Re: No. of wheels and runways

Date:         29 Nov 97 15:40:02 
From:         darren@daz-technology.demon.co.uk (Darren Rhodes)
Organization: Daz Technology
References:   1
Followups:    1
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On 20 Nov 97 02:53:42 , luisma@spainmail.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!