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|Posted: 11/27/2014, 07:11 am Post subject: Back to McMinnville- From Imagery Analysis to Interpretation
| The final IPACO report on McMinnville showed that, in both images, there was a strong return of a probable suspension thread at angles of 10.29 and 11.21 degrees from the vertical respectively, these observations supporting the overall conclusion of :
"The low values of the tilt angles between the suspension thread and the verticals of both McMinnville pictures are quite compatible with the presence of a soft wind on the site, and with the hypothesis of a rather light suspended object."
For the moment at least, that is probably about as far as we go in the imagery analysis per se. However, the thread observation allows some further hypotheses to be established when considered in the context of other findings in the report and some basic aerodynamics .
LOCAL WIND. Local topography has effects on surface wind conditions that can be quite marked. For example, it is not uncommon to have two wind socks on an airfield pointing in very different directions simultaneously. Objects such as trees and buildings produce local wind effects very different from those indicated in the vectors of the overall meteorology. In this instance the object is suspended very close to several buildings. Thus the direction of the local wind can be reasonably expected to differ from the recorded motion of the regional air mass. However,it may be significant to note, that in this case the recorded overall light wind from the West is in fact in reasonable accordance with apparent local wind conditions deduced from the imagery-i.e. predominantly from left to right across the scene.
SOME OBJECT SHAPE IMPLICATIONS. The object can be seen to be circular in shape with a convex upper surface approximating a somewhat truncated shallow dome, thus presenting a cambered profile. Such a shape is identified aerodynamically as an axisymmetrical aerofoil. The characteristics of such a "Flying Saucer" are unstable in free flight, with the Centre of Pressure being well forward of the Centre of Gravity, typically causing such an object to tend to accelerate in positive pitch resulting in a“back flip” if left to its own devices in forward flight. One full sized example was the 18ft diameter Avro Canada "Avrocar". Here the Centre of Pressure (or aerodynamic centre) was found to be at 28 per cent of the root chord (i.e. along the axis of symmetry pointing into the airflow ) and “therefore had a negative static margin and was both statically and dynamically unstable in aerodynamic flight “, as stated, for example, in Air International June 1974 pp.300. N.B. A frisbee is a different case, since its mass has angular momentum; also helicopter and gyro-plane rotor "discs" are different systems entirely. Nevertheless, the suspended object appears as an airfoil shape and almost certainly therefore shares characteristics with typical airfoils. Some apparent asymmetry regarding the protrusion on the top of the disc has led to speculation that the actual suspension point might not be central. Indeed, a displacement of only a few percent of the radius would cause a significant tilt to the disc.
STALLING. An airfoil "stalls" when the angle that it is presenting to the relative airflow ( Angle of Attack – AoA or alpha) exceeds about 15 degrees. At around 15 degrees the normal laminar flow of air over the wing breaks away, becoming turbulent, whilst lift rapidly decreases. However, unless designed otherwise, this collapse normally starts one wing tip first. The developing loss of lift causes this end of the wing to drop, incidentally increasing alpha locally and accelerating the situation. The centre of pressure having now moved away to the other side of the centre line of the C of G, accelerates the roll rate. Overall drag also moves toward the falling wing tip bringing in a yawing moment – difficult to detect visually with a “saucer”. If uncorrected in a normal aircraft this leads to an incipient , then developed "spin". Total drag goes on increasing as alpha further increases beyond 15 degrees.
THE APPARENT BEHAVIOUR OF THE MCMINNVILLE SUSPENDED OBJECT
Alpha. Mensuration performed in IPACO on the angle subtended by the disc's axis of symmetry to the vertical on both images comes out as: Direct measurements on the pictures gave the following results:
C1 = 19°
C2 = 17°
These angles are of course the same as those of the angle of the disc to the horizontal and thus, logically to the local airflow (i.e. alpha). Furthermore, assuming from the suspension thread measurements that the predominant wind vector is coming from the left in each image, the disc is showing a positive alpha to the relative airflow at values of 19° and 17° respectively. This vector is, in fact close to that of the recorded general wind direction from the West. As such a disc would be expected to increase its alpha with respect to the relative airflow, this direct measurement supports the presumption of local airflow from right to left across the image and further supports the case for the model behaving aerodynamically as a suspended model axisymmetrical wing or “flying saucer”.
In addition, since drag also increases with alpha, one would expect that the displacement of any suspension thread from the vertical would also move towards the right of the image as alpha increased .
In this case : C1 alpha =19° and the angle of the thread from vertical = 11.21° to the right
And : C2 alpha =17° and the angle of the thread from vertical = 10.29° to the right
Therefore both of these values are also apparently changing qualitatively in accordance with the suspended model airfoil construct.
Suspension Point. The precise suspension point location is unknown. However it is reasonable to assume that its effective suspension point is towards the top of the dome , if not possibly even higher up on the odd shaped dorsal protuberance. Two factors come out of this assumption:
Firstly, the disc cannot pitch up much beyond 90 degrees relative to the thread as it would be constrained by direct contact with the thread at around 11 degrees from the vertical : this would equate to not exceeding around 80 degrees alpha.
Secondly, as the disc pitches up for established aerodynamic reasons , thus rotating clockwise as viewed in the image , this rotation would be countered by an increasing anti-clockwise moment -attributed to the relatively increasing moment arm between the top suspension point and the centre of gravity of the disc. It is not unreasonable to expect these two opposing forces to cancel out at some point, leaving the disc hunting about some intermediary alpha value . This might be around the values exhibited by the disc, but without knowing wind velocity, disc mass, c of g and its distance from the effective suspension point, it could not be quantified. However, the observed behaviour of the disc in pitch is arguably not inconsistent with the hypothesis of it being a small object suspended on a tether.
In both images the disc is manifestly in a stalled state ( alpha measurement being> 15 deg.) on the end of the suspension thread.
As explained above, the stalled state of an airfoil typically generates a strong roll in the direction of the first wing tip to stall. This of course, in an axisymmetrical airfoil will be at an edge of the disc orthogonal to the relative airflow. As the disc rolls, so the resultant lift vector will tend also to produce a translation of the disc in the direction of the roll. Reference to the McMinnville imagery indeed shows in MM1 what may be the object in the process of rolling, at a directly measured 21.2 degrees away from the camera when imaged: this roll angle is therefore reasonably more than that induced by just the simple swinging angle of a pendulum mass in and out of the image plane. . However as the axisymmetrical airfoil swings away, the relative airflow vector moves towards the direction of the translation. This vector now tends to pick up the falling wing tip as the swing develops, as does the now increasing lateral vector of the suspension point to c of g lever moment assisting the roll reversal. This supposition is supported by the fact that when the object is calculated photogrammetrically to be further away from the camera in MM2, there is a measured rotation of 3.62 degrees back towards the camera .
This mensurated object rolling ( or banking) and associated swaying can be envisaged as being somewhat akin to how a kite sometimes hunts from side to side if it is rigged at too high an angle of attack and starts tip stalling, although of course in this case the tether is on the top of the surface and largely supporting its mass. Arguably the characteristics of this overall movement are thus more determined by aerodynamic considerations than by simple pendulum mechanics.
Suspension thread. The presence of a suspension thread is statistically established as strongly evident and common to both images. As has been described, the static and dynamic instability of the suspended disc in a light breeze will have various acceleration effects which might leave some evidence in other parts of the overall suspension system. It is possible that the suspension thread itself might undergo flexion, in which case there might well be a degree of point scatter around the statistically very significant negative peaks displayed by the thread detection process. Examination of these peak shapes indicates that such a result might well be contained within the area under the summation curve.
The Power Line. IPACO were at pains to point out that the observations made” do not constitute a scientific proof, but at least an interesting oddity. “
The power line in the area over the “UFO” gave the appearance when compared with the later LIFE image of being somewhat lower at the time of the Trent photography. The lines are however visibly kinked and distorted ( they could also be indicating possible “bounce” between the two Trent images) mitigating against any useful catenary based analysis, beyond finally noting “In both cases, the lower power line appears lower in Trent’s pictures than in LIFE’s picture, over an area centered above the UFO, which inevitably leads to think that this might have been the result of the model’s weight…”
In summary therefore, examination of the mensuration results obtained by IPACO was stated (in part) as “The clear result of this study was that the McMinnville UFO was a model hanging from a thread”.
To this can now, arguably with some justification, be added, “ which appeared to display the aerodynamic properties that such a suspended disc might reasonably be expected to exhibit, under both directly measured and derived parameters.”