Flying the Saqqara Bird

by Martin Gregorie

Paula Mercado's drawings

These drawings are fairly accurate apart from the fanciful markings. In fact the Saqqara Bird has no markings apart from the eyes, beak and stripe, which are all painted on. Colour photographs show the bird to be either uniform white or a pale earth colour. Evidently the light is not good enough for accurate color rendering in the museum. I scanned the drawings, converted them to DXF format and loaded them onto a CAD system so that they could be overlaid on the photos for comparison. Here are the results:

Comparison of top views
© Diagram copyright Martin Gregorie

As can be seen, there is good agreement between photograph and drawing except that the fin is tilted in the wrong direction. The body is also offset from the wing's centre line and is not square to the wing. Paula Mercado comments on the wing's offset but not on its skew. There is some evidence, e.g. iron nails used as fasteners, that the wing has been remounted relatively recently. Iron was very rare and expensive in ancient Egypt and so is unlikely to have been part of a wooden artifact. This makes a good case for the wing offset and skew to be the result of imperfect reassembly. It is entirely possible that Paula Mercado has recognised this and corrected her drawing to reflect its original state. It's also possible that the fin offset is the result of the drawing being reversed during printing.

Comparison of side, rear views
© Diagram copyright Martin Gregorie

The agreement is not so good in these views. The fin tilt is in the correct direction but is about twice reality. It is drawn as being 20 degrees off vertical when 10 degrees is a better fit to reality. The body's side view and cross sections are all drawn too deep. This accounts for a lot of the distortion in the cross sections, which are flatter sided than photographs indicate. Compare them with the rear view:

Last, but not least, there is reasonable agreement between drawing and photograph over the amount and curvature of the wing's anhedral, though the thickness taper in the drawing is much too fine and gradual.

The model

Construction

Paula Mercado's drawings are the best I've seen so I built the model from the CAD files I had made to compare her drawings with the photographs.

The body is based on a keel of 4.5 mm medium balsa which runs the full length of the body, forming the vertical fin at the rear. The main body was shaped by adding 10 mm balsa side pieces and carving and sanding them to shape. A wing saddle was cut into the top of the body and slots cut beneath it to hold lead sheet ballast.

Two wings were made. The first, referred to as the scale wing, was laminated from three layers of 1.5 mm balsa so it would hold the curved anhedral shape. The wing is 4.5 mm thick at the centre, tapering to half that at the tip. The scale wing section is a thickish flat plate with rounded leading and trailing edges. The second wing, referred to as the flat wing, has no dihedral or anhedral, hence the name. It has the same thickness distribution as the scale wing. This wing has a typical flat bottom hand launched glider section with a sharp leading edge, high point at 33% chord, and flat upper surface behind the high point. Both wings are mounted in the wing saddle with a nylon bolt.

As it seemed unlikely that the model would fly without some sort of horizontal tail surface, I made two tailplanes. The first, the scale tail, was made from 0.4 mm plywood sized and shaped to approximate the top view of the Bird's skewed rear fin. The combination of vertical fin and this tailplane is aerodynamically equivalent to the Saqqara Bird's fin. The second, large tail, was shaped to match the wing's shape. It has 25% of the wing area. It is made from 1.5mm balsa, carved and sanded to a streamlined section. The tails are mounted on the top of the vertical fin by a nylon bolt. Packing can be placed between the tail and the top of the fin to alter the model's vertical trim.

The model was given two coats of sanding sealer, sanded smooth and painted an ash white colour with matt acrylic paint. The eyes are black, the eye surroundings are darker grey and I added red belly stripes.

The two descriptions of the Saqqara Bird I've seen that quote its weight (Atlantis Rising Issue Number 5 and The Augusta Chronicle, 15 Feb 1998) give it as 0.5 oz (14 g) and 1.11 oz (31 g) respectively. The average of these two figures, 22.5 g, although deeply suspect is close to an estimate made another way. Light sycamore is roughly three times as dense as balsa, so I assumed that a weight of 21 g was a reasonable estimate for the Bird. In the light of this a set of lead weights were made that, when installed, brought the weight of the model to about 21 g. The flying weights of various combinations of components are as follows:

Complete model weights
Description Weight CG
(% wing chord)
Scale wings, no tail 7.10 g 82
Scale wings, scale tail 7.24 g 98
Scale wings, big tail 7.63 g 100
Flat wings, no tail 6.55 g 80
Flat wings, scale tail 6.69 g 98
Flat wings, big tail 7.07 g 100
Scale wings, big tail, ballasted 21.35 g 75

The finished model is shown below. The pictures were selected for easy comparison with the original photos of the Saqqara Bird:

Completed balsa model of the Bird of Saqqara
three views
© Photos copyright Martin Gregorie

The model fitted with the large tail: scale wing (left), flat wing (right)
two views
© Photos copyright Martin Gregorie

Flying

The model was flown outdoors on a calm day to check its flight characteristics and stability. Flight performance estimates were made from indoor flights so that air movements could not affect the measurements.

No tail and scale tail, no ballast.

The model will not fly without a tail or with the scale tail fitted.

The result is always a pitch up if the model is launched at its gliding speed and a pitch down if launched faster, followed by a tumbling motion. Adding ballast to the nose to move the balance point forward has no effect. The result is the same with either wing fitted, except that when the scale wing is fitted the model often tumbles laterally as well.

The model does make a splendid weather vane if it is mounted on a pivot set into its belly under the midpoint of the wing.

Big tail, no ballast.

Fitting the big tail enables the model to glide. Both wings require up trim on the tail, i.e. the rear of the tail must be raised. The result is a fast, not very flat glide.

As might be expected, the flat wing gives the best glide and the model is spirally stable because the high wing provides just sufficient lateral stability to prevent spiralling in if the model is thrown hard and high.

The scale wing is nearly as good in straight line gliding, but if the model is thrown hard and high it will slowly drop a wing and spiral into the ground. Both changes are expected; the scale wing section is rather inefficient and the anhedral built into it makes the model laterally unstable.

Even unballasted, the model's flying speed is fairly high. It does not soar and cannot be launched "with a slight jerk of the hand" (Atlantis Rising Issue Number 5). Attempts to do so merely result in the model flopping to the ground in a level attitude. It needs a full-arm launch at a fairly high speed to fly at all.

The best measured glide angle with the scale wing and big tail is 1 : 2.3, i.e. it flew 3 metres with a 1.3 metre loss of height in still indoor air. By contrast a modern sailplane manages better than 1 : 40.

Big tail, scale wing, ballasted to 21.35g

When the model is ballasted to 21g to simulate a model made from Egyptian fig wood its flying speed becomes so high that it is very difficult to make launches that are accurate enough for repeatable flights. In this configuration it flies more like a brick with fins than an aircraft.

When a satisfactory launch was achieved the glide angle was similar to the unballasted model. This result is in line with aerodynamic theory.

Conclusion

In my opinion the Saqqara Bird was probably made as a child's toy or a weather vane. As such it is an interesting artifact and is certainly not an example of Pharaonic High-tech or ancient lost knowledge.

© Copyright Martin Gregorie

Martin Gregorie has designed, built and flown Free Flight gliders for more than 30 years. He is an active competitor at international competitions. He achieved 5th place in the 1983 World Championships in Australia and was a member of the third placed New Zealand team in the 1993 World Championships in California. Both these results were obtained with gliders that he designed, built, adjusted and flew himself.

Catchpenny Mysteries © copyright 2002 by .

 

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