Performance

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LAST month I discussed ‘Handling’ of aerobatic aircraft. I tried to explain good handling characteristics and to relate these to design characteristics found on modern aircraft.

The idea was to set ground rules so a sensible discussion could be had on what ‘nice handling’ actually means, in terms of the effort required to move the aircraft round its three axes and how it responds. The other big subject in any comparison between different aircraft type is ‘Performance’. Top Gear viewers will be well aware of how the performance of a motor car is judged. There are just two questions: “What is its 0-60 time?” and “How fast does it go fl at out?” Just occasionally, they mention miles per gallon, but we all know that is not what it is about. Aeroplanes, however, are more complicated. For a start, they climb and dive. Also, they roll. Now, I know cars roll too, but usually only once and the rate is not an issue! For an aerobatic aeroplane, we want to know many things that come under the general heading of ‘performance’.
A lot of questions spring to mind:
» How much power has it got?
» More important, how much thrust has it got?
» How much does it weigh?
» How much drag is there on corners?
» How fast does it roll?
» What is the climb rate?
» What is its Vne?
You may be able to think of more questions. Some basic analysis is clearly necessary if we are to be able to settle on the really important criteria, and then some more thought on how to compare different aircraft from the limited data available.

POWER AND THRUST
Many western aircraft use Lycoming engines and among the smaller aerobatic machines, the engines are generally rated from 150 to 200 horsepower. Some, like the Cessna Aerobat series, have less. The rated horsepower is what a well maintained engine should get close to at its maximum rotational speed; usually something in the region of 2600 to 2700 rpm.

Though two aircraft might be fitted with identical engines, however, they will not necessarily have power plants that generate the same thrust. This discrepancy comes about because they may well have different propellers, and these vary in efficiency. Fixed-pitch propellers are almost universal on engines of 160hp or less, and come in a variety of geometries. There are ‘cruise props’ that work most efficiently, not surprisingly, at cruising air speeds. But they are noticeably less efficient at take-off and climb speeds. There are ‘climb props’ which reverse these characteristics, providing good thrust efficiency at low speed, but then requiring low manifold pressure setting in the cruise or descent so as to avoid over-speeding the engine.

Constant-speed propellers are designed so that the propeller works more efficiently more of the time. They allow maximum rpm to be achieved on take-off and when flying at very lowspeeds, so that the inefficiency of the cruise prop is avoided. They also allow the aircraft to be fl own very fast without the problem of over-speeding.

So it is necessary to think of the engine and propeller as a combined unit developing thrust. Of course, constant speed propellers require governors and hubs with pistons etc, so they weigh more than fixed-pitch props. This thought brings us to thinking about the weight of the aeroplane and how it affects performance.

WEIGHT
The heavier the aircraft, the more inertia it has. Thus, for a given amount of thrust, a lighter aircraft will accelerate quicker than a heavier one. OK, that’s pretty basic. But remember that an aerobatic aeroplane flying a sequence of figures may have to speed up, slow down, speed up again, over and over again. Inertia becomes more of an issue in this case than for a Boeing or an Airbus, where inertia only has to be overcome once in each sector.

In light aircraft, there are relatively few variables when it comes to the subject of weight. For an aerobatic aircraft the convention is fairly simple: “Don’t carry anything you really don’t need!” The list of things that can be left behind is quite long, starting with more fuel than you really need and perhaps finishing with the starter motor and battery if you are happy to hand-start the engine at all times.

There are, of course, some un-answerables. Such as, why would you fit an ‘aerobatic’ version of a Cessna 150 with a full airways IFR panel? It’s also funny, sometimes, to see how much money an owner will spend to save 10lb by fitting a lightweight alternator, while still carrying a couple of stone surplus around his tummy, but I digress...

LIFT AND DRAG
To get airborne you must generate lift, and lift always comes with some associated drag. The ‘efficiency’ of the wing is really a function of how much drag we have to suffer to get a certain amount of lift. For any given wing, this varies with speed, but for different wings at the same speed it varies because of differing wing sections and thicknesses.
So wing efficiency is important, none more so than in airliners, but what about in aerobatic aircraft? Here we come up against the biggest obstacle for the wing designer: a really aerobatic wing will do almost as much work inverted as upright.

There seems to be a rule, certainly I can think of no exceptions, that says that a wing that will fly the same inverted as upright has to be less efficient in both regimes as one that is optimised for flying upright only.

Consequently, basic aerobatic trainers which spend little to no time inverted are still built with wings incorporating design features to make them efficient in that regime, but these wings suffer from inordinate amounts of drag on the few occasions that their more adventurous pilots get them upside down.

Among cantilevered monoplanes, aerobatic designs are usually built to withstand more loading and therefore have thicker spars. This in turn causes more drag than a thin wing, even if they are both symmetrical. Wings generally taper towards the tip and have a root/tip ratio. Thick wing tips can cause a lot of extra drag in high lift situations like cornering, even though they are simpler to build.

ALL TOGETHER NOW
With so many variables under just the three headings of thrust, lift and drag, how can we hope to compare different aircraft with varying weights, wing profiles, engines and propeller?

The first approximation is to consider power/weight ratio (p/ w). This makes no allowance for different propeller types, but generally an aircraft with a higher p/w will accelerate better and climb quicker.

A more comprehensive single parameter to quote is maximum level speed. Forget Vne, because that figure on its own leaves a lot of unanswered questions, but think a bit more about max level speed.

First, it is telling you about thrust, not horsepower. It is also telling you about weight and drag, to a limited extent. Certainly lift is equal to weight. But it is not telling you everything. For example, two aircraft might weigh the same and have the same maximum level speed, but one might be less powerful with a more efficient wing, another might be more powerful but have a thicker wing. To judge which one will give more overall aerobatic performance, we need more information.

VERTICAL FLIGHT
As a workable approximation, it is reasonable to suggest that when an aircraft is flying in the vertical the wing is at a very low angle of attack and generating very little lift.

In this situation, the efficiency of the wing becomes much less important. If the wing is generating no lift it will be inducing no drag. There will be form drag from the wing but even this will reduce as the vertical speed decays. In this situation, the aircraft with more power and a less efficient wing will come into its own.

Vertical penetration, therefore, defined as the height which an aircraft will climb vertically from maximum level speed, is a much more universal measure of performance than either power/weight ratio or maximum level speed.

Unfortunately, these latter two figures are often quoted, especially for factory produced types, while vertical penetration can only be discovered by trial and error.

The best comparison test is to fl y level at maximum take-off power, pull up to (almost - no inverted fuel system) vertical and then ease forward to level flight at minimum speed.

The initial quarter loop should not be too fierce, just as if making a normal loop. This test might seem pretty well impossible in an aircraft that cannot loop from max level speed. The answer is that you can still make good comparisons, even though you do not reach vertical. You are trying to judge the ability to convert speed into height without a prolonged climbing period, so even a zoom climb at 45 or 60 degrees will give a reasonable comparator.

Naturally, a test such as this should be conducted at a safe height and you must be very aware of the proximity to stalling at the end. It is not for the inexperienced to prove their aircraft in this way, but such manoeuvres should be well within the capability of any aerobatic instructor.

ROLL RATE
To a basic degree, it is fair to say that a higher roll rate makes an aerobatic aircraft easier to fl y through many rolling figures, but there are limits. So let’s think a little bit more.

Last month I touched on the difference between consideration of roll rate and roll acceleration. Both are important, but there are no published figures on the latter, so we will have to think for now about just roll rate.

Roll rate can only be compared between different aircraft if you define at what speed you measure it. As differing aircraft can have quite different maximum and minimum speeds, it would be unwise to specify a particular standard air speed in knots. Additionally, it would be silly to try to measure roll rate at very high speeds, when full aileron deflection could cause structural problems.

Hence, it seems to me to be most sensible to quote roll rate in terms of that achieved at full aileron deflection at the maximum speed that this control input is allowed; ie manoeuvre speed – ‘Va’. My assumption is that most flight manuals or adverts that quote a roll rate are using this as a base. At lower speeds, roll rate would be less of an advert, while no factory test pilot should be rolling at full deflection above Va.

The big question is whether more is always better. My own view is that more is better, but only up to the point of reaching about 400° per second. After that, pilot reaction time is such that precision becomes almost impossible, even with a lot of practice.

Clearly, once you have established your vertical penetration, knowing how much rolling you can do on the vertical line before falling backwards is also a useful thing. Maybe you shouldn’t be trying this, however, if you only have access to the club Aerobat.

Remember the figures from the Pitts Model 12 (LOOP November 07): 2000 feet penetration and 300-320° per second roll rate. I hope to have other figures for you to compare over the next few articles.

CONCLUSION
More performance lets you do more things, have more fun, experience more ‘g’, beat yourself up more and maybe smile a little more too. But it is not the be-all and end-all. Making the most of the aircraft you have available now is much more important than day-dreaming about what you would have if you won the lottery!
Be safe and enjoy your flying. 

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