# Saqqara Bird flight testing

## Introduction

The tests were carried out on a fairly calm day in a sheltered place in order to estimate the glide ratio of the Saqqara Bird.

Why attempt to measure the best glide ratio? Simple. The best glide ratio can be used to calculate the longest distance an aircraft can glide from a given altitude. This provides a direct measurement of how efficiently it can convert potential energy (height) into work (distance travelled). The most efficient gliding aircraft is the one that goes the furthest from a given height. Less obviously, the glide ratio is also a useful measure of the efficiency of powered flight: the higher an aircraft's glide ratio is, the less power it will need to maintain level flight. Lets put some numbers on it:

• A modern jet airliner has a power off glide ratio of around 20, as Robert Piché so ably demonstrated when a fuel leak left him without power in an Airbus 330-200 over the Atlantic at 30,000ft. He glided the remaining 65 miles to the Azores where he landed after shedding excess height in a series of circles. That is a glide ratio of 13:1. The actual achieved glide ratio would have been rather better than that because this figure doesn't allow for the height they had to throw away before landing.
• In the world of sport aviation a sailplane that has a light enough wing loading to climb rapidly in a thermal and a glide ratio of better than 36:1 can easily make cross country flights, typically covering 300 km in 4 to 5 hours.
• My F1A class competition model gliders have a glide ratio of around 16:1, a deliberately low glide ratio because they are optimised to circle slowly in thermals. The competition is won on time on the air, not distance travelled.

Sadly, at 2.4:1 the Saqqara Bird is not remotely in this class. Its gliding ability is similar to that of a bloke in a wing suit, who will typically be getting 2.5:1. Even the Space Shuttle, at 3.7:1, has a better glide performance.

## Flight test data

The primary data needed for these tests was captured with a steel tape measure. Although most of the flights were recorded with a video camera, only a small selection have been included because they are fairly repetitive. The videos of these tests can be reviewed by clicking their Test id.

### Replica 1

The tests on this replica were flown with its anhedral, scale section wing installed. A 25% tailplane was fitted and the replica was ballasted to 21.9g, slightly lighter than the best estimate of the original Bird's weight. It proved difficult to set the launcher to exactly the correct gliding speed, so two sets of measurements were taken with slower and faster than optimum launches.

 Test condition Test id Distance (mm) Launch height(mm) Glide ratio Slower launch b1s01 3010 b1s02 3000 b1s03 2980 b1s04 2880 b1so5 3005 Average 2975 1365 2.18:1 Faster launch b1f01 3800 b1f02 3600 Average 3700 1365 2.71:1 Average 2.45:1

### Replica 2

The tests on this replica were flown with all three wings (scale section with anhedral, scale section with dihedral, modern section with no dihedral) installed. A 25% tailplane was fitted but the replica was flown at its unballasted weighs of 8.6g, 8.0g and 8.9g respectively. At this weight the launcher could be set to the correct gliding speed, so the results shown are simply the result of averaging the results for each configuration.

 Test condition Test id Distance (mm) Launch height(mm) Glide ratio Dihedralled scale section wing b2d01 3150 b2d02 3200 b2d03 3600 b2d04b 3300 b2d05b 3330 b2d06b 3370 b2d07b 3290 Average 3320 1365 2.43:1 Anhedralled scale section wing b2a01 2900 b2a02b 3180 b2a03b 3310 b2a04b 3400 b2a05f 3410 b2a06b 3350 b2a07 3340 Average 3270 1365 2.40:1 Flat modern section wing b2m01f 3700 b2m02 3700 b2m03 4000 b2m04 3860 b2m05 3980 b2m06 3640 Average 3813 1365 2.79:1

## Discussion

The launcher settings were judged by eye. Since I wanted to measure the glide ratio of my Saqqara Bird replicas but didn't have a lot of space to play with, i.e. the height of the launcher, it was essential to launch the replicas as close as possible to their optimum gliding speed. Since a stable glide path has a constant slope, this was done by looking at the flight path: if the launch speed was too high the replica Bird would initially have too flat a glide which would arc over as its speed dropped toward its gliding speed. Similarly, if it was launched a little slow it would show an initial steep descent that would flatten out as it picked up speed.

Setting the launcher for Replica 1 was difficult: its extremely hard to hand launch a replica when, as was the case here, it was ballasted to a similar weight to the original Bird. Setting the launcher correctly wasn't much easier. By watching the flight path it was easy to see that the first launches were too slow: the descent angle was steep and the model flew so nose high that it almost stalled. By contrast, the second launch series was obviously fast as you can see the glide path getting steeper the further it went, yet these were the two closest power settings available on the launcher.

Fortunately the appropriate launcher setting for the unballasted flights with Replica 2 had been worked out in preparation for filming the sequence shown in William Shattner's Weird Or What?. This setting gives fairly staight glide path, though close inspection of the video clips hints that it may be slightly fast.

However, comparison of the glide ratio between Replica 1 and Replica 2 when fitted with the same wing shows good agreement with aerodynamic theory, which tells us that an aircraft's best glide ratio doesn't vary when its weight is increased even though it must fly a faster to support the extra weight. In this case the weight was increased from 8.0 to 21.9g, a 173% increase, while the glide ratio increased from 2.40:1 to 2.45:1, an increase of just 2%.

### Notes on the video clips

These clips were all shot at 15 fps with a small Nikon digital camera and then edited with Avidemux. All had footage before and after the actual flight test removed. In addition b2a05f and b2m01f were rotated 90 degrees to make them upright. bf101, b2d02, b2tl01 and b2tl02 were slowed down to 3fps so that the model's flight attitude during the test could be examined.

Tests b1f01 and b1f02 give an idea of how the original Saqqara Bird might have flown if it had been fitted with a suitable tailplane. Its easy to see that Replica 1 is gliding rather faster than Replica 2. This is needed in order to carry the extra weight: aerodynamic theory says that Replica 1 would need to fly 65% faster since its weight is 2.73 times that of Replica 2.

Comparison of the clips b2a04b, b2a05f and b2a06b show just how misleading a simple examination of a video clip can be. They look as if b2a05f is a much longer flight than b2a04b while b2a06b seems to have scarcely flown at all. In fact, the flights were 3.41m, 3.40m and 3.35m respectively, making the best less than 2% longer than the worst.

Its also obvious that Replica 2 is much less stable in roll with its scale anhedral wing than it was with either the dihedralled or the flat wings. Again, this fits both theory and model building practise: all model designers know that a model with anhedral (drooping wingtips) is unstable and cannot be successfully flown without either remote control or an autopilot. Exactly the same is true for full-scale aircraft as well. Clips b2a01 and b2a05f clearly show the model rolling about 40 degrees to the right or left while travelling less than two metres. All flights with the other sets of wings kept them more or less level until they landed.

### On needing a tail

Any model flyer knows that any aircraft with the same general layout as the Saqqara Bird needs a tailplane if it is to fly stably, but no tailplane has ever been found for the Saqqara Bird and a careful look at photos of its tailfin show that its very unlikely that one was ever attached there, and certainly not where various speculative drawings have sketched one in. Just to prove the point, I flew Replica 2 without the tailplane I added for these flight tests.

As expected, the replica tumbles without its stabiliser. The most interesting thing about these tests is that the tumbling direction, which wasn't evident until the frame rate was reduced to 3fps, evidently depends on individual launch details rather than aerodynamics. If the latter was the dominant factor the tumbling direction would be consistent.

Here are videos of the two launches.

• b2tl01. This is a pitch-down tumble, with the aerodynamics of the first 180 degrees of rotation forcing the model into a dive with little or no loss of speed: the model is still travelling backwards after rotating around 200 degrees when it goes out of shot.
• b2tl02 A pitch-up tumble. This time the aerodynamics of the initial pitch-up produced a slight climb which is noticably slowing the model down: this time it has rotated almost 270 degrees by the time it goes out of shot. Observed but not caught on video: it had lost virtually all its kinetic energy by the time it had spun through a further 180 degrees. It seemed to almost stop in the air before, still tumbling, it fell to the ground.