- Martin Gregorie, © 2012 all rights reserved.
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:
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.
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.
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%.
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.
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.