Designer Techniques No. 22

Boulton Paul P.111A

by Andy Conway - Autumn 2001

When asked why build a stubby delta for the slope and will it fly mister?


Well when you look at it, there is an unusual shaped fuselage for a start, a nice elongated teardrop with the ends chopped off, also plenty of wing area: - yes, this looks like a good project.




Regarding the history of the Boulton Paul P.111A, the Boulton Paul deltas were the first in Britain to be designed purely for academic research into the delta planform, to determine the basic aerodynamic stability of the delta shaped wing.  This was due to the research data gained from the Germans after the war.  Interest was shown into the advantages inherent in the delta wing.  The first flight took place in October 1950.  The wing was designed with the ability to increase the span by adding two other sections giving three different spans to the wing, this 34ft, very nearly to a point at the tip.


My first encounter with this model was in the late fifties, when P E Norman who was one of the first in building ducted fan models, produced a plan for an I.C. powered ducted fan model of the P.111A (he also produced a very nice MiG 15).  When asking around if anyone had a copy of this plan I had no luck, so I had to start from scratch, but as you all know any good modeller will have some reference books to hand.  After a good three view was obtained this was scanned into the computer and a working drawing produced for construction.


What scale?  Well I was looking for the model to be one piece but small enough to fit in the back of the car; after a debate how much room we had for transporting 5-6 models to PSS events, 1/10 scale was chosen as this gave me the following sizes: span 36" length 32" (the middle of the three on the full size).  This is a handy scale as you have a good chance of locating commercial parts i.e. pilots and transfers .  The target weight was 2.251b, which gives a wing loading of 15oz sq.ft.  I like to have all my PSS models in the range 14-19 oz/sq.ft as this works best for the type of jets that I have designed.


The main problem was on which wing section to use, fully symmetrical or a reflex as we'd normally choose for a delta wing, on the slope don't we!  Well I did not, why you ask because it didn't look right, as it faired into the fuselage, so I used Eppler 182 for the root and my all-time favourite Eppler 374 for the tip, with no wash out.  By the way these were not chosen scientifically as I'm not that knowledgeable; I just, as they say, go with the flow - i.e. if it looks right it is O.K. [pun intended]


Next thing to consider built up wing or foam, well as this was going to be two different wing sections this really meant using foam.  Pink or white, well as I had just acquired a nice block of foam, no it was not nicked!.  Just check out your local building sites and ask the site foreman nicely if they have any waste in their skips, it's amazing the sizes they throw away.


The wings were hot wired by using the method common for cutting delta shaped cores.  Put a nail into the building board attach cutting wire and stretch out about 36" for an 18" core the other end is attached to 1" dia. dowel this now allows you to swing through an arc, hey presto a set of delta cores, these were covered with 1.6mm. balsa.


The fuselage was made from a central elongated box to carry Rx and servos, the 3mm formers being attached at approx. 4" stations up to where the wing leading edge would be.  These formers now had the stringers attached after the two servos had been installed to provide elevon control and then skinned with 2.4mm balsa.  The front section was hot wired from pink foam and sanded to shape and skinned with 1.6mm balsa; this was then attached to the remainder of the fuselage.  The wings were now slotted at the appropriate points and pushed into the central box giving plenty of gluing area, now it was time to glue on the front end.  The fin was built up to a symmetrical section and also skinned with 1.6mm balsa.


When it came to the point of what colour scheme to use you have two choices.  The first prototype was all silver and the second was all yellow with some black trim, and nice large roundels.  I chose the bright yellow one, easy to see in all conditions as has been proven at the last two events, when there was some thick mist, so I was still able to follow the model.  Which brings us to the main point - WILL IT FLY MISTER?


Well the answer to that one is fantastic!  It will fly on the right slope in 6 mph winds and up to 40 mph if you can stand upright, inverted is as good as upright !



Andy Conway holds aloft his Boulton Paul P.111a at the Hole of Horcum


So here we have a true delta, that performs on the slope, when a lot of people thought it would have too much drag but it has proven to be good at pylon racing also.


One thing that was still required was some reflex, not a lot, as Mr Daniels might say, but I still think I MADE THE RIGHT DECISION REGARDING WING SECTIONS.


I have only one bad comment and this is my own standard of finish to this model as I had covered it with film and this was just not right, but I now have an opportunity to rectify this as I had a nasty crash into a fence post at the last PSS event.  That's the thing with these early jets, they did not have post-avoidance radar.  I hope this has been an insight into the meanderings of an old modeller.


Let's hope we have a better season next year.