A return to model building and flying. Part 29
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I have been waiting to show this for some time, now I have the space I can. This always makes me smile.
Vid-Clip 1.3MB 0m:17s .wmv
Long ago in a February 1970 Aeromodeller, I read an article by Ian Barrett on the development of a vertical take off and landing control line model. Since that time I have wanted to build one for a couple of reasons, first, it's a fascinatiing idea in it's own right, and second I have always wanted to build a scale (or semi scale) model of the Convair XFY-1 Pogo and a Lockheed XFV-1 Salmon. Maybe you can see where I am going with this before I start. If I can build a VTOL control line model and master flying it, the door is open to the scale models.
I managed to find some short vid-clips of the Pogo in action (.mov format).
Some of the ground work is already done in that I now have a simple easy to make three line system already working, and I know prop hanging is possible, although I have not quite managed it yet. I think now is the time to get serious about the VTOL concept.
I have read the Vertigo article many times over the years, but somehow something always got in the way of attempting it. The original concept went through several changes, starting with a biplane layout and changing to the delta seen in the picture. It was powered by an E.D. Racer 2.5cc diesel, with an (of the day) RC throttle. Modern engines develop a lot more power per cc, so some experimenting will be needed in that respect. Since I started to take it seriously again, I read it yet again very carefully. One huge ommission that hit me strait away is the length of lines that where used, the only reference is, "on short lines." I will have to play that one by ear. And solve the big problem that is beginning to haunt my dreams, how to keep the throttle closed when the lines are slack, without affecting the throttle response? The original Vertigo had this problem which was solved with a weighted throttle arm. This may work in the vertical position, but I have not thought through how it will possibly behave in normal flight yet.
Well this is my version of the Vertigo. It's as simple as I can make it because I have no idea how long it will last of even if it will work. There is nothing quite like stepping into the unknown. I chose the AP.15 as a power source because all the vulnerable bits are tucked out of the way like spraybar and silencer. I would be wildly optimiistic to think that this model will not bump into the ground a few times.
Vertigo Maiden Flight
Well the first flight was either going to be fun or very short lived. After a couple of flops to the side I realised that I was paying so much attention to the throttle I was forgetting to look at the elevator. Unlike a conventional take off, if the elevator is biased in one direction at all, that's the way the model will go, and if the engine has not enough power to take off it will just fall to one side. Just ramming the throttle full open proved the best way to clear the ground, the AP.15 has more than adequate power to lift the model vertically at quite some speed, infact so fast that a few impromptu aerobatics were involved until I could get things under control. On the second attempt easing in slight down elevator prior to takeoff produced a semi-orderly transition to level flight. It had taken off and was flying after a fashion, that was the first obstacle overcome.
I tried pot luck with the line length and made up some 11 metre 0.2mm/0.008in lines. if you have ever tried to make up multi strand stainless steel lines of this diameter, you will know that was a feat in it's own right. I reasoned the lighter the lines the better. and I can't see a model of this size pulling harder than 15kg. I tested some of this wire up to 5kg without it breaking, so it's breaking strain is higher. In practice I think I got it right as I had no problems with respect to the lines, apart from the obvious one of having to handling then with care because they can be almost invisible at times in grass, or anywhere else for that matter.
The big problem was the CG position, it was way too forward; that, combined with forgetting to blank off one of the vents for the pressure feed to work causing the throttle to hesitate in a slight nose down attitude, resulted in the model flying into the ground several times before the penny dropped. These sort of situations are ideally suited to old style flexible nylon props, as they are capable of absorbing a lot of punishment without breaking, I am extremely glad I have a few, or things could have got expensive. Adding lead to the rear end improved the control but full up elevator was not enough to make the model rotate into a vertical position, I find this manoeuvre more difficult to achieve in calm conditions than in a wind, and it was a calm day. The throttle response was also a little too quick and the engine a bit too powerful to adequately control the slow speed.
I have to say I am quite impressed with the AP.15 performance. it's easy to start, throttles well, is quiet at peak revs, and has lots of unique nice features. That's quite a package for a modest price new. Mine was unused second hand, so I am even more pleased. The only thing wrong with mine was the spinner-nut was threaded off true, which meant the mating surface did not fit flat on the prop hub. I machined some metal off the back so it sat true, but it looks slightly odd if you look closely. A nut and washer would have done the job just as well, but the spinner nut looks nice. The only other thing that may cause a problem is the use of a short reach plug, these are not exactly easy to get hold of, as well as one mans short reach being different to another, manufactures and retailers seem very coy about giving such basic information with their plugs. Without taking the head off it is difficult to ascertain the depth of the thread. At present I am using an Enya No3 with two copper washers as spacers, which seems to be doing the job.
A broken wing joint finally brought the proceedings to a close after one too many collisions with the ground, and I could go home and ponder what I had learned so far. I do know I need a bigger wing with more elevator area and movement, the engine will cope with the extra weight easily enough. This will allow me to arrange things to move the CG rearwards without adding lead. So far I have come to the conclusion that the existing design will have to be altered dramatically to get it to balance in the right place
My decision to not spend too much time on construction has been vindicated, The fancy stuff can can wait until I know what I am doing. Watch this space!
Vertigo Mk II
The Vertigo MkII. Built over tree days and not much care taken with anything. The strange uncoordinated colour scheme is the result of using up a lot of the odd bits of solarfilm I have lying around, that I am trying to get rid of. These short lived models seem an ideal use for it.
Major differences are more wing area, bigger elevator, and the engine recessed into the wing as far as it will go to get the centre of gravity back rearward without adding more lead. It still balances just behind the engine so will probably have to add more lead, but at least it should not be so much.
Vertigo Mk II Flight
Just how wrong can you be? The first flight proved one thing very quickly; the damned thing was extremely tail heavy. When I finally got it airborne it was almost uncontrollable, the slightest twitch on the controls would change it's direction completely. Not only that, but level flight was almost impossible, with the model pitching up and down in a most violent manner, each time stalling the wing to the point where I thought it would fall out of the sky, or just pitch down full bore into the ground. After frantic few seconds that seemed like very long minutes, I managed a semi-controlled crash landing.
The model is surprisingly resilient, and no damage was detected. I had to attack several problems at the same time with what I had to hand. first was adding a few links to the throttle line as it was opening the throttle as soon as I picked up the handle. this did not cure it completely, and in retrospect I know why. The Mk I had a soldered joint on the throttle linkage that acted as a weight pulling the throttle shut with the model in the vertical position sitting on the ground, I had replaced the old linkage with a new piece of wire, and obviously the weight was not there, so the slightest tension on the throttle line was enough to actuate the throttle. Lesson number one hammered home!
As fitting lead to the nose would be difficult, I sliced the elevator along its length reducing the area by about a third. The second flight was still hampered by the sensitive throttle, only one in three attempts to get airborne were successful, but was a tad more controlable, even though it still had the vicious high speed stall in level flight, which strangely disappeared as I throttled down and the airspeed dropped. Transition the the vertical was was easy to achieve but the elevator was still much too sensitive, resulting in another crash landing.
After managing to bore a hole through the plywood engine mount using a small screwdriver, I managed to bolt two pieces of lead to the nose. This almost transformed the models flight characteristics to, 'Flyable without heart failure!' Not quite though, as the high speed stall is still present but very reduced.
The one major problem I have left is, when the model rotates to the vertical, if the nose rotates to the right beyond a certain angle, looking from the pilots perspective, it is almost impossible to get the the nose vertical (or even horizontal) again, resulting in flying into the ground yet again. This only happens in right rotation, not left; the only thing I can think of that may be influencing this is, this is the side of the nose I bolted the lead to?
Much food for thought then, and a big thumbs up for AP engines, it's still going strong even after being pummelled into the ground in a most unkind manner many times; and also a Kavan nylon prop which is also surviving the mistreatment without complaint.
This may sound strange but this project, to me, sums up what aeromodelling
was, and still should be all about. Build something, and keep working at it
until it flies how you want it to. This can still be achieved with an ARTF RTF
model, so there is no excuse not to do this. The very basic principals of
flight are not to difficult to grasp and apply to all types of flying model. To
paraphrase something I read in an Aeromodeller many years ago which sums it all