Why do Spitfires turn better than Hurricanes?!

[TomcatViP]

+1

We shld not forget that planes at those ages had a fairly low P/W ratio (power vs weight).

Hence once the weight component was added to the thrust line it multipliable the propulsive power

[Viper2000]

The original design for what became the Spitfire had straight taper. The elliptical planform happened because the man from the ministry kept demanding more guns.

There wasn't enough depth to accommodate them. The two options were to increase chord outboard or to increase t/c. Mitchell was a clever man, so he opted for the former.

However, the subtext of this was that he didn't anticipate a production run of 20,000+, mostly built in shadow factories.

He thought that Supermarine would probably make a few hundred at most, and therefore the extra work entailed in a nightmare of compound curves was quite a neat way of making work to keep his company in business.

I suspect that had he not died before his time, the Spitfire would probably have been rapidly been replaced by a more practical follow-on aeroplane with straight taper, or perhaps polytaper; though perhaps more interesting still is the possibility that fighter work might have been entirely handed to Hawker so that Supermarine could concentrate on their bomber, which was effectively a 4 engined heavy with Mosquito speed...

Meanwhile the P-47 was very fast going downhill, but had quite a low tactical Mach number, and was also rather a nightmare to manage due to the extra workload and failuremodes inherent in the turbo.

[Sternjaeger II]

Originally Posted by TomcatViP

No problem. The ultimate fighter Pilot is the utmost finest Engineer (this is an awfully wrong statement )

Do you remember the Venturi effect when the air is accelerated through a narrower section ?
Now take a wider look at a wing section. If you focus either on the upper or the lower surface, you will see that regarding to the free stream of air unperturbed by the airfoil, there is a section increase as the thickness of the airfoil increase and the a similar decrease after the point of inflexion.

The direct effect of this (appart from the direct generation of LIFT) is that the air is accelerated and then as the section increased, expended with a rather brutal Pressure increase.

Now imagine that (I am actually singing it) you are flying at a speed nearly 2/3 of the speed of sound.

Due to the imaginary geometry described above, you can understand that the air is accelerated trough this partially materialized venturi. The increase in speed being directly proportional the the section decrease. Hence the more thickness the more the air flow is accelerated

When the speed and the wing's thickness are high enough, the airflow ard the wing reach Mach 1, the speed of sound. As the air goes further back along the chord, the air is expended (the distance btw the free airstram and the airfoil increase) and the air is decelerated bellow Mach one trough a pressure shock. This pressure shock is what we call a shock wave.

Now let say simply that due to he fact that the pressure distribution is modified because of both the shock waves above and bellow the wing, the LIFT moment is modified with a negative upward (relative to the chord) pitch down moment.
What you can see is that the more the wing is thick, the earlier the air ard the airfoil section reach the critical mach number.

Hence the thin airfoil series of the 50's fighters (Starfighter, F105, Mig 21 etc..).

This is why the compressibility affect the wing and not so much the generally thinner tail section.

Interestingly too this is what makes the Spitfire so fast in a dive. What you can see now is the sublime irony of mother nature that turned a wrong design assumption (the elliptical wing) in a wining parameter. I tell you Germany cldn't hve win !

Note : The 47 had an elliptical wing too. And it was also awfully fast in a dive!

~S!

uhmmm it's still a matter of proportions, especially when getting to supersonic speeds.

When doing wind tunnel studies, the transonic/supersonic experiments need to be made on a life size model to make an accurate observation of the shock phenomena, which behave and change dramatically according to the size of models.

I would like to hear Viper's opinion on the matter, do you reckon tail surfaces would hit compressibility before wings or viceversa?

[Viper2000]

Originally Posted by Sternjaeger II

I would like to hear Viper's opinion on the matter, do you reckon tail surfaces would hit compressibility before wings or viceversa?

I think the only reasonable answer to a question like that is "It depends...".

Really this "hit compressibility" concept is horrid. Air is compressible all the time. If you go fast then compressibility becomes more important. But I'd generally be inclined to consider compressibility in calculations for M>0.2 if I wanted accurate answers. Really the Mach number at which you elect to consider compressibility is a bit like the temperature at which you decide to account for variation in the Cp of dry air - it's arbitrary, and depends upon the computational resources available and the effort you're prepared to put in.

I'd generally expect the tail to be lower aspect ratio than the wing. Therefore it can be thinner. I'd also expect it to be at lower absolute CL than the wing. So, if we assume the same amount of sweep for wing and tail, I'd expect the tail to remain subsonic to a higher freestream Mach number than the wing.

But the generalisations associated with the above are dramatic. Control deflection, variations in wing downwash angle and so on could easily make quite a big difference.

In the WWII context, with manual controls you're likely to find that the pilot runs out of bicep before shockwave development over the tail starts to bite.

Here's a quick list of the factors causing "Mach tuck":

• Migration of the wing's centre of pressure
• Changing wing downwash angle
• Rapidly increasing stick force for constant elevator deflection
• Reduced elevator effectiveness due to shock developement over the tail

You can see that 3 out of the 4 don't require shock development over the tail.

OTOH, the underlying problem is the tail; because that's where the control surfaces are, and the problem is a control problem.

So the brute-force approach towards the end of WWII and immediately thereafter was to hydraulically boost everything. Once you do that then the new problems are dealing with the failure modes and providing Q feel so that the pilot doesn't break the aeroplane.

Then you might get into trouble with lack of elevator effectiveness, because the elevator can't affect the pressure distribution upstream of any shock which may have formed over the tail. But really it's unreasonable to think of the flying tail as a "solution" to this problem, because going to a flying tail in 1940 wouldn't have solved fighter stability & control problems. Without irreversible screwjacks to drive the thing, you'd find that either it fluttered off or else the control forces were impossibly high.

Really the concept of the "flying tail" is a sort of marketing thing rather than reality. There's nothing magic about it. The simple reality is that if you're supersonic the you need the movable bit of your control surface to do all the work by itself, and you therefore have to size it appropriately. Note that delta winged fighters do just fine with elevons - no need for the surface to have its own private leading edge to work - it just needs to be big enough and to be driven by a sufficiently strong set of jacks.

In subsonic flight the elevator can be quite a lot smaller because it affects the lift curve slope of the whole surface it's attached to, which is great if you've only got a relatively limited actuating force available. Hence the rapid ubiquitous adoption of the elevator in subsonic aeroplanes quite early in the evolution of the aeroplane.

The other thing which people got used to is the idea that in general a nice set of aerodynamic controls will give you stick free stability which may well mask any nasty stick-fixed behaviour your design may exhibit. During WWII the added friction associated with pressurised cockpits started to show up some of the stick fixed problems that had been lurking under the rug for decades, and this caused people to start working on tweaking the control system itself via bob-weights etc to affect the subjective handling characteristics of the aeroplane, rather than just expecting the pilot to tolerate what he was given.

This gradually took us towards full FBW/manoeuvre demand systems, which allow you to design the apparent handling of aeroplanes to be almost independent of their aerodynamics. Which is great, though it does open all sorts of philosophical cans of worms since you now have to think about what the ideal set of control laws would be, rather than just asking the test pilot whether he can fly the thing or not...

The loss of stick free stability also rather reduced the importance of the fixed portion of the tail from a handling perspective, and since the elevator had to be big enough for the supersonic control task, it was almost always going to end up big enough for the subsonic case and therefore why not just bin the fixed tail?

Actually there are several reasons, especially for larger aeroplanes; essentially a fixed tail with an elevator is likely to be lighter, though you may have to also vary its incidence for trim, which takes some of the advantage away (though the trim change can be an order of magnitude slower than the control trim, since effectively the trim change has to damp the phugoid whilst the control change has to damp the short period oscillation, and therefore the trim actuators can be smaller).

But I digress; time to get a cup of tea & get back to my thesis...

[Sternjaeger II]

sorry man, that's pilot jargon, the correct question would be "do you reckon that the effects of air compressibility would affect first the control surfaces or the wing?".

Thinking about the effects of compressibility puts the whole research and introduction of delta wings and canard wings into a very interesting perspective

[Viper2000]

Originally Posted by Sternjaeger II

sorry man, that's pilot jargon

No no no! Please watch the following training videos immediately.

Pilots use banter.

http://www.youtube.com/watch?v=5rKYL0tW-Ek

Only engineers are allowed to use jargon.

http://www.youtube.com/watch?v=bOV0v1Uq5CY

[TomcatViP]

Originally Posted by Viper2000

I think the only reasonable answer to a question like that is "It depends...".

Really this "hit compressibility" concept is horrid. Air is compressible all the time. If you go fast then compressibility becomes more important. But I'd generally be inclined to consider compressibility in calculations for M>0.2 if I wanted accurate answers. Really the Mach number at which you elect to consider compressibility is a bit like the temperature at which you decide to account for variation in the Cp of dry air - it's arbitrary, and depends upon the computational resources available and the effort you're prepared to put in.

I'd generally expect the tail to be lower aspect ratio than the wing. Therefore it can be thinner. I'd also expect it to be at lower absolute CL than the wing. So, if we assume the same amount of sweep for wing and tail, I'd expect the tail to remain subsonic to a higher freestream Mach number than the wing.

But the generalisations associated with the above are dramatic. Control deflection, variations in wing downwash angle and so on could easily make quite a big difference.

In the WWII context, with manual controls you're likely to find that the pilot runs out of bicep before shockwave development over the tail starts to bite.

Here's a quick list of the factors causing "Mach tuck":

• Migration of the wing's centre of pressure
• Changing wing downwash angle
• Rapidly increasing stick force for constant elevator deflection
• Reduced elevator effectiveness due to shock developement over the tail

You can see that 3 out of the 4 don't require shock development over the tail.

OTOH, the underlying problem is the tail; because that's where the control surfaces are, and the problem is a control problem.

So the brute-force approach towards the end of WWII and immediately thereafter was to hydraulically boost everything. Once you do that then the new problems are dealing with the failure modes and providing Q feel so that the pilot doesn't break the aeroplane.

Then you might get into trouble with lack of elevator effectiveness, because the elevator can't affect the pressure distribution upstream of any shock which may have formed over the tail. But really it's unreasonable to think of the flying tail as a "solution" to this problem, because going to a flying tail in 1940 wouldn't have solved fighter stability & control problems. Without irreversible screwjacks to drive the thing, you'd find that either it fluttered off or else the control forces were impossibly high.

Really the concept of the "flying tail" is a sort of marketing thing rather than reality. There's nothing magic about it. The simple reality is that if you're supersonic the you need the movable bit of your control surface to do all the work by itself, and you therefore have to size it appropriately. Note that delta winged fighters do just fine with elevons - no need for the surface to have its own private leading edge to work - it just needs to be big enough and to be driven by a sufficiently strong set of jacks.

In subsonic flight the elevator can be quite a lot smaller because it affects the lift curve slope of the whole surface it's attached to, which is great if you've only got a relatively limited actuating force available. Hence the rapid ubiquitous adoption of the elevator in subsonic aeroplanes quite early in the evolution of the aeroplane.

The other thing which people got used to is the idea that in general a nice set of aerodynamic controls will give you stick free stability which may well mask any nasty stick-fixed behaviour your design may exhibit. During WWII the added friction associated with pressurised cockpits started to show up some of the stick fixed problems that had been lurking under the rug for decades, and this caused people to start working on tweaking the control system itself via bob-weights etc to affect the subjective handling characteristics of the aeroplane, rather than just expecting the pilot to tolerate what he was given.

This gradually took us towards full FBW/manoeuvre demand systems, which allow you to design the apparent handling of aeroplanes to be almost independent of their aerodynamics. Which is great, though it does open all sorts of philosophical cans of worms since you now have to think about what the ideal set of control laws would be, rather than just asking the test pilot whether he can fly the thing or not...

The loss of stick free stability also rather reduced the importance of the fixed portion of the tail from a handling perspective, and since the elevator had to be big enough for the supersonic control task, it was almost always going to end up big enough for the subsonic case and therefore why not just bin the fixed tail?

Actually there are several reasons, especially for larger aeroplanes; essentially a fixed tail with an elevator is likely to be lighter, though you may have to also vary its incidence for trim, which takes some of the advantage away (though the trim change can be an order of magnitude slower than the control trim, since effectively the trim change has to damp the phugoid whilst the control change has to damp the short period oscillation, and therefore the trim actuators can be smaller).

But I digress; time to get a cup of tea & get back to my thesis...

Any how I managed to write it down it seems that the tail has to be looked with suspiciousness.

When it comes to fluid mechanics don't be obsessed by tail's story



Just to make thing more clear and easier : the path to supersonic speed at those time went troughs symmetrical airfoils then to the all flying tail (AFT) unit. I am not sure that a full array of Bell and NACA engineers would hve been fooled such a way to design the X1 with a conventional tail if it could hve not fly faster than Mach 0.66 (compressibility).

idem for the F86 with thckness ratio decrease then AFT

[Sternjaeger II]

Originally Posted by TomcatViP

Any how I managed to write it down it seems that the tail has to be looked with suspiciousness.

When it comes to fluid mechanics don't be obsessed by tail's story



Just to make thing more clear and easier : the path to supersonic speed at those time went troughs symmetrical airfoils then to the all flying tail (AFT) unit. I am not sure that a full array of Bell and NACA engineers would hve been fooled such a way to design the X1 with a conventional tail if it could hve not fly faster than Mach 0.66 (compressibility).

idem for the F86 with thckness ratio decrease then AFT

I don't think you understand what I mean man, but I'm afraid I'll have to give up on this, since you don't really seem to read the answers, you just wait to reiterate your theories..