Aerodynamic Design of Handling Qualities

[djpacro]

Classical processes for the design of light aircraft to meet handling qualities specifications are described in this slideshow: http://aerosociety.com/Assets/Docs/Events/671/Gordon Robinson.pdf

 

Simply copying the layout of another type works OK.

 

The section headed "Bite the Bullet" can involve a lot of labour (if warranted) but can also be done very effectively and without a lot of effort by using the estimation techniques provided in NASA CR-1975.

 

For example, development of the classic charts of tail size vs cg plotted for required static margin at aft cg and elevator authority at forward cg. Then the note:

 

Wide range of ‘acceptable’ static margins~3% to ~15% of MAC.

• Tail area depends on wing x location as well as static margin.

 

• Tail contribution to static stability proportional to tail arm, but damping proportional to square of tail arm.

It then moves on to consideration of Control Anticipation Parameter (CAP) as a better way of selecting tail size and moment arm etc than a simple static margin. The arithmetic is no more difficult but gets to the heart of good handling by consideration of parameters determined from vast amount of data from pilot assessments of handling qualities. And all quite straightforward using simple methods of estimating data from NASA CR-1975.

The attached diagram shows how the end result of the calculation may look. The blue line on the left is derived from forward cg control limits - typically elevator required to rotate on take-off or flare for landing. Lines on the right derive from aft cg stability considerations. Side by side seating usually results in a smaller required cg range than tandem seating - obviously consider fuel and baggage in the cg range required then simply select the size of the horizontal tail to suit.

 

 

[Admin]

Nice to see some of your teachings making their way on to the site DJ...thanks from all of us

 

 

[djpacro]

 

It is also useful to look at the Short Period "Bulls-Eye" for longitudinal handling characteristics. i.e. put your design in the centre ring.

 

(From: http://www.princeton.edu/~stengel/MAE331Lecture17.pdf)

 

 

[djpacro]

I find it interesting revisiting some of the history of this, from repository.tudelft.nl/assets/uuid:a7d15d2e.../LR-756.pdf

 

The pace of aircraft development slowed almost to a stop at the end of the First World War. ... shortly after ... the first development of static stability theory. ... In the 1920's, theory showed that lift due to angle of attack acting at 25% of the chord is a property of all lifting aerofoils. ....RAE Farnborough began the development of static stability theory, publishing the expression equating the slope of the wing pitching moment versus lift coefficient to the CG margin. This is the non-dimensional distance between the CG and the "aerodynamic centre" (AC), the name he chose for the 25% chord point. The theory was developed in the next few years to include the effects of the tailplane and wing downwash field to determine the overall AC of the whole aircraft, the neutral point (NP). Work on the "diving pull-out" added the influence of pitch rate in manoeuvres to define the manoeuvre point (MP). From this evolved the relationship of control displacement and force with speed change and steady manoeuvre.

These initial elementary results are sufficient today to perform all the pitch stability and control calculations needed for simple low speed aircraft.

Worth considering that last sentence.

 

By determining the NP and locating the CG at a minimum distance ahead of it, typically 5% for example, both speed and manoeuvre stability would usually be sufficient to enable adequate handling to be obtained.

Some people haven't progressed beyond 1920's thinking.

 

One major stimulus for this work was the need to solve the problems of the Supermarine Spitfire, which suffered from a serious lack of longitudinal stability throughout World War 2, In its original form it had a CG range of 2.7% chord, adequate then for its limited fuel and weapon load stored on the CG. Eventually its weight and power had doubled, and it was often operated in an unstable condition that sometimes lead to fatalities. ... To some surprise, apparently conditioned by the old fallacy about the incompatibility of stability with manoeuvrability, the handling was found to be superb. ...The beneficial effect of good short period stability on combat effectiveness was demonstrated by aircraft such as the Hawker Hurricane, Typhoon and Tempest. ... Onions on this continued to vary, but it is noteworthy that two leading test pilots with more experience than most in World War 2 combat and test flying (Yeager in the USA and Beaumont in the UK) were firmly in favour. It was a further 20 years before formal support was given by the minimum short period frequency requirements in the updated Mil.Spec.8785 ...

Aerobatic aircraft also need good short period stability for the same reasons as a gun platform.

[djpacro]

Also extremely interesting from page 28 of that previous reference. Commonly expressed myths in aviation literature can be identified as opposite of the following factual statements:

 

Movement of the CP has no effect on stability.Direction of the tail load has no effect on stability.

The angular difference between wing and tail has no effect on stability.

 

The change of tail arm as the CG moves is of little significance.

 

The direction of the "lift-weight couple" has no effect on the change of trim when the engine stops.

 

An aircraft with low stability has a more sluggish pitch response than one with high stability.

 

Nose drop at the stall is not significantly affected by aft CP movement.

Sorry for the thread drift.