Why is the Spitfire Mk IX still unstable??

[MiloMorai]

The Pilot Notes says the stability about all axis is satisfactory and the a/c is easy and pleasant to fly.

 

The P-51 was not easy to fly with a full rear fuselage tank and had flight restrictions similar to the Spitfire.

 

An Olympus Mons is being made out of a mole hill.

[Yo-Yo]

The Pilot Notes says the stability about all axis is satisfactory and the a/c is easy and pleasant to fly.

 

The P-51 was not easy to fly with a full rear fuselage tank and had flight restrictions similar to the Spitfire.

 

An Olympus Mons is being made out of a mole hill.

 

The flying qualities of two different planes hqving the same stability factor (%% of MAC to the neutral) can be very different regarding of damping, stick forces, etc. Moreover, the neutral point depends on CL (so the speed of 1g flight), and power setting.

[bongodriver]

Already been discussed in my very first post starting this thread. It does not increase the area of the tailplane and therefore does not shift the stability margin.

 

you can discuss it all you like of course, it is however incorrect, the horn extensions on the elevators do increase the area of the elevator, there are simply no modifications to enlarge the tailplane, only Grifon powered Spits had a larger tailplane and it didn't even happen with the first versions.

 

It simply adds another form of balancing to the elevator..

 

Yes, aerodynamic balancing.

 

Why is an airplanes elevator balanced? To shift the control forces to something more controllable under unstable conditions....

 

Is one of the possibilities, it also is used to reduce flutter, it all depends really, some aircraft have no balancing on control surfaces at all.

 

Let's see:

 

Oh yes lets.....

 

 

1. A million different bobweights of various sizes tried and experimented with...

 

a single inertia weight with experiments at different weights on a few tests, none ever making production on a MkIX and only extensively used as a stop gap measure on MkV's due to bad loading habits creeping in to front line squadrons, there are even documents stating that the inertia weight is not to be fitted with a horn balanced elevator.

 

2. Multiple attempts at other forms of balancing...

 

2 attempts, mass and aerodynamic horn, the latter being the continued choice.

 

3. Multiple elevator redesigns and an obsession with elevator manufacturing tolerances on test aircraft.....

 

obsessive redesign is sort of the point of tolerance testing, however only 2 designs made production.

 

4. Post war area added to shift the stability margin (the correct fix and what should have been done in the first place)

 

I really don't think it took 70 years to create an elevator magic bullet.

No evidence of this is provided in any form, none, nada, diddly squat, zero, sweet FA!

 

5. Post war aircraft all have with restricted CG limits compared to wartime variants; even have specific longitudinal instability behaviors noted on some the type certificates!

 

Post war aircraft are regularly flying with wartime limitations in place, even the engine handling limitations are the same as wartime.

 

And most damning...

 

Drum roll please.......

 

No change in the relationship between the CG limits and no design changes to move the Aircraft's Aerodynamic Center from the Mk I to the Mk IX.

 

:suspect:

 

And yet we have pages and pages of staunch defense that the NACA conclusion was correct and the aircraft was longitudinally neutral to unstable at normal to aft CG.

 

Fixed that for you, as it has been shown that within 7.0" inches aft of datum the spitfire is stable on all Merlin powered marks, the normal range of a Spit falls well within this range with exception of late Grifon powered variants.

[Decibel dB]

Welcome to the discussion.

 

The front cockpit is moved forward in the tandem Spitfires otherwise the conversion

 

It was in fact my first question and no was his answer. The only modification was in the back

[NineLine]

It was in fact my first question and no was his answer. The only modification was in the back

 

I just did a quick compare of some drawings and pictures, and it does look like in the case of the TR9, the front cockpit is indeed moved forward some.

[Decibel dB]

I didn't have a measuring tape but two MK IX park side by side in the hangar and my eye couldn't make a difference with the distance from the front cockpit. Not between both Mk IX, there was indeed one on the MK V. The two seater conversion was one of my discussion with the engineer and I have no reason not to believe him when he say that on the MK IX the only mod was on the back but occasionally ballast was use . But again I can be wrong

[NineLine]

I didn't have a measuring tape but two MK IX park side by side in the hangar and my eye couldn't make a difference with the distance from the front cockpit. Not between both Mk IX, there was indeed one on the MK V. The two seater conversion was one of my discussion with the engineer and I have no reason not to believe him when he say that on the MK IX the only mod was on the back but occasionally ballast was use . But again I can be wrong

 

My test was real quick with internet drawings and pics from the internet where angles arent quite the same, it might be just an illusion... but not so on topic anyways :)

[Crumpp]

I just did a quick compare of some drawings and pictures, and it does look like in the case of the TR9, the front cockpit is indeed moved forward some.

 

And it is listed in the manufacturer's list of design changes that have to be done to convert a Spitfire to a TR9.

 

:smilewink:

[Crumpp]

The only one, short, natural and the right way to determine Mk IX stability having EXACT data for Mk V, that is, for sure, obvious for you, is:

Answer the question if 9 has the same wing (planeform and airfoil), stab area and its arm relatively to the wing MAC.

If so, the neutral point of the airframe is the same, and we are able to create the airframe model having stability charachteristics exactly as Mk V using measured data.

Changing CoG position corresponding the Mk IX docs.

Get it as is... :), profit.

And i am really amazed how many letters were typed during the discussion... :)

 

So, i very much appreciate your battle proving the engineering way of thinking! :)

 

Thank you Yo-Yo!

 

I am amazed at the quality and depth of FM's you are able to produce on a home PC. You should call Flight Safety, I think they could learn a thing or two from you! :thumbup:

[Cripple]

The Pilot Notes says the stability about all axis is satisfactory and the a/c is easy and pleasant to fly.

...

 

I don't want to rehash the point I made earlier, but that we have to qualify the terms "stability" and "easy" by adding "for a trained pilot, and compared to other aircraft of that period". We could also add "(not Joe Console who has just graduated from playing with his thumbs").

 

I believe elsewhere it has been noted that it would be a lot easier to control the Spitfire "quirks" (as I shall diplomatically call them) with a full scale column that with a desktop joystick.

[bongodriver]

I don't want to rehash the point I made earlier, but that we have to qualify the terms "stability" and "easy" by adding "for a trained pilot, and compared to other aircraft of that period". We could also add "(not Joe Console who has just graduated from playing with his thumbs").

 

I believe elsewhere it has been noted that it would be a lot easier to control the Spitfire "quirks" (as I shall diplomatically call them) with a full scale column that with a desktop joystick.

 

Obviously for a trained pilot, why would they be drawing conclusions for people not likely to be flying an aircraft?

Obviously the comparison is for contemporary aircraft, the test was done in wartime and NACA did not have time travel technology to determine if they would be making recommendations for game console players.

 

Yes, apparently the key to success is to have a full mock up of each cockpit in game, that shouldn't alienate a majority of flight simmers at all, not that the requirement for scale hardware is strictly correct, you can be delicate with a short stick and get the same results.

[NineLine]

And it is listed in the manufacturer's list of design changes that have to be done to convert a Spitfire to a TR9.

 

:smilewink:

 

Well I didnt dig that deep as we arent getting a TR9 that I know of :)

[Cripple]

Obviously for a trained pilot, why would they be drawing conclusions for people not likely to be flying an aircraft?

Obviously the comparison is for contemporary aircraft, the test was done in wartime and NACA did not have time travel technology to determine if they would be making recommendations for game console players.

 

Yes, apparently the key to success is to have a full mock up of each cockpit in game, that shouldn't alienate a majority of flight simmers at all, not that the requirement for scale hardware is strictly correct, you can be delicate with a short stick and get the same results.

 

It's not actually "obvious". In my experience what we personally believe to be obvious often needs to be made explicit in order to avoid confusion over semantics.

 

My point is that we, the prospective players, are not (in the majority of cases) trained pilots - let alone ones who are trained on vintage tail-draggers. Ergo, what the 1940s pilot manual might describe as "easy" should not be taken at face-value and considered easy by 21st century gamer standards.

 

(By way of an analogy, I suspect most of would find manipulating two analog thumb-sticks to be "easy" in a way our hypothetical time-travelling NACA employee would not.)

 

 

Actually, there is really no need for the facetious comments to try and make your point. In fact, I agree that concessions must be made to avoid alienating prospective simmers - a point I made elsewhere about not expecting desktop "pilots" to have 300+ quid of Warthog on the desk, and rudder pedals under it (it just becomes a lot easier if you do). That however, is a designer/FM/accessibility issue, not ours.

[bongodriver]

It's not actually "obvious". In my experience what we personally believe to be obvious often needs to be made explicit in order to avoid confusion over semantics.

 

OK, sorry, hopefully it is much clearer now?

 

My point is that we, the prospective players, are not (in the majority of cases) trained pilots - let alone ones who are trained on vintage tail-draggers. Ergo, what the 1940s pilot manual might describe as "easy" should not be taken at face-value and considered easy by 21st century gamer standards.

 

(By way of an analogy, I suspect most of would find manipulating two analog thumb-sticks to be "easy" in a way our hypothetical time-travelling NACA employee would not.)

 

Again, sorry. I personally am not a console gamer and agree the thumbsticks would be practically impossible for me to get my head around, I don't just fly vintage tail draggers, I teach other people to fly them too, I should know the difference between aviation 'easy' and layman's 'easy'.

 

Actually, there is really no need for the facetious comments to try and make your point. In fact, I agree that concessions must be made to avoid alienating prospective simmers - a point I made elsewhere about not expecting desktop "pilots" to have 300+ quid of Warthog on the desk, and rudder pedals under it (it just becomes a lot easier if you do). That however, is a designer/FM/accessibility issue, not ours.

 

Blimey, another apology, not meant to be facetious, I type much of my stuff while smiling because I think I'm being funny....someone has to think so eh?

[Pilum]

Two questions to the OP regarding your claim that the Spitfire Mk9 is unstable:

 

1) The Spitfire Mk9 had 17.5 lb ballast and a 5 lb ballast mounting in the tail. Why would the designers put ballast in the tail of an unstable plane?

 

2) You say you are in possession of a report on the Spitfire Mk9 showing it to be unstable. What report is this?

[Crumpp]

I don't want to rehash the point I made earlier, but that we have to qualify the terms "stability" and "easy" by adding "for a trained pilot, and compared to other aircraft of that period". We could also add "(not Joe Console who has just graduated from playing with his thumbs").

 

I believe elsewhere it has been noted that it would be a lot easier to control the Spitfire "quirks" (as I shall diplomatically call them) with a full scale column that with a desktop joystick.

 

Good points Cripple in placing the comments in context, I agree that is important.

 

However I think understanding what Yo-Yo wrote is the key to understanding the general comments found in the Spitfire Operating Instructions:

 

Yo-Yo says:

The flying qualities of two different planes hqving the same stability factor (%% of MAC to the neutral) can be very different regarding of damping, stick forces, etc. Moreover, the neutral point depends on CL (so the speed of 1g flight), and power setting.

 

Flying qualities is Coefficient of Lift dependent. As speed increases, our coefficient of lift decreases. Lets look at the slope of the static longitudinal stability curve of the Spitfire and see what happens.

 

When the curve is moved up or down, the point it intersects the 0 line is our trim speed.

 

In this case the red line approximates the curve shifted up to represent the aircraft trimmed for a speed of 250mph IAS. Forgive my not so good line shifting....it is not meant to make specifics only to show the concepts.

 

The slope of the Spitfires stability curve gets larger the lower the coefficient of lift. Our stability increases and at 250mph IAS, the aircraft has good static longitudinal stability and with the light stick forces, I would certainly agree it is not only satisfactory at this speed but it must have been quite pleasant to fly around.

 

That does not have anything to do with the weak static longitudinal stability experienced elsewhere in the envelope or the dynamic longitudinal neutral to unstable found at normal to aft CG.

 

[Yo-Yo]

Two questions to the OP regarding your claim that the Spitfire Mk9 is unstable:

 

1) The Spitfire Mk9 had 17.5 lb ballast and a 5 lb ballast mounting in the tail. Why would the designers put ballast in the tail of an unstable plane?

 

2) You say you are in possession of a report on the Spitfire Mk9 showing it to be unstable. What report is this?

 

Nobody tells about INSTABILITY of Mk 9, yeah? The initial statements of mine were 1) Mk V is NEUTRAL (see the curves in the NACA report) at least with fixed controls (we can not simulate free controls till all our customers have FFB :) ).

2) Somebody, I can not remember now who, mentioned that Mk IX due to its new engine got CoG more forward.

3) Ok, I answered that if so, one would want to lighten pitch control because of increased stability, for example, increasing the horns.

4) Somebody found that the horns area was increased...

 

Anyway, I think that all these speculation have absolutely no sense because there are accurate data of Mk stability and CoG position for V and IX regarding the constant aerodynamical focus position.

[Pilum]

Nobody tells about INSTABILITY of Mk 9, yeah? The initial statements of mine were 1) Mk V is NEUTRAL (see the curves in the NACA report) at least with fixed controls (we can not simulate free controls till all our customers have FFB :) ).

2) Somebody, I can not remember now who, mentioned that Mk IX due to its new engine got CoG more forward.

3) Ok, I answered that if so, one would want to lighten pitch control because of increased stability, for example, increasing the horns.

4) Somebody found that the horns area was increased...

 

Anyway, I think that all these speculation have absolutely no sense because there are accurate data of Mk stability and CoG position for V and IX regarding the constant aerodynamical focus position.

 

I think you misunderstand Yo-Yo: I never claimed you said the Spitfire Mk9 was unstable. But unless Crumpp changes the title of the OP then someone for sure IS claiming the the Spitfire Mk9 is UNSTABLE.

 

The questions are directed to Crumpp since he is the one claiming the Spitfire Mk9 is unstable. IMHO the answer to question 1 is you don't add ballast in the tail to an unstable plane. You add ballast to decrease the STABILITY of a STABLE aircraft. Ergo the Spitfire Mk9 is STABLE. Q.E.D.

 

I think the answer to question 1 also answers question 2 so I'm not holding my breath on that one........

[Yo-Yo]

Yeah, I think I missed something amongst 17 pages... nevertheless, I can assume that due to more powerful engine even Mk 9 can be at least neutral at TO power. :) Let's see...

[bongodriver]

Nobody tells about INSTABILITY of Mk 9, yeah? The initial statements of mine were 1) Mk V is NEUTRAL (see the curves in the NACA report) at least with fixed controls (we can not simulate free controls till all our customers have FFB :) ).

2) Somebody, I can not remember now who, mentioned that Mk IX due to its new engine got CoG more forward.

3) Ok, I answered that if so, one would want to lighten pitch control because of increased stability, for example, increasing the horns.

4) Somebody found that the horns area was increased...

 

Anyway, I think that all these speculation have absolutely no sense because there are accurate data of Mk stability and CoG position for V and IX regarding the constant aerodynamical focus position.

 

I raised most of those points, yes the late war horn balanced elevators (Spitfire mod 789) are a significant factor that allowed the CG to move further aft to 9.0" in a MkV, the same elevator used in late MkIX's

I understand you say the MkV is Neutral and there is no issue with that, it is Crumpp's claim that the spitfire is unstable in normal CG range that is the questionable theory.

[Yo-Yo]

I raised most of those points, yes the late war horn balanced elevators (Spitfire mod 789) are a significant factor that allowed the CG to move further aft to 9.0" in a MkV, the same elevator used in late MkIX's

I understand you say the MkV is Neutral and there is no issue with that, it is Crumpp's claim that the spitfire is unstable in normal CG range that is the questionable theory.

 

No, you mixed the reason of horn area increasing. If you move CoG FORWARD you increase stability. Thus, more elevator deflection is necessary for the same g at the same speed, i.e. more stick force per 1 g. More hinge moment compensation - less stick forces.

[bongodriver]

No, you mixed the reason of horn area increasing. If you move CoG FORWARD you increase stability. Thus, more elevator deflection is necessary for the same g at the same speed, i.e. more stick force per 1 g. More hinge moment compensation - less stick forces.

 

Not quite what I was getting at, I have argued from the beginning that stability increases with forward CG, something that Crumpp has denied in the case of the MkIX, I also understand the function of the elevator balance.

The elevator mod I mention does allow an increase in aft CG range according to provided documentation though, the same modification is present on the Fighter collections MkVB EP120.

[Crumpp]

I think you misunderstand Yo-Yo: I never claimed you said the Spitfire Mk9 was unstable. But unless Crumpp changes the title of the OP then someone for sure IS claiming the the Spitfire Mk9 is UNSTABLE.

 

The questions are directed to Crumpp since he is the one claiming the Spitfire Mk9 is unstable. IMHO the answer to question 1 is you don't add ballast in the tail to an unstable plane. You add ballast to decrease the STABILITY of a STABLE aircraft. Ergo the Spitfire Mk9 is STABLE. Q.E.D.

 

I think the answer to question 1 also answers question 2 so I'm not holding my breath on that one........

 

When an design is changed or reconfigured, adding ballast simply corrects the CG back to its original location to prevent possible adverse loaded condition.

 

It has nothing to do with changing the CG to new limits and simply maintains the stability and control characteristics of the original aircraft.

 

For example:

 

Airplanes New Empty weight moment = 1150

 

New empty weight CG is 2 inches forward

 

Arm for our ballast attachment point is 100 inches

 

[1150*(+2)]/100 = 23lbs of weight attached to our ballast point to move the empty weight CG back to its original location.

Edited October 22, 2015 by Crumpp
removed empty weight...you can temporarily ballast an aircraft

[bongodriver]

When an design is changed or reconfigured, adding ballast simply corrects the CG back to its original location to prevent possible adverse loaded condition.

 

It has nothing to do with changing the CG to new limits and simply maintains the stability and control characteristics of the original aircraft.

 

.

 

of course you have just confirmed everything you have been arguing against, the MkIX is more nose heavy and becomes too stable, they add ballast to return it's 'desireable' stability and control characteristics.

[Crumpp]

I know you think that but it is ok, really.