F-Fifteeney (oz14511)

About this Plan

F-Fifteeney. Free flight cartoon scale model, for twin EDF.

Direct submission to Outerzone.

Update 20/2/2024: Added article, thanks to RFJ.

Quote: "Development: Prototype 1: The first prototype (P1) was stick and tissue, with crude parallel ducts. The model was made in two halves, front and back, banded together in an attempt to add survivability. Four Sanyo 270mAh cells provided the power source.

The design was very stable and showed a lot of promise. The fins had to be reduced in area, to prevent weather-cocking, but everything else seemed fine, though there were two major design flaws, namely the aircraft did not hold its trim, due to the two-part construction used, and there was not enough power.

Unfortunately P1 was badly damaged during test-flights, following an experiment to see if a hard throw would help the climb rate! The project was now running behind schedule, so the expedient of using one piece, mostly all foam, construction was decided upon.

Prototype 2: P2 used the built-up wings, salvaged from P1. The ducts had a slight taper and utilized a full length motor-fairing cone. This time, four Sanyo N-500 AR cells were used.

The change in duct design, not only looks better with the smaller outlet, but improves performance. The original parallel duct with the short motor-fairing cone (as supplied with the Union fan-unit), allowed the airflow to expand (and thus slow down) within the duct, due to the increase in cross-sectional area just aft of the motor. Theory suggests that this causes turbulence inside the duct and a loss of thrust. The tapered duct and full length motor-fairing, keeps the cross-sectional area constant, from the fan, all the way to the outlet.

Although it takes extra effort to construct the improved duct design, it is well worth it: for once, practice seems to agree with theory.

The outlet diameter could have been made just a little bit smaller still, which would have improved the top speed at the expense of static-thrust. For free-flight models, it may be best to choose higher static-thrust, to help with safe, reliable, hand launches.

Despite having a heavier battery, the all-up weight of P2 remained the same as P1, due to the extensive use of foam. On-going development and test flying will clear the aircraft for carrying external stores.

Construction: This example of up-to-date heavy metal, is not too difficult to build, if construction is broken down into the sub-assemblies as detailed below. However, you have a challenging target weight of 12 oz, which calls for some stringent weight-watching.

Wings: Start by cutting all the ribs to the same shape as the centre-section rib, so it is worth making a ply template. Try to find some light, but stiff, quarter grain 1/16 balsa..."

Supplementary file notes

Article.

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