Date: 29 Nov 97 15:40:02 From: firstname.lastname@example.org (Darren Rhodes) Organization: Daz Technology References: 1 Followups: 1
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On 20 Nov 97 02:53:42 , email@example.com (Luis Manuel Perez Llera) wrote: >Does anybody know if there is any regulation stating that the >weight-per-wheel is limited in aircrafts due to runways care? In other >words, the reason for the modern aircrafts wear more wheels than older >ones is the danger of wheel explosion or the runway strength? Sorry for the late response. They're are no regulations per se, but if you want to sell an aircraft it must be capable of using the runways that that your customer uses. Every runway has a unique strength- this catergorised as a Loading Classification Number (LCN) or a Pavement Classification Number (PCN). The reason every runway/airport has different LCN/PCN values is that almost runways are constructed differently. Some may have flexible surfaces or rigid surfaces. Also the depth of the structural layers will vary. I am no expert in runway design but I guess these differences are based on the underlying terrain and also the fact that many runways have been extended from smaller runways. Anyway from LCN/PCN value an aircraft designer can determine the equivalent single wheel load (ESWL) required. Then he can determine the type and size of tyre/wheel to use including the tyre pressure and also the number required. The term ESWL is used since the ESWL derived from a four wheel bogie is not the vertical load on the bogie divided by four but a somewhat higher value since the proximity of the wheels in the bogie does not spread the weight out applied to runway completely. Based on wheel disposition and bogie geometry empirical charts can used to determine the correction factor. Clearly there is a compromise here as the undercarriage engineer wants a large bogie which will add weight and results in stowage problems that the airframe designer doesn't want. ESWL tables for manny tyres and also correction factors for bogie geometry can be found in an excellent book, "Synthesis of Subsonic Airplane Design" by Egbert Torenbeek, Delft University Press. Finally (sorry if this is long winded) - I disagree with your comment that more modern aircraft have a greater number of wheels. Most major runways were upgraded with the introduction of the B707/DC8 and the B747. The B747 still requires the highest LCN/PCN values since its ESWL values are the greatest. Although aircraft like the B777 have two six wheel bogies, Boeing is plannning a family of aircraft - the MTOW has already grown to 286 tonnes from the starting 240 tonnes for the -200IGW. The -300 pushes this to 300 tonnes and the -200X being touted is pushing 335-340 tonnes. I believe this will indeed require an additional centreline wheel pair like the A340-300 and MD-11. Hope this answers the questions!