Originally posted by Shaw
Once again, sorry for the long reply time. As I've said before, I just don't get many opportunities to check this discussion.
Not a problem, I’m also very busy lately so I can’t check in very often, so I too would like to apologise for the delayed response.
Originally posted by Shaw
There are several exceptions I would take to your comments above:
Rightly so, there are some notable exceptions, so I’ll expand on them where it might be helpful to others.
Originally posted by Shaw
A prop aircraft most certainly CAN perform a nose-low turn (low yo-yo) at full power and best sustained turn speed without increasing speed. I've personally done it in both props and jets, and I'm no magician. The reason is that the prop fighter is NOT at the stall boundary while performing a max sustained turn.
This is the first exception, so let’s see how it occurs. Here are three EM diagrams for WWII fighters that show that for the particular configuration involved in each case, the best sustained turn is indeed at the stall boundary. The RAE engineers state that the best turn with no loss in energy “is flown as close to the stall as possible”. The diagrams shown below are for the British Spitfire, the German Me109 and the American F2A. With reference to the NACA report on the Navy F2A-3 No 01516, the diagrams were correlated with flight tests, the engineers claiming “satisfactory accuracy”. All of these diagrams were published by RAE Farnborough and NACA between 1940 and 1943 and shows the maximum turn rate and smallest turn radius that can be achieved (in a sustained turn) with zero specific excess power (Ps = 0), occurs on the stall boundary.
This first image shows that the angle of straight climb (which is the same thing as the Ps = 0 curve) curve increases all the way to the stall boundary, which means that as you get closer and closer to the stall, the sustained turn rate increases, and the sustained turn radius decreases, until the best sustained turn is reached at the point marked M on this first diagram. In practice, depending on the stall/control characteristics of the aircraft and associated risks, one would optimise sustained turns by flying as close to the stall as possible.
Here you see a similar situation for the Me109, where the best sustained turn increases all the way to the stall, but in this case, you can see the angle of straight climb curve (Ps = 0) just beginning to level off at the stall boundary.
In this EM diagram for a Navy F2A airplane, you notice that the best sustained turn still occurs at the stall boundary, but that the angle of straight climb curve (Ps = 0 curve) is much flatter than the previous examples, which means that flying closer to the stall boundary reduces the sustained turn radius while the sustained turn rate is not as significantly improved. In this case, the optimum sustained turn, remains at the stall boundary.
In the next two EM diagrams we see what happens when the altitude increases, and the power drops as in figure 35. Figure 34 shows that a similar shift in the maximum sustained turn location occurs when flaps are employed.
These diagrams now support your previous statement that the maximum sustained turn does not occur at the stall boundary, in these cases it clearly doesn’t. Generally, with more power and less altitude, the Ps = 0 curve increases towards the stall boundary and as altitude increases and power drops the best sustained turn moves away from the stall boundary. The F2A EM diagrams are a good example of where the maximum sustained turn is at the stall boundary and then moves away from the stall boundary significantly as the altitude increases and power drops. This is particularly interesting because the analysis for the F2A confirms both of our assertions, depending on the configuration involved. However, in a simulation context I think the points I made in earlier posts are (due to the modelling) more valid.
Originally posted by Shaw
I'll grant that most prop fighters will gain speed in a full-power split-S even at max-AOA. A low yo-yo in this case may be only a degree or 2 below the plane of the bogie's turn...although most prop fighters can descend much faster than this and still maintain speed.
Agreed, in the example shown above (Fig35) with the F2A at 27,000ft it can maintain its best sustained turn speed at 120mph indicated, by pulling all the way to the stall boundary while descending in the turn at a little over 6 degrees. That confirms your point, but if you take the fight lower with more power available the situation becomes similar to the Spitfire and Me109 diagrams for 12,000ft where the best sustained turn was at the stall boundary. At that point any degree of descent would result in an increase in speed, unless the AoA is increased even further resulting in a disproportionate increase in drag and energy loss. And if the aircraft had sudden or severe stall characteristics the excursion might be even more costly. I’m sure there are prop’ fighters in which it is possible, but not necessarily advisable.
Originally posted by Shaw
Granted it's much closer than the jet is to that boundary, but the prop can still increase AOA and drag above that required for best sustained turn speed. Your diagram is misleading on this point.
In the sense that it only illustrates one set of conditions, as these diagrams must do, I would agree. But it does correctly illustrate the sustained turn behaviour under those conditions, and is generally correct for high powered prop fighters at low to medium altitude, that remain inside the envelope. If I change those conditions by increasing altitude and reducing power, the EM analysis correctly predicts the behaviour you have described. So I wouldn’t say misleading, just not the whole picture. For example in the diagrams below, it can be seen that the Ps = 0 curve exhibits the same behaviour and characteristics as the ones I posted earlier that were originally published by NACA and RAE Farnborough.
Now if I increase the altitude and reduce the power, the EM diagram looks like this:
Which shows that in that situation the aircraft can hold its best sustained turn speed while descending in the turn at about 4 degrees below the horizontal.
Originally posted by Shaw
Best sustained turn speed for either props or jets is roughly the same as best climb rate speed (Vy). I think you'll agree that Vy is normally well above stall speed (or stall AOA) for props.
Ok, it’s my turn to take an exception. Yes, they are roughly at the same speed, but Vy occurs well above the 1g stall speed, as you say, not above the stall speed at the load factor at which the best sustained turn occurs! I can illustrate that with an EM diagram for two fighters as modelled in AHII as shown below. Notice that the data plate for these two fighters shows that the best sustained turn speed, and the best climb speed are almost the same, as you point out, with only a 2mph difference for the Ki84 and only a 5mph difference for the F6F. However, although the best sustained turn for the Ki84 is well above the 1g stall speed it is at the 3g stall speed! Generally, the best climb speed occurs well away from the 1g stall speed, the best sustained turn may be at roughly the same speed, but at a higher load factor and on the stall boundary, with the exceptions noted earlier regarding altitude and power.
The diagrams published by NACA and RAE Farnborough and the indicated behaviour, was known, analysed, and correlated with flight tests, with no shortage of practical minded and experienced pilots able to validate the work. The NACA engineers state that the results of flight tests were correlated with this analysis with satisfactory accuracy. The RAE engineers state that the best turn with no loss in energy “is flown as close to the stall as possible”. Even more importantly, most of the readers on these boards are mainly interested in how this behaviour is reflected in Aces High, and the sustained turning behaviour shown in these diagrams is easy to verify in AH, or almost any other simulation, by carrying out the appropriate flight tests.
For anyone interested, this is an example of the style of Energy Manoeuvrability diagram I’ll be using for the AH2 fighters from now on, including the information on the data plate. If anyone is interested in seeing other data included let me know.
Hope that helps…
Badboy
