Flying Instructors & ExaminersA place for instructors to communicate with one another because some of them get a bit tired of the attitude that instructing is the lowest form of aviation, as seems to prevail on some of the other forums!
If spins are not a prohibited maneuver for your type aircraft, then I 'spect overstress would be more likely during an ill-advised type entry (i.e. snap roll variety) or the resulting dive recovery if mishandled, following the spin recovery itself.
I would say you're fairly safe while actually in the spin.
(Be careful, I've seen some light aircraft transition to a spiral from a spin with nose low, no change in pro spin controls, and the resultant airspeed increase. You can feel the 'G' building when this occurs.)
OK465 pretty much has it, certainly Cessnas and Pipers that are cleared for spinning are VERY prone to ending up in a spiral dive instead of a spin, recovery from a spin is unlikely to over stress the aircraft unless the pull out is delayed or MUCH too slow, spiral dive recovery is much more likely to cause over stress.
Proper spin training will teach you not only spin recovery, but how to recognise if it is spinning or spiralling - it can be surprisingly difficult to tell the difference just by how it looks outside.
Refer SAE Paper 700222, Loading Conditions Measured During Aerobatic Maneuvers by Albert W. Hall from NASA Langley Research Center. In this context, that work basically confirmed that the existing structural airworthiness requirements are satisfactory. i.e. an airplane must be in utility category to be approved for spinning and it will be strong enough.
There is a little bit of data in this Australian report and some-one has kindly made it available online (it was done by BASI and I have seen it on the ATSB website but they continually re-arrange the pages so this is a more reliable link). http://www.8kcab.com/G-LOC%20Report.pdf
As has been said here, the "normal" spin entry, and spin itself are relatively gentle maneuvers around 1G. Certainly, there are ways to enter a spin (like a snap roll) where local forces on parts of the aircraft cold exceed the "average" G load on the aircraft. This would be a reason that some Utility category aircraft can be approved for spins, but not aerobatic maneuvers.
It can be the recovery from a spin (or other upset type maneuvers) which can get you in trouble. Extreme nose down attitudes can cause a speed buildup very quickly, and immediate pilot action is vital. I can tell you that a wingover in a Cessna 182RG can result in an alarming speed buildup.
During certification testing, I was required to spin a Cessna Grand Caravan, with an external load. I did a lot of inquiring about spinning such an aircraft before hand. The aircraft was equipped with an accelerometer. I did 14 spins at the extremes of C of G, and at differing weights. The varied C of G produced dramatically different recovery characteristics. Forward C of G consistantly resulted in an extreme dive. Several of the recoveries were very close to Vne, and at 2.8G.
This would not concern me too much for a normally aspirated direct drive engine, and I doubt that other types of engines are commonly installed in spin approved aircraft.
My experience has been that the engine bouncing around in the engine mounts can be a greater concern, but the forces required to do this are not so much a result of a "normal" spin, but more aggressive aerobatics.
Loads during a spin are complex, poorly understood, but generally unlikely to damage the aeroplane. They should be well within the design loads.
It is however NOT a 1g manoeuvre. I've seen a couple of g in both directions, and somewhat more than 0 sideways on occasion. The loads can be quite oscillatory.
If an aeroplane is going to be broken in the spin however, other than the obvious ground imapact risk, it's almost always going to be either overspeed or overstress in the resultant dive and pull out.
That said, I have seen a propeller damaged; it was a Tucano, doing oscillatory spins, full power - so high powered turboprop, with a high mass prop, in a very oscillatory spin mode. It's a pretty rare set of conditions and I'd not get worried about it in the vast majority of GA types.
Yes, for most spins, it's the build up of speed during the recovery which is the danger to the aircraft structurally. The challenge is, while the plane is pointing just about straight down, allowing it to accelerate enough to not "secondary stall" it upon recovery, but not so much that you overspeed it.
This results in one of the other things we look at during certification, upset. If the pilot does something silly in the plane, and ends up pointed straight down, is there enough margin between the speed they were flyinig, and Vne to allow them room to get out of it without overspeeding.
The added challenge with the dive is that as speed builds, the desire to get out builds, and the G force being applied to get out increases, so the stall speed increases, and you're back to the secondary stall problem again.
The accelerometer installation for my spin testing was a bit more straight forward than you suggest. You want something which is really easy to read, when things are changing fast!
(That is the reading after my first spin testing, I purposefully did not reset the recording pointers in flight)
As others have said, it is the recovery from the dive that could overstress the aircraft. If you can control the speed to less than Va, then you should not exceed the aircraft's g limitations with a straight pull on the stick, although you might stall again. Over Va and you could break something.
You still need to be careful even at less than Va, because if you roll and pull at the same time you can cause damage. If you recover in an inverted attitude, roll to the nearest horizon immediately after stopping the spin, then pull.
Can an airplane in a fully developed spin build up speed on it's own accord without control inputs, as a result of a forward CG or nose- down attitude? Is a spin really ,,stable'' ?
This is what happens in many Cessnas, when it happens it is no longer a spin but a spiral dive - this is the main visual difference between the two, a spin will have low oscillating airspeed, a spiral dive will have the speed increasing.
Interestingly, having accomplished my gross weight, forward C of G spins in the Caravan, it was then time for aft C of G sipns. The recovery was profoundly different, and has me thinking that it might be the spin recovery ability which defines the rear C of G limti for the Caravan.
Unlike the steep nose down dive recovery of the forward C of G spins, the aft C of G spin recoverys were pitch control full nose down, hold and wait with opposite rudder also applied. After a half turn and some pitch bobbing, the recovery seemed to begin. Indeed, it had recovered in the required one additonal turn, but it is weird holding the controls fully forward for that long, particularly in light of the vigorous dives of the preceding flights!
Though the control applications were the same for the two extreme spins, the length of time they were applied, and what happened next were very different!
That said, yes, 100 series Cessnas generally enter a spiral dive on their own, during most spin recoveries. Make sure you can quickly recognize the difference between the two, and can apply the correct recovery technique - because they are different!
My main concerns with spinning are entering the steep spiral mentioned above, or the engine stopping.
I've heard a lot of people say they've had an engine stop on them, but i've spun about 25 times now without the engine stopping. How common actually is it? Does spin direction really make any difference to this? I've spun both ways without problem.
Every time I spin I keep expecting the engine to stop, or even sputter, but so far no problems at all.
I have never had an engine stop or sputer during spins in the dozen or so types I have spun. If the engine did sputter, I would really wonder why. A gravity feed fuel system might be afffected by a momentary reduction in G during the push over, but only once, while spinning an aft loaded C 206 did I momentarily get to zero G. I cannot imagine in an erect spin or recovery in a GA aircraft getting to zero G long enough to make an engine sputter.
I have sputtered engines momentarily during aerobatics, and certain very specific flight test maneuvers, but it's a non event. One of the required tests in the DA-42-L360 was a -0.5 G push over for a specified number of seconds. That did produce a sputter, but they came right back with positive G. Observers on the ground reported seeing clouds of white smoke during the zero G. Turns out that the engines each threw out about a liter of oil!
My main concerns with spinning are entering the steep spiral mentioned above, or the engine stopping.
Never had a problem with the engine stopping in a spin either, though i have had a Tiger Moth engine stop in a stall turn. If you are, or intending to do, spinning/aeros, both engine restarts and spiral dive recoveries should be covered fully and you should be happy with both before doing them solo.
RTN, just looked at your profile and seen that you are an instructor! I am amazed, with the concerns you have voiced I had you down as a low houred PPL, as an instructor you should really be comfortable with spiral dive recovery and engine restarts and should be teaching both (though not actually doing restarts in the air on SE). If you are teaching spinning with the concerns you express I suggest you get some more training, certainly UH at Kemble do an excellent advanced spinning course and there are others around that will cover this as well.
The only time I have ever had to restart an engine in flight has been when I have deliberately shut it down, and stopped it, which does not happen casually. For a GA piston powered plane, a restart following fuel flow interruption should be a non-event, and indeed involve no pilot action in many cases.
When I say it concerns me, I didn't mean I was frightened or thought I couldn't handle it, just that it could be a fright for the student.
Also thinking about inadvertent spins in a type not cleared for intentional spins, and the outcome being a very steep spiral. Recovery is simple enough, but I'd imagine it would be very easy to pull too hard and over stress something.
I am confident and trained in spinning and basic aerobatics, I've never had an engine stop but I have spoken to a few people who say it stops almost every time they spin. I'm just wondering what they are doing to cause this since I have not experienced it myself. The advice I had from my aerobatics instructor was to pick your field before you start the maneuvers just in case.
pick your field before you start the maneuvers just in case
That's good advice for most types of flying in single engined aircraft!
it would be very easy to pull too hard and over stress something.
Expect that it will be much more easy to over speed than over stress to the point of a safety of flight issue, for modern GA aircraft. The design tolerance for overspeed is only 10% (though you can go that 10% with no damage at all, if you're very gentle in smooth air). Where the design tolerance for over stress is 50%. You really don't want to be in either of those corners, but most spin approved aircraft are in the utility category, which allows 4.4G. That means that it will be 6.6G before you acutally make an unflyable aiircraft. Between 4.4G and 6.6G you will probably bend something expensive and scary. Most "regular" pilots would be shrieking in horror before they applied 4.4G. I have only once, and it was deliberate in a 150 Aerobat. It takes more pull than you think!
That all being said, there is no excuse for an instructor to get anywhere near those corners in a spin approved aircraft. The aircraft is designed with a margin of speed to account for a speed build up after upset. Any spin approved aircraft I have ever spun has lots of speed capacity to allow for some speed build up in the dive recovery, without creating a risk. If you're in the top half of the yellow speed range after a spin (in an approved aircraft), you've done it wrong!
a few people who say it stops almost every time they spin
Respectfully, these people either need more spin training, or should be presenting that aircraft to a mechanic before the next flight!