Observer (oz9134)

About this Plan

Observer. Radio control sport model. High wing model for a .20 size 4-stroke engine.

Quote: "Well, it could be scale, but this .20 size 4-stroke high wing model is really a sport plane in disguise, and was inspired by the original Bostonian version of it. Observer, by Alex McLeod.

Here is a 56 in spritely sport airplane that can nimble around the sky on a .20 four stroke engine yet look like a WWII observation aircraft.

While looking through one of the model mags my eye happened to fall upon a set of plans for one of the cutest little rubber powered Bostonian model I had ever seen. It was ideal, if scaled up and changed around somewhat, for the little OS .20 four cycle engine that I had see one year at Toldeo.

the RC version is very gentle flying, yet an agile performer when more than regular control movement is used. Several Observers have been used as trainers and I know of one that has been flown with floats.

After drawing up the plans and obtain-ing one of the little 4-cycle jewels, I was sidetracked by another project, and, believe it or not, never built an Observer of my own. I was, however, one of the few members of our club who didn't have one, when they saw how much fun the one built by Ted Sharp was. Some of the accompanying photos show two of the five built here. I did, however, build a half size one for a small VL electric geared motor, and fly it free flight. It flies well that way too. The plans for it might some day by published if there is any interest shown in it.

The R/C version is a very gentle flying yet agile performer when more than regular control movement is used. Several Observers have been used with success as trainers and I know of one that has been flown with floats. The flat bottomed airfoil shown provides a faster model but the undercambered rib shape slows the model down for use in small fields or school yards. The combination of lots of wing area with a light airframe and a small engine that pulls well, produces an easy flier.

Even though I haven't built one, I did do the test flying of the prototype and found it to exceed my most optimistic hopes. I expect to build two of them, one with a .19 R/C PAW diesel and the other with a .15 size electric motor. There is every reason to believe the design will lend itself to these two power sources as well. Certainly a .20 4-cycle is enough power for the airplane and that was proven when it was a bit overpowered with a Surpass .26 4-cycle. If some of you are heavy builders (the airplane that is) you may very well need lots of power and have to go to a .40 4-cycle, but that's up to you.

Construction: I like to use spruce in my models because I can cut down on the size of the wood and, therefore, also the weight. It's easier to han-dle a model built of spruce (or other harder woods if you cut your own) since it is less likely to crunch when you handle it.

The plan shows balsa construction with sheet sides on the fuselage and 1/8 square inside for rigidity, but if you wish to build an open framework from 3/16 square spruce and no sheet in the fuselage behind the trailing edge of the wing, it will work fine. They have been built both ways, and even with other variations. They all fly well.

Fuselage: Start by building two sides as shown on the plan or of 3/16 square spruce or whatever wood, then join the sides in the center section of the fuselage with the two 1/8 ply formers E and F making sure everything is perfectly square. When this is done, bring the tail together and cement it. Install the other cross pieces, firewall and the remaining formers.

Install the pushrods and throttle linkage, then the fuel system and sheet in the nose. Be sure to epoxy the blind nuts for the motor mount behind the firewall. Side or inverted mounted motors both work well and depend on how you want the nose shape to look. Upright mounting, I think, would destroy the illusion of the full-sized airplane but do what you like here. A thin ply sheet wrapped over the inverted motor works well and is simple while the same sheet can cover the curved top of the nose, or use 1/22 sheet balsa as shown.

Be sure to glue in the ply and blocks for the torsion type landing gear. This is a very light, strong arrangement that takes a lot of punishment. One of the weakest places on the model is the wing mount on top of the cabin. This can be strengthened by cutting a wing saddle from 1/8 ply and gluing it to the top of formers E and F with gusset blocks and 1/4 ply wing hold down, bolt threaded blocks for 1/4-20 bolts. If you use the undercambered airfoil be sure to glue matching blocks on the wing saddle to fill the gaps..."

Observer, Flying Models, July 1994.

Note this is a low resolution plan.

Direct submission to Outerzone.

Supplementary file notes

Article, thanks to RFJ.

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