ON PAGE 3 of this thread 'alloycowboy' cited a very old but very good PDF about designing 'arrestor hooks and damping them satisfactorily' from
1954. Yes a lot of water under that bridge since then but a good insight into the difficulty as 'alloycowboy' correctly suggests. Usually all arrestor wires are held inches above the surface either by 'fiddle bridges' (on carriers) or 'rubber grommets' (on runways). However the weight of main wheels trampling the wire down flat before the arrestor hook arrives can be an issue as discussed in the recent F-35C report about that. Anyhow I thought it useful to excerpt some text from the old PDF document:
http://naca.central.cranfield.ac.uk/rep ... m/2980.pdf (3.3Mb)
A Study of the Aircraft Arresting-Hook Bounce Problem
By J. THOMLINSON, Ph.D. May
1954
http://naca.central.cranfield.ac.uk/rep ... m/2980.pdf
SUMMARY: The kinematics of an arresting-hook unit are studied in order to determine, within the limits of the assumption of a perfectly rigid hook unit, the damper force necessary to control hook bounce. The necessity for a smooth deck and the desirability of small trail angle for the hook unit are demonstrated from several aspects. The design requirements for a hook damper unit are discussed in all their functional aspects and methods are given for determining the up-swing motion of an arresting hook unit immediately following engagement of an arresting wire. The behaviour of arresting wires after being depressed by the passage of aircraft wheels is also outlined.
1. Introduction. The operation of deck landing depends to a large degree on the ability of the aircraft arresting hook to engage a cross-deck centre-span. of an arresting gear. It is most desirable, for many reasons that the hook upon coming within reach of the deck shall engage the first centre-span which crosses its path; or expressed another way, the hook on reaching the deck shall not bounce, or if this ideal is unobtainable then the bounce (in terms of clearance between the deck surface and the underside of the hook) shall be measurable only in fractions of an inch. If this objective is achieved then the arresting wire will be engaged by the hook before the aircraft wheels touch down and disturb the arresting wires, since a hook installation is usually designed so that the hook lies some 2.5 ft or more below a line which is tangent to the underside of the main wheels and parallel to the deck or ground, when the aircraft is in its approach attitude. If, however, the hook, having failed to engage an arresting wire before the main wheels touch down, is then confronted by a wire which has been disturbed by the aircraft wheels, then the chances of the hook engaging such a wire may be greater or less than that of engaging an undisturbed wire (see Appendix V). In the case of a nose-wheel aircraft with its main wheels on the ground or deck, the chances of engaging a wire are greater when in a nose-up attitude than when in a nose-down attitude, because in the nose-up attitude the hook suspension is trailing at a smaller angle with respect to the deck, than when in a nose-down attitude, with a result that the hook is in a more favourable attitude for engagement with the wire, since the small trail angle is less conducive to hook bounce. This condition is one of first importance when considering arresting gears as an overshoot safety measure on land runways.
One has only to witness a few deck landings of aircraft fitted with hook installations having, alternatively, good and poor anti-bounce properties, in order to appreciate the existence of a problem having a fundamental bearing on the safety of deck landing operations. However, the factors which contribute to this bounce phenomena are not at all obvious, and realistic theoretical treatment becomes most intractable....
...Fig. 1 illustrates a typical layout of a hook installation and shows the parts with their names as will be used in the subsequent text. The element known by common usage as the hook damper, is not necessarily a damper in the strict mathematical sense, and in the U.S. Navy is known by the more lengthy but more exact title of: Arresting-hook shock absorber and hold-down device....
...It is clear therefore that the bounce properties of an arresting hook during landing cannot be explained in terms of the simple percussion examples described above, these effects, if any, making only a small contribution to the hook bounce behaviour. The next section shows that the initial hook bounce is caused by a wedge action between the hook suspension and the deck, the ‘wedge’ being the angle between the deck and the descent path of the aircraft....
__________
...Finally, a plea is made for simplicity of design with due regard for ease of servicing, maintenance and inspection, bearing in mind that with the aircraft in a static attitude, whether it be a nose-wheel or a tail-wheel layout, the hook in the ‘down’ position is not at the limit of its travel, particularly with a tail-wheel aircraft....
...During arresting proof strength testing, when the aircraft is taxied at speed into an arresting gear (on a land installation), an indication of the effectiveness of the damper can be seen during the taxying run with the hook down. However, under these conditions it is usually not possible to get the hook into its fully down position, since the wheels must be clear of the deck or ground to achieve this.....
...
11. Conclusions:— With the ever increasing approach speeds of successive generations of deck landing aircraft and in consequence of this, the reduced area of touch-down following on which satisfactory arrested landing can be made, it is imperative that arresting hook bounce shall be reduced to an absolute minimum in order to insure engagement with an arresting wire within this limited area. A critical and searching examination of the hook bounce problem shows gaps in the knowledge of the fundamentals of the problem.
The probable use of arresting gears on airfields again makes it essential that the understanding of the hook bounce problem shall be developed to as high a standard as possible.
Two clear-cut conclusions emerge from the present study, namely, that the trail angle of the hook should be as small as is reasonably possible, certainly not more than 65 deg if possible, and that the surface of the touch-down area shall be free from obstructions. Both these factors have become self evident in a qualitative manner, particularly the latter, from experience during the past years, and the present study, it is considered, enables quantitative values to be established for the purpose of design and general assessment. The study also demonstrates that even though the above two conditions are met to a high degree, the absence of bounce can only be assured if high damper loads are employed.
No conclusions are submitted here concerning the effect of the flexibility of the hook suspension....
_________________
...There is a danger with contemporary aircraft and arresting-wire layouts that the time of recovery of the arresting wire maybe longer than the time for the hook to reach the wire after the passage of the aircraft main wheels. Under such circumstances the hook will only engage the wire if the hook is in contact with the deck (which demands good anti-bounce characteristics) and even then only if the traiI angle of the hook is small. Fortunately this latter condition is fulfilled in a tail-down landing but not so in the case of a nose-wheeled aircraft rolling on all three wheels. This last point is of importance when considering arresting gears as overshoot preventers at the ends of runways.
In order to ensure recovery of the cable before the arrival of the hook–for a given engaging speed–it is obvious that the cable tension should be as high as possible; the distance between rope supports as small as possible; and the wheel track and wheel axis to hook distance as big as possible. Practically all these requirements are in conflict with requirements in respect of other considerations. Thus the only recommendation which can be made with certainty, in the case of a carrier landing, is to ensure engagement with a wire before the wires are disturbed by the aircraft wheels. The hook suspension is usually of a sufficient length to ensure this happening providing the hook does not bounce after first contacting the deck. Hence a further emphasis is placed on the requirement of a ‘no bounce’ hook installation.
A tail-wheeled aircraft having its arresting hook aft of the tail wheel is a common configuration of special interest. If the tail wheel is rolling on the deck and depresses the wire then engagement of hook and wire is only possible if the hook is in such an attitude that the hook beak is able to ‘scrape up’ the cable off the deck. There are contemporary aircraft where this is not possible, but the occurrence of the tail wheel depressing the wire before hook engagement is considered to be so rare with conventional layouts during deck landing that it can be neglected; the aftermost position of the hook installation being most desirable in its ability to prevent excessive pitching during the subsequent arrested motion. Nevertheless when considering an arresting gear for runway overshoot conditions the hook position aft of the tail wheel is undesirable and may be unacceptable unless the hook suspension tail angle is sufficiently small."
A4G Skyhawk: www.faaaa.asn.au/spazsinbad-a4g/ & www.youtube.com/channel/UCwqC_s6gcCVvG7NOge3qfAQ/videos?view_as=subscriber
14 Dec 2011, 08:49
