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CG and spins

Old 8th Jul 2019, 13:56
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CG and spins

Hi,

This question may not be in the right place, maybe even a different forum altogether would be better? If so, please advise.

I read in Jeremy Pratt's 'Private Pilots Licence Course' that
'A forward CG position makes an aircraft more reluctant to spin. A rearward CG makes the aircraft more likely to spin and the resulting spin will be flatter. A flatter spin i.e. one with a higher nose attitude is generally considered to prolong the recovery as well as making recovery more difficult..'

Why would aft CG make an aircraft more likely to spin?

Also, I was under the impression that aerobatic flight is often undertaken with max aft CG, for more control. If true, this seems to contradict the statement in quotes.

Can anyone help here?

Thanks
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Old 8th Jul 2019, 15:13
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The more forward the C of G, the more aerodynamic effect is required from the horizontal stabilizer to maintain balanced flight (more nose up trim is required). Thus there is more down elevator available when you want to recover a spin at that C of G. At the aft C of G limit, the elevator is already trimmed down somewhat, so less range of elevator travel is available to apply nose down elevator control, and the airplane is balanced such that it is not so eager to lower the nose on its own (tail heavy), so more nose down elevator will be required.

In cruise flight, it is desirable to fly with a more aft C of G, as there will be less elevator deflection, and less drag - but the plane is also less spin tolerant.

While doing spin testing on a modified Cessna Caravan, I found the forward C of G spins were easily recovered, aft C of G spins were alarming. The plane recovered as required by regulation, but the spin was very flat, and full nose down elevator had to be held in for an extended period to get the nose below the horizon. If you spin behind the aft C of G limit, recovery from a spin is no longer assured for you, that really is the basis of the establishment of the aft limit.
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Old 8th Jul 2019, 16:29
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A very clear and comprehensive answer, thankyou. That makes it a lot clearer. So a forward CofG will point nose down in a spin, and there is plenty of elevator deflection left to recover because the elevator had to be trimmed upwards to push the tail down/nose up. Elevator for Aft CofG starts out trimmed down anyway, so you run out of deflection to recover from a spin, which will be flat because the aft CofG isn't weighing the nose down. Got it.

The Cessna sounds awful, did you have a parachute? I'm even more paranoid about weight and balance than I was now! Probably a good thing.
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Old 8th Jul 2019, 17:41
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At least some gliders will recover from a spin of their own if the CoG is far enough forwards. Apparently that happened with one of ours when someone pushing the weight limit spun it as part of a test flight. It was a single-seater.
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Old 8th Jul 2019, 19:12
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I have heard talk of a few planes that are allegedly impossible to spin; maybe some of these 'self recover' from the stall before hand, or just don't even drop a wing, ever? Never sure what to make of it when someone tells me something like that, I'm a total novice, so the fact that it sounds impossible to me doesn't mean much.
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Old 8th Jul 2019, 19:28
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I believe the Beagle Pup had a large lump of metal ballast bolted to the tail or ventral to assist in spinning, clearly it would move the C of G rearwards when fitted.
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Old 8th Jul 2019, 22:40
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"Also, I was under the impression that aerobatic flight is often undertaken with max aft CG, for more control."
For less stability, and more manoeuvrability?
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Old 8th Jul 2019, 22:48
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Easy explanation, consider a dart like used in the sport. Weight is in front to stabilize.
Visualize reversal, tail feathers in the back like normal but this time also the weight.
The weight is now after pushing the dart rather then forward and pulling it.
Destabilizing but increasing maneuverability which is pretty much defined as the ease of changing direction.
World class aerobatics pilots will know exactly where the sweet spot is for their aircraft.

As far as your fears about CG, consider that the aircraft has been designed and approved to be perfectly safe between the two CG limits.

Last edited by B2N2; 8th Jul 2019 at 22:58.
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Old 8th Jul 2019, 22:49
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Originally Posted by Maoraigh1 View Post
"Also, I was under the impression that aerobatic flight is often undertaken with max aft CG, for more control."
For less stability, and more manoeuvrability?
Please don't use "less" "more" stability - it's meaningless without additional qualifiers.

Aft CG gives lower apparent static stability, lower manoeuver stability, generally less well damped short period longitudinal and lateral/directional static modes and Dutch Roll mode, it may well make the long period longitudinal mode better damped; the spiral mode is more complex because that is primarily dependent upon the RATIO of static lateral to directional stability.

It's the lower static and manoeuver stability values that tend to favour aerobatics, the reduced damping on the DR can be a nuisance, and the reduced SPO damping can interfere with accurate pitch control however.

A lot of that is trade jargon which most pilots don't need to know - but my important point is that to say something is "more" or "less" stable is generally over-simplistic and potentially misleading.

G
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Old 8th Jul 2019, 23:14
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G,

Not everyone is an engineer and you certainly don’t need that level of knowledge to enjoy flying as a private pilot.

Aft CG gives lower apparent static stability, lower manoeuver stability, generally less well damped short period longitudinal and lateral/directional static modes and Dutch Roll mode, it may well make the long period longitudinal mode better damped; the spiral mode is more complex because that is primarily dependent upon the RATIO of static lateral to directional stability.
This is certainly not an answer you should give an aspiring private pilot.
Its just meaningless gobblygook to them without any point of reference they can latch on to.

The instructor needs to teach to the level of the student.

Last edited by B2N2; 8th Jul 2019 at 23:25.
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Old 9th Jul 2019, 00:31
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This is certainly not an answer you should give an aspiring private pilot.

(Personal view, only)

Well, yes, and no.

One of the problems in this Industry is that pilots, in general, are trained to comparatively low levels of technical understanding. That's fine, so far as it goes as, the pilot's main thrust is to fly it rather than build, test and certify it. Unfortunately, the philosophy serves to encourage/perpetuate knowledge mediocrity, especially over recent decades where the whole thing at Industry training level has been dumbed down, in part due to cost minimisation desires.

Like many here, I know who G is and his background. One would expect that techo advice he gives will be both technically accurate and operationally pertinent. We are fortunate that we have a cadre of such folk in the PPRuNe sandpit for just those reasons.

Its just meaningless gobblygook to them without any point of reference they can latch on to.

So, yes, the pilot might be able to get away with minimal knowledge for the great majority of routine operations and history supports such a view. It certainly follows that there is an effort involved and required to improve one's knowledge level.

The instructor needs to teach to the level of the student

Might I suggest, with respect, that a more useful statement might be along the lines of "to teach to the present level of understanding held by the student with a view to improving that level of understanding" ?

Might I suggest that sound basic knowledge is valuable in its own right (we don't all need to have the PhD level of knowledge and understanding) ? There is nothing stopping the motivated listener/reader from either asking for additional explanation or going off and doing a little personal research to find out a bit more about the discussion. While, for folks who have no background, it might take a few hours to locate and read up on the basics, there is nothing in his post which is not readily amenable to research via the net - one of the most valuable consequences of the net's development, in my view.

A suitable level of basic understanding doesn't require the maths which goes along with the subject. Just the basic results and ideas are the main value for the typical pilot.
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Old 9th Jul 2019, 02:19
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I have heard talk of a few planes that are allegedly impossible to spin; maybe some of these 'self recover' from the stall before hand, or just don't even drop a wing, ever?
The only plane I have flown which is impossible to spin, and placarded so, is the Ercoupe. It has no rudder pedals, with the rudder being coordinated to the ailerons. And, the elevator is limited, so in most cases, you cannot pull it into a stall. If you do get it to stall or spin, and I have done both in it, you cannot hold it in, it just recovers on it's own. Other aircraft I have flown have control travels such that in some configurations, a pitch down in a stall is hard to achieve. But, as the aircraft can still be stalled in some configuration,s and still can be spun, they would not be classified as spin proof. Other types of modifications (like STOL kits) advertise that they improve stall spin resistance, and for my experience they do, but the aircraft can still be stalled and spun, and when you finally get it there, it can be a bit more sudden (it just held on to the bitter end longer).

A spin, in a suitable aircraft, is a benign maneuver, and well worth learning. A spin in a non spin approved aircraft may demand skill, and/or exceed limits during recovery. The Cessna Caravan upon which I did spin testing with an external load, recovered exactly as the design requirement states that it must. though in doing it, I came very close to both speed, and G limits during the recovery from the dive. The Cessna 206 is similarly unpleasant to spin.

Forward C of G spin in the Grand Caravan:

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Old 9th Jul 2019, 08:18
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BN2 had you read my post more carefully I said....

Originally Posted by Genghis the Engineer View Post
Please don't use "less" "more" stability - it's meaningless without additional qualifiers.

<SNIP>
A lot of that is trade jargon which most pilots don't need to know - but my important point is that to say something is "more" or "less" stable is generally over-simplistic and potentially misleading.

G

Re: unspinnable aeroplanes. I flew a programme years ago on the French HM1000 Balerit which was also unspinnable. I was not party to the testing, but was also assured by those involved that the same was true of the British CFM Shadow. Such aeroplanes do exist, they're just extremely rare.

G
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Old 9th Jul 2019, 08:44
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Thanks G, I understood some of your explanation, and will look up the rest. It basically fits with the analogy of the dart with nose/tail weight from B2N2, which works as an aid to visualisation, although obviously there is way more to it.

By coincidence it's a CFM Shadow pilot who most recently asssured me that his aircraft wouldn't spin, no matter what he did to it, and he couldn't get it to drop a wing either. I'd be happy to own one actually; apart from the 2 stroke engine I think it would suit me and my budget very well. The other aircraft I was told about with a reputation as being impossible to spin is the Cri-Cri, although I read an account of a test flight somewhere a while ago where the pilot inferred that he had acheived some kind of spin in it, but didn't comment further. I can't find the article now, unfortunately.
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Old 9th Jul 2019, 09:13
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I would be surprised looking at the shape if the Cri-Cri is unspinnable, but I have been wrong before.

Re: the Shadow, I've flown most variants. I enjoy them, but will dissapoint you by saying that in my opinion the very best of them is the Rotax 503 engined Shadow CD. My biggest gripe would be the sheer number of materials and relative fragility which means it absolutely needs to be hangared.

But if you have a hangar and don't mind putting work into any maintenance tasks, which possibly includes developing some new skills - they're great little ships. Don't be afraid of 2 stroke engines, particularly the 503 which is a superb piece of engineering that just needs you to stay on top of the scheduled maintenance (but you should on any other engine too.)

G
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Old 9th Jul 2019, 09:18
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Be careful with claims like “this aircraft doesn’t spin” as a spin is an aerodynamic occurrence as the result of an asymmetric stall.
He may not be able to spin as the CG is too far forward.
I’ve flown at a flight school a lifetime ago where there was one 70’s Cessna 172 that simply wouldn’t spin no matter what you tried.
They had another one where every single stall led to a wing drop to the right.
Function of airplane and flight control rigging or lack thereof and tired old airplanes and probably bent.
I’ve also done hundreds of spins in the DIamond DA-20C1 Eclipse which is pretty spin resistant but when provoked will spin like the proverbial washing machine.
The only way you can make an aircraft impossible to spin is if you make it impossible to stall like with a canard design where the front wing stalls first and drops the nose and therefore prevents the main wing from stalling.
Now....people have been creative and have found other ways to hurt themselves in aircraft that “don’t spin”.

Might I suggest, with respect, that a more useful statement might be along the lines of "to teach to the present level of understanding held by the student with a view to improving that level of understanding" ?
@ Tullamarine and Genghis,
Quoted section is what I intended to say and whatever I put down came out different then intended.
Did not intend to but may have come across as insulting to G.
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Old 9th Jul 2019, 10:11
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Originally Posted by cats_five View Post
At least some gliders will recover from a spin of their own if the CoG is far enough forwards. Apparently that happened with one of ours when someone pushing the weight limit spun it as part of a test flight. It was a single-seater.
To add, I also know of a very serious competition pilot adding weight to the tail of their glider until they had a neutral elevator at 70 knots.
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Old 9th Jul 2019, 10:51
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My single glider is optimised for the lightest pilot in our syndicate by having lead weights (factory) fitted inside the tail wheel so as to reduce drag at normal cruise. In addition it has a water tank in the fin that allows us to add up to 8 litres of water in 1 litre increments. One has to do weight and balance calculations carefully. Spins well. Needs correct recovery. (It also has water ballast tanks in the wings with a total capacity of 200 litres.)
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Old 9th Jul 2019, 11:26
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B2N2 - no offence taken, I just thought you'd somewhat missed the point of my post.

Re: spin resistance in individual airframes. There is experience that the range of control movement - particularly of the elevator and rudder can be significant. An aeroplane which is rigged down at the minimum available range of nose-up elevator and rudder in the direction you're trying to get it to spin can be very reluctant to spin. The big risk is that if the limited range is also the case in the opposite directions it can also be very reluctant to recover. One should be therefore very nervous of spin reluctant individual airframes.

G
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Old 9th Jul 2019, 16:16
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Genghis,

I have experienced the opposite problem. Asa 17 year old I flew a C150 which had come back from heavy maintenance after a landing accident and had been previously noted to markedly drop a wing on entry to the stall. It was my pre-FHT during my RAF Flying Scholarship course and we were required to enter and recover from a fully developed spin, which was still in the 35 hour PPL course back then. As far as I can recall, I entered the spin as per the book but the aircraft rolled very rapidly in the pro-spin direction. Something very strange then occurred and my instructor looked across at me and asked if I noticed anything unusual. I replied that we were upside down!

He agreed and asked me how I intended to recover. I said "I think I need to pull back on the stick!"

He nodded. I did so and the aircraft recovered quite quickly with opposite rudder, but at some stage the engine stopped. It restarted on the key. My instructor then said "Let's go back to the circuit and recover our composure". We did. I passed my FHT on the next trip. I don't know what subsequently happened to the aircraft but I think the wing rigging was found to be somewhat awry, hence the wing drop. Strangely, as a relatively ignorant young pilot, it didn't phase me nearly as much as it had done my instructor.
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