Aerodynamic Buffet of the Spitfire

In an airplane that is dynamically neutral in longitudinal stability, excellent stall warning is essential to safe flight.

The Spitfire had that in droves and this not a thread to bash the design. It speaks for itself and there are many good characteristics resulting from the airplanes stall warning. As mentioned, in a dynamically neutral airplane with only 3/4 inch stick travel from cruise flight to stall a deep buffet zone contributes enormously to making the airplane controllable.

To further narrow the topic, it is not stall characteristic but rather the effect of the buffet zone itself.

The compromise of a deep aerodynamic buffet zone is this is not a place you fight or operate the aircraft. In normal operations outside of practicing stalls or accidental stalling, the buffet zone is to be avoided.

Aerodynamic buffet is caused by flow separation. The wing sections experiencing buffet are in fact stalled but not enough sections have reached that point to stall the aircraft.

Designers have to balance the buffet zone. Too little buffet zone means the pilot can maximize the airfoil potential but gives the pilot no warning of an impending stall.

Too much buffet zone ruins a pilots ability to track a target effectively and quickly robs the aircraft of precious airspeed shrinking the performance envelope.

Attached is the Aerodynamic Buffeting standards based on load factor induced by the buffet. The NACA report delivers measured results in various configurations.

The largest factor in configuration appears to be whether the gun ports are open or closed.

Also included is the NACA test pilots comment on the gliding condition buffet effects on his ability to control the aircraft. This agrees with the buffeting standards for a moderate buffet. It is interesting to note that the gun ports determine the severity of the buffet in gliding condition. The gun ports closed results in a measured moderate buffet. The gun ports open results in a measured slight buffet.

In the rapid turning department, the major difference is gun ports closed results in a more severe buffet and consequently more rapid airspeed decay. Gun ports open condition also resulted in a rolling instability developing, that is another topic.

The report is included so the reader to examine and confirm.

Only the more gentle rapid turn measurement with gun ports sealed is included in the attachments.

I hope this helps and gives the reader some insight into the buffet zone behaviors we see in DCS.

"Benign flying characteristics" in this case would refer to the aircraft giving a more pronounced buffet warning with a wider margin of flying on the limit than another aircraft that would give minimal or no warning

Exactly. The pilot had no doubt when he reached the buffet zone in a Spitfire and this characteristic is the essential backstop required to fly it with the longitudinal stability and control characteristics.

Even though the bob-weight was incorporated into the Spitfire Mk IX production, it does not change the stability characteristics. It does change the control characteristics by increasing the force gradient giving the pilot more resistance.

The bob weight was a short term fix for the MkV and it's CoG problems, only MkV's were fitted with bob weights, subsequent marks had a new elevator design bar some very early Mk IX's

thanks for the file. It's an interesting read.

Fig 17 shows how small elevator deflection is required in order to get into an accelerated stall in the Spit. Buffet starts at about 5 degrees of elevator up deflection and the pilot never exceeds 10 degrees (out of a max of 20+ degs)

The bob weight was a short term fix for the MkV and it's CoG problems, only MkV's were fitted with bob weights, subsequent marks had a new elevator design bar some very early Mk IX's

Ok,

The instability is a function of the static margin, not the control surface or stick force per G gradient.

They could change the Stick Force per G gradient by changing the elevator design. Do you know exactly what they did?

In the rapid turning department, the major difference is gun ports closed results in a more severe buffet and consequently more rapid airspeed decay. Gun ports open condition also resulted in a rolling instability developing, that is another topic.

Interesting, I suspect with the gun ports open (i.e. not covered) there could be considerable turbulance over the wings and the ailerons. They are in fact quite considerable in size and located on the upper part of the leading edge, rather than in the centre.

Here's a picture of a Mark V, showing the gun ports and the large cannon stubs to good advantage.

Fig 17 shows how small elevator deflection is required in order to get into an accelerated stall in the Spit. Buffet starts at about 5 degrees of elevator up deflection and the pilot never exceeds 10 degrees (out of a max of 20+ degs)

It is no wonder pilots related "think it and the Spitfire does it".

I think Yo-Yo's assessment is spot on.

It is interesting to compare the P47 and other designs. I have several of the NACA stability and control reports.

The difference in the Spitfires control characteristics and the P-47D is dramatic.

Ok,

 

The instability is a function of the static margin, not the control surface or stick force per G gradient.

 

They could change the Stick Force per G gradient by changing the elevator design. Do you know exactly what they did?

Late elevators had an elongated horn.

nobody is suggesting instability is due to the control surfaces or force per g gradient, but an aircraft suffering from weak longitudinal stability will suffer further from having light control forces, this really only manifested as a problem in the MkV hence the bob weight quick fix.

Here's a picture of a Mark V, showing the gun ports and the large cannon stubs to good advantage.

Great picture! Yes, you can tell by the placement of the gun ports they will cause issues!!

Great picture! Yes, you can tell by the placement of the gun ports they will cause issues!!

Clearly nothing significant though.

Kurfurst,

The most important factor is the gun ports placement in relation to the leading edge. The higher on the leading edge of the wing, the higher the section coefficient of drag.

Drag and lift coefficients of pressure have a fixed direct relationship....when drag goes up, lift goes up.

That means the section lift coefficient reaches the buffet zone faster and the stall point over the gun ports.

Mystery solved on the Spitfire dramatic differences between gun ports and closed! Thanks for the picture.

Dramatic is hardly the word, a noticeable difference perhaps.

Late elevators had an elongated horn.

Did they increase or reduce the size of the elevator balancing? If they added surface area, then went in the wrong direction....I highly doubt they did this.

You sure they did not reduce the surface area which might serve to make the horn balance seem elongated?

All except the initial production Mk. IXs had elevators with extended mass balance horns. Whereas the early Spitfires had a 45-degree break in the hinge line, this new pattern had an additional 45-degree break, resulting in the tip of the elevator pointing straight forward.

As a rule of thumb the early pattern is seen on Mk. IXs converted from Mk. Vs, and early original Mk. IXs. I’ve seen both patterns on aircraft in the MA serial range (Castle Bromwich – mid-1943 vintage).

only MkV's were fitted with bob weights

You do know you can look in the airplane and see them on the differential??

Ok, the horn balance makes sense as they changed the tail angle of incidence adding 2 degrees. That changes the balance relationship so the area had to be changed.

Do not get confused, the tail produces a down force. Essentially they reduced the coefficient of lift the tail was producing in providing that down force for cruise flight or the design coefficient of lift.

The increase in angle requires more surface area to produce the same balance force on the pull up.

I'm not confused

You do know you can look in the airplane and see them on the differential??.

I'm only saying they were only fitted to MkV's, I have seen no evidence of them fitted to any other mark.

I have seen no evidence of them fitted to any other mark

Well, Look in the airplane....

They were incorporated into production. It's the counter-weighted lobes coming off the differential, bongodriver.

The bob weight was a short term fix for issues suffered by the Mk V only.

https://books.google.co.uk/books?num=19&id=2KdZBAAAQBAJ&q=59#v=snippet&q=59&f=false

Well, Look in the airplane....

That is all you have to do...

You passage does not say they did not incorporate it into production either. It just says they found an aerodynamic solution as well.

Since they were bending airplanes at a consistent rate in the Mk IX on pull outs from bombing runs, it sounds like the design needed more work in the longitudinal stability and control department.

Again, balancing does not change the static margin or the instability. It increase the stick force per G gradient improving control by increasing resistance.

The topic of increasing the stick force per G is covered in the design. Without new information there is no need to keep covering the same ground.

Well, Look in the airplane....

That is all you have to do...

Next time I have a spare Mk IX laying around the house I will do that, though the last conversation I had with Sammy Sampson of the Battle of Britain memorial flight where he told me bob weights were fitted to MkV's only is sufficient for me.

in the mean time I have been assuming you have all the evidence required?

You passage does not say they did not incorporate it into production either. It just says they found an aerodynamic solution as well.

Actually it does not say 'as well' at all, it clearly states an aerodynamic solution was found period.

Notice the problem 'disappeared' once the solutions were applied.

Since they were bending airplanes at a consistent rate in the Mk IX on pull outs from bombing runs, it sounds like the design needed more work in the longitudinal stability and control department.

Wild speculation as usual.

Again, balancing does not change the static margin or the instability. It increase the stick force per G gradient improving control by increasing resistance.

again this is exactly what was intended, the stability of the spitfire was not an issue except for the MKV, it was the control sensitivity that was being addressed.

The topic of increasing the stick force per G is covered in the design. Without new information there is no need to keep covering the same ground.

You are the one providing all the ground to be covered.

Yeah, buffet would be nice. But not as important for the IX as for the early marks, considering the heavier engine.

Here is the P-47D buffet zone characteristics. Dramatically different from the Spitfire Mk IX.

Here is the F6F-3 Hellcat for comparison.

Here is the P-47D buffet zone characteristics. Dramatically different from the Spitfire Mk IX.

Dramatically indeed.....considering there is no chart shown for the Mk IX but instead only the Mk V from your NACA Spitfire bible.