Stability and Control characteristics of the Early Mark Spitfires

[Crumpp]

Thank you again Lane for posting those documents.

Problem is on a few in the discussion even understand the topic. For many it is a emotional affair riddled with fear their favorite gameshape will be somehow ruined.

I did not read the report but did look over the graphs from Lane's post. Here is what those graphs are telling us about the Spitfires Longitudinal stability in various conditions of flight.

Negative stability is divergent. Co-efficient of moment and Co-efficient of lift have an inverse relationship in a stable airplane. You can look at the Co-efficient of moment against Co-efficient of Lift plots in the other report Lane posted and see the slope is positive.

What does that mean in plain english. As the wing approaches CLmax, the Coefficient of Moment is well into the negative values and wants to push the nose down. Therefore, the pilot must pull the stick back to keep the nose up.

In a positive slope, as the wing approaches CLmax, the Coefficient of Moment is high in the positive values and wants to push the nose up. Therefore, the pilot has to push forward on the stick to keep the nose down.

This is confirmed behavior in our test flight document below.

[Crumpp]

Originally Posted by CaptainDoggles

I just want to point something out here: Adding instability does not necessarily make the aircraft more maneuverable.

Perfect example is the B-2 Spirit. Extremely unstable, but not particularly maneuverable.

Exactly. Instability does not do anything except make the aircraft harder to control.

It is a myth that instability has positive effects on maneuverability. In fact, it narrows the maneuvering envelope because the pilot cannot precisely control the aircraft.

[Crumpp]

NACA report makes it quite clear that there was some doubt about the correct cg position

31.4% MAC is quite clear.

[CaptainDoggles]

Originally Posted by Crumpp

Negative stability is divergent. Co-efficient of moment and Co-efficient of lift have an inverse relationship in a stable airplane. You can look at the Co-efficient of moment against Co-efficient of Lift plots in the other report Lane posted and see the slope is positive.

This is what I got from the report as well, but what I wasn't sure about was the difference between columns 1 and 2. Did they just run 2 trials at each flight condition and those are the results?

Additionally, it seems odd to me that in the "Engine Off" case, it appears stable in trials 5, 6, and 7 but #8 is not stable. All the other variable appear to be the same (flaps+gear are up, altitude is the same, trim set to -7.5).

Anybody know why that would be the case?

[taildraggernut]

Originally Posted by CaptainDoggles

I just want to point something out here: Adding instability does not necessarily make the aircraft more maneuverable.

Perfect example is the B-2 Spirit. Extremely unstable, but not particularly maneuverable.

You do know it's a bomber? It's extremely unstable longitudinally and if it wasn't for the computers toning down its controls it would be extremely sensitive in pitch.

[taildraggernut]

Originally Posted by Crumpp

Exactly. Instability does not do anything except make the aircraft harder to control.

It is a myth that instability has positive effects on maneuverability. In fact, it narrows the maneuvering envelope because the pilot cannot precisely control the aircraft.

So if this is true why was the Spitfire 'known' for it's ease of control?

Interesting, yet when they wanted to increase the Spitfire roll rate they had to make it less laterally stable by clipping it's wings, so not such a myth about instability, seriously think about it, stability is a resistance to change, stability will never be conducive to manouverability, neutral to slight instability would be the ideal (which is approx where the Spitfire is) and high instability starts to require computers for control.

[ATAG_Dutch]

Originally Posted by Crumpp

Instability does not do anything except make the aircraft harder to control.

It is a myth that instability has positive effects on maneuverability.

Even your average R/C flyer could tell you just how much rubbish the above statement is.

You don't win aerobatic championships by flying an inherently stable trainer.

Nor can you fly an aerobatic championship winning aircraft hands off for very long.

As to the Spit being 'easy to fly', the key word which is always missing is 'relatively'. Otherwise anyone with a pilots licence could've been selected for fighter training . They weren't. There was a rigorous selection procedure which many pilots did not get through and were sent to other, non-fighter training units.

I still fail to realise what your thread is trying to prove.

[taildraggernut]

Heres a couple of links that clearly show the relationship between stability and maneuverability i.e. the relationship is inverse, this is what USN student pilots are being taught, I am fairly sure the USN currently are using 'adopted' standards

http://navyflightmanuals.tpub.com/P-1231/P-12310068.htm

http://navyflightmanuals.tpub.com/P-1231/P-12310031.htm

So lets get all this in perspective, longitudinal instability does 'not' mean the aircraft is difficult to control, and it 'does' mean the aircraft is more manouverable, it's all about how far on the scale you go, and the Spitfire just sits on the slightly unstable end, the 109 would sit slightly on the stable end.

I will just quote Crumpp again so you can draw your own conclusions wether he really knows what he is talking about or just has a sinister agenda.

Originally Posted by Crumpp

Exactly. Instability does not do anything except make the aircraft harder to control.

It is a myth that instability has positive effects on maneuverability. In fact, it narrows the maneuvering envelope because the pilot cannot precisely control the aircraft.

[TomcatViP]

NO NO and NOOOOO

Two of the most manoeuvrable fighters today are perfectly stables: Mig29 and Su27.

It's a matter of philosophy from the manufacturer and the air forces using it.

Today, aft positioned CG is used to lower the drag in high AoA configuration : less elev surfaces deflection (especially in the case of elevons), less tail plane surface, less drag when deflected. Have a look at the range of tailplane incidences available on modern fighters (when they do not act as airbrake).

Moreover I suspect you are mixing stability (or the lack of it) and relaxed stability (what you call instability). The former is what ease the plane in flight for the pilot and, in the case of modern plane with relaxed pitch stability, is always implemented by the flight controls (the pitch crl of F16 and airbus is ctrled by the number of inputs basically - you don't have to pull and push), and relaxed stability (akka positive longitudinal instability) used in modern fighter and Airbus airliner to reduce the drag, accelerate the pitch rate or both.

I don't see why a pilot would want an unstable aircraft especially in pitch when you have to do lengthy flight in clouds, bad weather or simply T.O at dusk . There was the same prob with the Camel during WWI. they made it instable "in purpose" to give him a way to fight the superior airfoil section used by the Germans but at a very high cost : pilot SA dramatically dropped. And there we had teh same result: Experienced pilots were at ease in that situation but rookies had all their attention drown in flying the plane. Doesn't it remind you something ? ... Like section leaders landing back to base with both his wingmen shot down as depicted in a famous 1969 movie?

... And the 47 and the 51 over Europe have told us that you don't win a war with experteen but with a range of perfectly trained young pilots at ease in planes easy to master.

Regarding the Cessna, if you really push and turn the yoke My memory tell me that the 172 is quite manoeuvrable. I won't hve had a dogfight with a 29 but still you feel secure in mountainous terrains.

[ATAG_Dutch]

Originally Posted by TomcatViP

Regarding the Cessna, if you really push and turn the yoke My memory tell me that the 172 is quite manoeuvrable. I won't hve had a dogfight with a 29 but still you feel secure in mountainous terrains.

Yes, but release the yoke and the Cessna returns to normal flight due to its stability.