Ok, yesterday was our last outdoor contest of the year. The weather was cool (30s with frost on the ground when we started and 50s when we left), with light breezes blowing away from the pits, for once. However, only three of us showed up and we settled into test flying many models.
For four hours, I tested ONE model. Testing is something I virtually never do. I can do glide trimming in my back yard, but any testing that involves any sort of power application needs to be done at the flying field. Since I live so far from the flying field, the only time I go there is on a contest day. So, I dedicated yesterday to testing one model – my High Performance Sportster (August 1948 Model Airplane News). I had built this before the NOFFA contest in Muncie last month. It was a flyer right from the start and I only had to make slight adjustments to the thrust and about 1 gram of noseweight. The model turned out to be quite floaty in the near-perfect weather that we had for those two days. It put up more maxes than it did dropped flights and I won OT Stick – with a brand new model.
the bare bones of the High Performance Sportster. The 20″ span model looks a lot like a small Gollywock.
That’s not to say it was “perfect” (there are always improvements that can be had) – it seemed wobble or roll quite a bit – maybe the CG was back to far and it was on the edge of a stall the entire time. Also, I had used a printed 8″ diameter prop with 13″ pitch. Combined with two loops of 1/8″, this created a slow climb – no zooming here. But the long duration of power, even in cruise, put it in a great position to just cruise around and wait for the little thermal and that rear CG created a model that was very susceptible to those thermals and it would just go up.
As I prepared for yesterday, I figured attendance would be low, so I took to “modifying” the HPS in preparation for testing. The main thing I wanted to do was to test a lower-pitch prop. The 8″x13″ prop has a P:D Ratio of 1/6:1 – that’s quite high – but it does explain the slow climb-out and long motor run. A long motor run is great, but slow climb can result in short (not max) flights since the glide portion could be short due to low altitude. So I printed an 8″x11″ prop with a much more typical P:D of 1.3:1. I also made a new noseblock (printed) so I could just swap the entire front end (I made sure both prop & noseblock assemblies weighed the same). The idea here was to test with the original prop, then swap front ends and wind to the same torque and see how the flight differed.
I also swapped out my BMK Band-burner DT for a BMK Remote Band-burner DT. This took a little work: I print little boxes to hold the DT and Battery so they can be moved from model to model. But I had set this DT “backwards” or in reverse so the leader line came in from the front (see the image above) instead of the rear. The box for the RDT unit is the same mounting size BUT due to the front entry of the leader line,the antenna pointed down. I had to re-run the line to come in from the rear.
Note that the model has a pop-up wing DT system. I did this to keep excess structure and weight off of the tail and move that weight to much closer to the CG location. In fact, all of the DT system is well in front of the CG – and I still had to add nose weight. I did not want to slide the wing further aft as that compromises the model, making the tail moment shorter. Anyway, I got the RDT installed and working. AND – I discovered (maybe I already knew this?) that you do not have to put the DT on the timed count-down to activate the DT remotely; it just has to be plugged in and when you hit(and hold) the button, the DT burns through the rubber. This is ideal for testing.
In four hours at the field yesterday, I probably put in close to 20 flights some short and some long. I started with the “known” condition – I used the original 13 pitch prop to verify that the setup hadn’t changed and things were relatively the same. Cold dense air is different than warm buoyant air and the model did act differently. But one of my first flights was a 1:56 – not bad for no thermals. (Oh, this demonstrates what I said before: it had a great motor duration, but never climbed high enough to allow for gliding over the 2-minute mark.) But the model was “less good” than it had been. I am pretty sure the lack of thermals and a slight breeze revealed that the model really was not trimmed the best – at least not trimmed for these new conditions.
I swapped out the prop & noseblock and eased into trimming the thrust. This took several flights. It also revealed a great feature of the RDT. I could launch, watch the climb, see it wasn’t well trimmed and hit the RDT, bringing the model down after 30 seconds or so and move on. And more than once, I had high-torque launches that were going tight right and would have hit the ground hard, but I could push that button and save the model. These two uses saved much time and saved the model more than once, proving the RDT to be very useful and almost necessary.
But the model was quite different in this weather, regardless of the prop used: it seemed to be more loopy than it had been, both under power and in the glide. in the glide, I decided that maybe the model had too much incidence and I sanded down the stop that holds the front of the wing up. Similarly, under power, it seemed to zoom, stall, recover, zoom again, etc, etc. I think this was a fault in my construction design. I think the front of the wing was not being held down firm enough and high speed would lift the front of the wing, increasing incidence. As it stalled, the force on the wing would allow it to re-seat and reset to normal incidence.
Given all of this, I tested a lot. My last flight was with the 8″x11″ prop and a fully wound motor. The trim was just right and the plane climbed out very aggressively – the way one does when it has a little assistance from the air. I mean, it went up like it never had before, the prop and the air combining to carry it VERY high. I am glad I looked at my transmitter – it was OFF – the battery must be getting low! I quickly turned it on, checked the watch and it was just over 1 minute. The model was so high. I pushed the button and it DT’d . The model finally hit the ground at 2:06 just off the field – it took a MINUTE to come down.
And learned a lot. Probably the primary thing I learned was that if I want to optimize this model, I probably need to build a new one, or at least a new fuselage. I would lock down the wing and move the DT to the tail, where all DTs should be. Then I’d probably move motor peg forward one bay to shift some of that motor weight forward.
I learned that the RDT is a great tool for trimming – it will shorten your flights so you can move on to the next adjustment. I learned that you better keep that transmitter charged, just like all the rest of your equipment. I learned that testing can really help you improve a model’s performance – and it can reveal some of the bad choices you made during construction. I also learned that this little model is a performer – right up there with the best OT Sticks.
