HPA

Per the AMA indoor rules there are two Penny Plane events. They are: 1) Penny Plane; and 2) Limited Penny Plane (LPP). My favorite indoor event is LPP. Both rubber powered models are very slow flying and capable of 5+ minutes flights in 8-9 meters (25-27 feet) of flyable height. The models of each event must not exceed 20-inches in length and 18-inches in wingspan. The maximum distance allowed from the front of the thrust bearing to the rear of the motor hook is 10-inches. The minimum allowed weight of the model for both these events is the weight of one US penny, namely, 3.10 grams.  The Penny Plane event has no limits on the chord of the wing and the stab and allows for rolled motor sticks and bi-planes. The LPP event requires a solid motor stick and limits the wing to a maximum of 18-inches x 5 inches and the stab to a maximum of 12-inches x 4 inches. The prop of an LPP must not exceed 12-inches in diameter. The LPP event is limited to monoplanes.

LPPs provide a satisfying flight duration for the amount of work involved. US pennies weighed 3.12 grams, or maybe 3.11, until sometime in 1973. The newer ones weigh about 2.5 grams and are copper-plated zinc. If your model weighs the same as one of the new pennies, it's too light for Pennyplane.

The unlimited pennyplanes can get quite sophisticated. Ray Harlan had one that was a biplane with a variable pitch prop. It climbed for a while, then descended and flew around for a while at two or three feet, then climbed again. The flight I watched set a record. Something like 15 or 17 minutes under a fifty-something foot ceiling. I don't know what the current record is.