that makes me hesitant to spin it.

I have another theory... perhaps because they think that in the event of a mishap, you might be tempted to (prematurely) abandon their precious a/c to it's fate, rather than sort it out/continue trying to sort it out until the last possible moment

[blueandwhite]

Mark1234 - What a cynic

My instructor was always keen to avoid an inverted spin, even stall turns were done with 80 degree upslope. Aparntly the robin can go inverted easy.

[BackPacker]

My instructor was always keen to avoid an inverted spin, even stall turns were done with 80 degree upslope. Aparntly the robin can go inverted easy.

I've been taught that you can't spin until you stall. And an inverted stall in the R2160 (assuming you're trimmed for a fast cruise, which is what we use for aerobatics) is hard. In fact, with the seat at its rearmost setting my arms are not long enough to get it to really do an inverted stall.

In any case, you are nowhere near stalling, upright or inverted, in a stall turn. I'm very happy doing them with a 90-degree upline. What you do need to watch out for, though, is to exceed the 90-degree upline as that will stall the engine. That'll make the turn very sloppy and lead to a significant net altitude loss.

[Mark1234]

I'm inclined to agree with BackPacker that you'd need very long, very strong arms to get an inverted stall out of the 2160.. however, there is some truth in the concept that coming around the latter part of a stall turn you have a pretty good control setup for an inverted spin entry; forward and out stick, with in-rudder. Scuttlebut suggests that the pitts likes to spin out of there, though I've never met the tendancy... could well just be people winding me up