Stability and Control characteristics of the Early Mark Spitfires

[Crumpp]

Attached also the quote from Perkins and Hage to show how the use of bob weigh and CoG at aft center of gravity are connected

Why are you regurgitating the same stuff I just posted. Puzzling...

Anyway, yes, the bob weight artificially increases the stick forces so that the pilot can have more control.

That is band-aid to fix the longitudinal instability the NACA reported!!!

The fact Supermarine recognized that longitudinal instability and took measures to fix it invalidates any pointy tin foil hat theory the instability did not exist.

[Crumpp]

Now let's get back to the NACA report so there is a better understanding of the issue.

We will look at a condition of flight essential to a dogfighter. The ability to make abrupt turns.

The pilot must be able to precisely control the amount of acceleration he loads on the aircraft. All aircraft performance depends on velocity. In order to get maximum performance out of the aircraft above maneuvering speed, Va, he needs to be able to make a 6 G turn and not exceed that load factor to prevent damage to the airframe. Below Va, the pilot needs to control the acceleration so that he does not stall the aircraft making the abrupt maneuver as well being able to maintain a maximum performance turn.

Doing that in an early Mark Spitfire was difficult and something only a skillful pilot could perform.

First the NACA report. Abrupt 180 degree turns were conducted at various entry speeds to gauge the level of control the pilot had in maintaining steady accelerations. The turns were also done to the stall point in order to gauge the behavior and amount of control.

"In turns at speeds high enough to prevent reaching maximum lift co-efficient" means turns above Va.





"By careful flying" a pilot can hold a steady acceleration. That agrees with the Operating Notes warning for the pilot to brace himself against the cockpit to get better control when making turns.

Now lets look at the measured results.



Here we see in a rapid left turn performed at 223 mph the test pilot is unable to hold constant acceleration on the airframe. Very small variations in stick movement and stick force changes of 1-3lbs results in large fluctuations in acceleration.

Next let's look at the pilots ability to control the accelerations in the pre-stall buffet.



Here we see the pilot was able to load the airframe to 5G's in 1 second to reach the pre-stall buffet 3 times. The smooth positive sloped portion of the curve represents the aircraft flying while accelerations are increasing. The top of the acceleration curve represents the pre-stall buffet. The bottom of the curve represents the stall point.

The take away is:

1. The large accelerations change for very little elevator movement.
2. The very rapid rate at which the pilot was able to load the airframe to 5G's.
3. The equally rapid rate at which the airframe unloaded down to 2G's when the pre-stall buffet was encountered. In 1 second, the aircraft went from 5G's to 2G's due to buffet losses. This means a rapid decay in turn rate resulted.
4. The violence of the pre-stall buffet combined with the longitudinal stability and control caused large fluctuations in the accelerations on the aircraft.


Last part of the NACA we will cover for today is the stick force travel. The amount of stick travel as measured by the NACA was not acceptable.




Next let's look at the opinion of Stability and Control Engineers on the Early Mark Spitfires.









Tomorrow I will post some of the plethora of references to this same issue of longitudinal instability as found in the Spitfire Mk I Operating Notes from July 1940. You will see the same references or similar to the same issue the NACA measured in the Spitfire Mk II Operating Notes.

There is no doubt that the Air Ministry was aware of the longitudinal instability of the early mark Spitfires.

[Crumpp]

NACA did not have any drawings for the Spitfire and specifically state that their measurements might have been in error -

Correct, the NACA did their own weight and balance analysis so their numbers might not agree with the RAF's. That has nothing to do with flying the airplane outside of the CG limits.

without a weight and loading diagram of the specific aircraft tested there is no way to know whether the aircraft was teetering on instability because it was loaded beyond the usual tolerances.

You don't need a thing from the manufacturer to do a weight and balance analysis. Your leap of logic requires some pretty hefty suspension of belief. One would have to assume the NACA was incapable of doing a simply weight and balance analysis and constructing a potato chart. That is something every homebuilder in the United States does in his garage. A weight and balance analysis and constructing a potato chart is also something every FBO is capable of doing. It is a routine process in aviation.

You need a few simple tools and the knowledge to run the math is all. It is obvious you don't know what you are talking about but are only trying to muddy the waters in defense of a gameshape.

You do not understand the process and do not realize the datum point is just a random point picked to begin measurements.

So what if the NACA picked a line of rivets that is ~5 inches away from the one Supermarine chose on the back of the firewall instead of the front.....

The reference datum point can be anywhere the person doing the weight and balance analysis decide's to put it.

[Crumpp]

MAC as measured by RAE:

19.5+8.4 = 27.9/84 = 33.2%

NACA CG as flown = 31.4% MAC

The NACA flew the Spitfire with the CG 1.8% MAC FORWARD of the aft CG limit as defined by Supermarine.

[TomcatViP]

Don't loose your time commenting "comments", Crumpp. Hold the line.

Ended already my long reading for tonight. Enjoyed it as much as any other good entertainment as it shld be from any interested reader in aviation. Sad you don't sell any popcorn.

So now I am waiting for the interludes played by our looney toons

~S

PS: that story about the NACA not having any drawing is true... but they neither had any for the 109 they tested

[Crumpp]

that story about the NACA not having any drawing is true... but they neither had any for the 109 they tested

Yep, it is not like switching aviation fuel. Weight and balance analysis is simple and easy to do.

[MiG-3U]

Originally Posted by Crumpp

Why are you regurgitating the same stuff I just posted. Puzzling...

Nothing puzzling there, you just forgot to underline the most relevant part for this thread and I fixed it for you; the bob weigh was added because the CoG had slipped too far aft causing londitudinal instability just like Quill notes.

Originally Posted by Crumpp

The fact Supermarine recognized that longitudinal instability and took measures to fix it invalidates any pointy tin foil hat theory the instability did not exist.

Yep, when the CoG slips too far back, instability exist in pretty much every airplane, including Spitfire and Mustang. Bob weighs were used in some models of both fighters exactly for the same reason: CoG shift. Both aircraft were also stable when the CoG was forward enough.

Here is again A&AEE on stability of the early mark Spitfire:

http://www.spitfireperformance.com/k9787-fuel.html

(v) Stability - The aircraft is laterally stable at all speeds except in the immediate vicinity of the stall when it is unstable. The aircraft is directionally stable engine 'OFF' and 'ON' at all speeds, but on the climb this is difficult to assess owing to insufficient rudder bias. Longitudinally, the aircraft is stable with centre of gravity forward, but is unstable with centre of gravity normal and aft with engine 'OFF' and 'ON'.

CoG was at 5.8" aft datum line when the airplane was found to be longitudinally stable, that is 2" more forward than Naca tested. Note that Spitfire IX had CoG around 5" aft datum line at service load, no need for bob weigh.

[Crumpp]

Slow down and tell me where you get the 78.54 MAC on that sheet.

Do you know what percentage MAC is??

The reason the NACA used percentage MAC is because they did their own weight and balance analysis.

The ONLY number that is comparible...is the percentage MAC!!!

MAC as measured by RAE:

19.5+8.4 = 27.9/84 = 33.2%

NACA CG as flown = 31.4% MAC

The NACA flew the Spitfire with the CG 1.8% MAC FORWARD of the aft CG limit as defined by Supermarine.

If you want to use your 78.54 in MAC with our most narrow aft CG limit...

CoG range 5.4"-7.9" aft of datum point, no inertia device needed

(19.5 + 5.4) / 78.54 = 31.7% MAC

NACA CG as flown = 31.4% MAC

The NACA flew the Spitfire with the CG .3% MAC FORWARD of the aft CG limit as defined by Supermarine.

[Crumpp]

Longitudinally, the aircraft is stable with centre of gravity forward, but is unstable with centre of gravity normal and aft with engine 'OFF' and 'ON'.

http://www.spitfireperformance.com/k9787-fuel.html

Check it out...That is what the NACA said.....

[Crumpp]

Ok let's not go down the rabbit hole again.

You are using a weight and balance sheet that incorporates the longitudinal stability fix and is from February 1944 to prove the NACA conclusion was not correct.

Yes, the RAE addressed the issue of the longitudinal instability in the Spitfire around 1942. However, the Spitfires used in the Battle of Britain did not benefit from the fix.


This is Spitfire K-9787 and was tested in June, 1939.

http://www.spitfireperformance.com/k9787-fuel.html

If you click on the center of gravity link at the bottom of the page...

The weight and balance diagram is K-9788, the very next Spitfire off the production line.

http://www.spitfireperformance.com/k...cg-diagram.jpg

We can eliminate the February 1944 document from the thread as not applicable and conclude it is the result of the NACA findings.

Which brings us back too:

MAC as measured by RAE:

19.5+8.4 = 27.9/84 = 33.2%

NACA CG as flown = 31.4% MAC

The NACA flew the Spitfire with the CG 1.8% MAC FORWARD of the aft CG limit as defined by Supermarine.