Its kinda hard getting your hear around it but it happens on cars and motorcycles too at the limit with some types. When you are on the as coming out of a corner in a rear wheel drive rally car you are steering right on a left hand corner but the steering input is just to keep the angle of the car correct for powering out.
P-51 fuselage fuel tank
- Thread starter Barrett
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Thank You for posting those pages Bill,
Yes, plate II-1 is Static Stability and shows push or pull forces on the control stick for maintaining level flight with speed deviations away from level flight trimmed at 100mph, at different
CG, %MAC, as shown.
Plate II-1 cont'd shows a simple illustration of linear stick force per G at varying CG, %MAC.
Neither of these plates appear to represent the P-51 data.
Going back to the 4-43-23-1 Eglin test report of the P-51B aircraft with rear 85gal tank, we can see that the results were much more complicated. Although the results are not presented in a detailed or graphic way, we can read in section 6.c that, with a full rear tank, stick force reversal occurred above accelerations of between 3 to 5g. Presumably, below 3 to 5g the
stick force response was normal and, in fact, the criticism of handling only really seems to be aimed at tight turns and pull-outs. Going back to plate II-1 cont'd, we can see that stick force per G is shown there as a straight relationship, but here the relationship seems to have changed at about 3g. We know that the full rear tank is taking the CG, %MAC some 1.8" to 2.7" beyond it's normal rear limit. My guess is that the limitations and characteristics of the pitch control system pretty much bounded the normal 102" CG, %MAC limit, and so this extra weight so far aft was just causing the undesirable handling that is described. Fortunately, careful operation of the aircraft and crew training allowed the use of this great increase in capability.
Thanks for the extra info.
Eng
Yes, plate II-1 is Static Stability and shows push or pull forces on the control stick for maintaining level flight with speed deviations away from level flight trimmed at 100mph, at different
CG, %MAC, as shown.
Plate II-1 cont'd shows a simple illustration of linear stick force per G at varying CG, %MAC.
Neither of these plates appear to represent the P-51 data.
Going back to the 4-43-23-1 Eglin test report of the P-51B aircraft with rear 85gal tank, we can see that the results were much more complicated. Although the results are not presented in a detailed or graphic way, we can read in section 6.c that, with a full rear tank, stick force reversal occurred above accelerations of between 3 to 5g. Presumably, below 3 to 5g the
stick force response was normal and, in fact, the criticism of handling only really seems to be aimed at tight turns and pull-outs. Going back to plate II-1 cont'd, we can see that stick force per G is shown there as a straight relationship, but here the relationship seems to have changed at about 3g. We know that the full rear tank is taking the CG, %MAC some 1.8" to 2.7" beyond it's normal rear limit. My guess is that the limitations and characteristics of the pitch control system pretty much bounded the normal 102" CG, %MAC limit, and so this extra weight so far aft was just causing the undesirable handling that is described. Fortunately, careful operation of the aircraft and crew training allowed the use of this great increase in capability.
Thanks for the extra info.
Eng
While this will probably be wrong, and overly simplified: Is this almost like that the tail wants to keep going forward in a turn on momentum, and you end up doing a donut?
*scratches head* Is this something to do with neutral stability (no AoA change with speed when AoA is increased), deceleration (which drives up AoA), and potential effects of the tractor-prop on stability?
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drgondog
Major
Explain your logic for 'much more complicated', please.
Facts to support 'presumably' and 'only seem' would be nice.
explain 'seems to have changed' from straight line relationship. Conclusions regarding acceleration force of 3G?
I believe the phrase was 'control reversal' - well illustrated in Horkey's graphics between Push and Pull forces plotted for CG and airspeed.
I am not a pilot so I havnt a clue about how it works in the air. On the ground in motorsport the car or bike is "balanced" between all the forces acting on its centre of mass, even though the back wheels arent following the front and the whole thing isnt pointed in the direction it is actually going, its a controlled slide. The input into the steering is just to maintain that balance. If ever you go to a kart track, even with the beginners type powered by 5 HP lawn mower engines, the steering is really heavy until you really start to corner on the limit and then it feels like the steering wheel is disconnected.
Hi. This is quite complicated really, because it is a case where the aircraft is being modified to operate beyond the normal limits where it handles in a stable and easily handled way.
The base line here is that for stability and normal handling in upright normal flight, the horizontal tail produces a downforce to balance the aircraft weight and the wing lift, the lift being behind the C of G weight and the tail pushing down a bit to balance. A simple stable aircraft has a generous amount of stability if the horizontal tail has plenty of effect available compared to the Lift/Weight couple. Now, high performance aircraft generally reduce the margins of control to minimise structural weight and drag, but must still handle well. However, if you take any normal aircraft and put more weight towards the tail, it becomes less stable because the horizontal tail has to generate smaller control forces to balance the aircraft. The normal C of G limit defines the normally acceptable handling range so, if you break that limit, you get into handling problems. In reality, this is a very complex subject.
Eng
Hi Bill,
Unfortunately, we only seem to have the written reports that lack tabulated data to define the actual issue. I will try and explain your points raised in more replies.
Eng
The 2 graph's you posted by Horkey do not illustrate "control reversal". The graph with Push-Pull forces illustrates static stability. It shows how the stick forces vary with an aircraft trimmed for level flight at 100mph when speed is reduced or increased by pushing or pulling. The text describes how this was done and how the forces reduce with an increasingly
aft C of G, illustrated by the varying slope of the lines. Normal stability is illustrated, where the aircraft slows by pull force, and speeds up with push force. Although the text mentions negative stability, it is not illustrated. The undesirability of "too low a slope or negative stability" is mentioned but not shown. "Control reversal" is not mentioned and it is not indicated on this graph. The tests shown were conducted at 1g. This description has no reference to control reversal or variation or of stick forces at more than 1G loading.
The second graph (accelerated) illustrates stick forces under varying G loads at varying C of G. It shows linear relationships and no "control reversal". There is no description of the trimmed speed that the graph applies to.
Eng
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Hi,
I feel that the handling described in the Eglin test report 4-43-23-1 is much more complicated than the illustrations and text in Horkey's information that you posted.
Eng
It said words to that effect in the report I posted, ''considerable forward pressure on the stick'' was required to stop the aircraft from tightening it's turn which I assume means it'll rapidly enter a flat spin if not corrected.
Hi,
The Test report 4-43-23-1, Sect 3. b "violent (!) pullouts or tight turns must be carried out with caution as stick loads rapidly reverse; with the fuselage tank half empty these manoeuvres may be executed in practically the normal manner".
This would indicate that the "stick load" change from "practically normal" to "reversed" above half fuselage tank full.
Sect 6.c
Sect 6.d . These parts describe the change of handling at Full and Half tank contents. The emphasis is on the CHANGE of handling with Full Tank that occurs at "three (3) to five (5) "G" with Full tank."
Therefore, with the above (sect 3.b and 6.c/d) it would seem that the variation of C of G made a big difference in the handling.
Unfortunately, no detailed figures or graphical information.
Eng
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drgondog
Major
It is quite clear in Horkey's discussion in which the stick force steadily decreases from positive to zero, then continues to a 'push; on the stick as turn accelerations increase with aft cg condition.
See above.
No, but enough information that NAA developed Bob Weight for control stick to alleviate some of the 'impending reversal' notification. Unfortunately the necessary flight safety provisions ranging from Dorsal Fin and Reverse Rudder Boost Tab and Bob Weight chipped away at the superb handling qualities of the Mustang.
drgondog
Major
Yes, and No. Mathematically Horkey's plots as shown are of Mustang calculations. In a perfect fluid, they would be linear as shown. In the real world, constantly varying turbulence motion air is an everchanging influence on flight controls, particularly in a turn in which the aerodynamic loads are asymmetrical. The 'high' wing is at a relatively higher AoA, more CL and more drag than the lower wing - requiring rudder input to trim the turn as contrast with a skidding turn.
WRT the flight test itself, the original fuel tank did not have an internal baffle to retard rapid changes internally of center of mass of the AvGas as it reduced from full. That could be particularly 'interesting at about 60-65gal where the real time center of mass fluctuates due to accelerations.
So, that big pdf is the Technical order No. 01-60J-29 dated 18 September 1944 North American-Installation Of Elevator Inertia Weight-P-51B, P-51C And P-51D.
The modification introduces an approx 20lb lead "inertia weight" onto the elevator control run.
I believe this shows that the P-51 aircraft did not have such a weight in the elevator before this modification.
The addition of the weight is likely to have increased the stick loads to pull positive G, enough to prevent stick force reversal with a full rear fuselage tank on these aircraft.
Eng
The modification introduces an approx 20lb lead "inertia weight" onto the elevator control run.
I believe this shows that the P-51 aircraft did not have such a weight in the elevator before this modification.
The addition of the weight is likely to have increased the stick loads to pull positive G, enough to prevent stick force reversal with a full rear fuselage tank on these aircraft.
Eng
drgondog
Major
see post 213. That said, most of the B-15, C-10, mid block D-5 & Subs had the Bob weight delivered at factory before July 1944.
Well, it is interesting that the static stability is illustrated with a trimmed 100 mph condition and shows test points at 50 mph airspeed. Some P-51.
The lack of suitable tank baffling on a long large tank (and beyond the limit of C of G) seems like no-one reviewed the lack of baffles before cutting metal.
Eng
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I thought the dorsal-fin root extension was a net positive across the board, fascinating.
Well, I think Bill is commenting on changes that added weight that moved the C of G rearwards and compromised handling, which then caused the extra modification of the controls, which made the control forces heavier, which makes the general handling characteristics less pleasing.
Eng
