Sunday Flyer!

Well, have just checked my memory banks and I've put in a somewhat surprising 19 flights this weekend. And the weekend has not been completely without casualties. Not only did I notch up a the usual nose overs (although they are generally starting to become rarer), but there was a mid air collision with Ground Dancer's new spitfire, and I stalled the Tucano on approach. A pretty full program and I am actually exhausted. However, I have no doubt you at least want to hear the morbid details of the mid-air, and the Tucano crash. So, for you dear readers, here we go.

So, the mid air. What can I say. They happen. Ground Dancer finally got around to maidening his new WM Spitfire. Is an ARF balsa/ply spitfire including a 18A brushless motor. Having been away a bit recently, and having to pull the pin on trying to fly her last weekend when the CoG was found to be a long way from where it needed to be he was of course keen to get in a good day flying her. After a lot of false starts (making us think that our group may have a curse on Spitfire models) he finally got her in the air, and she flew beautifully. I taped his first flight (or much of it) on the video camera. Nice flying bird - much trickier to fly than the PZ Spitfire, or even the SBRC Spitfire I suspect. She held no height in corners, and had quite small wings. Excellent turn of pace though.

Anyway, he brought her in, and I said I would join him with the Zero in the air. Once I was up I set of to pursue the Spitfire, but knowing how fragile both planes were didn't want to get too close.

So - how did it happen? Not sure - I think I pulled a half loop to change direction to follow him and was closer to him than I thought, and he was going slower than I thought. The planes touched going in the same direction with the spitfire the right way up and the Zero inverted canopy to canopy- there was an ominous whack and both planes fell away - I realised I had mine under control, and called out to Ground Dancer to see whether he had his - he did - he called landing, I called landing - I was going to let him go first but I seemed to be dead stick so we more or less approached at the same time. He came up short of the runway, while I did a nose over, but reached the tarmac just.

Apart from breaking a prop on landing the spitfire was basically down safe. Zero - down safe. The Zero's prop cut a hole through the top of the covering of his port wing. It also, shattered the cockpit canopy, and had also knocked the pilot from his point and he was only just secured lying back in the cockpit. We decided it was a fatal hit on the pilot, but given the remarkable landing some sort of posthumous award was in order.

Anyway, a close call. I'm very glad GD's Spitfire got home in one piece. The zero was more or less undamaged except the engine mount loosen up a little with the impact - this gave her a nice authentic noise on her next flight.

So, onto the Tucano. This afternoon I ended up in Lyneham flying in 10 knot winds while my other half was at Uni. Those that know Lyneham know that the wind is very variable, and you get a lot of odd effects. However, none of those where to blame for what happened although perhaps after so much flying I was just not thinking clearly or quickly enough.

The Tucano was screaming along on downwind runs - easily 90-100km/hr. I had landed her once - the landing was a little inelegant but the landing gear did its usual trick. On the second approach I was just trying to get her into a nice glide curve. With the wind I just didn't realise she had reached stall speed, and she stalled from about waist high. As usual the undercarriage took much of the shock, but her engine mount also busted away doing some damage to the cowling at the same time - no doubt you saw the photo at the top of the post (sorry about graininess - too tired for a proper photo - that one just came from my phone).

Silly mistake, but no really serious damage which is the important thing - a reminder about stalls learnt very cheaply.

I gotta go - fair winds to you all.

Lucky - ground dancer was good about it, but he has had a run of bad luck on planes recently and I would have been really upset if I had prematurely ended the life of the spitfire.

Icy Conditions

Canberra is dissimilar to much of the rest of Australia in many ways (just ask the average Australian what they think of Canberra to see some of the ways that Canberra is different). One of the more quotable complaints about Canberra is the weather. For a country full of people used to hot and sunny summers, and warm and sunny winters Canberra is most unusual. Canberra routinely gets down to below freezing during winter, and while I know that is not anything of worthy note to an American or European, to the average Australian the typical reaction to below freezing temperatures is to turn on everything single appliance in the house that makes heat, not only in the hope of the short term benefit of warming the house, but also in the hope that the coal burned will release enough carbon dioxide to permanently alter Canberra's Climate. Either that, or they just go back to wherever it is they came from.

There are colder places in Oz, but probably not any colder capital cities in Oz. Even Hobart is rarely as cold. So, why all this prattling on about the weather - well be patient I am getting there.

The other morning we had a real pea souper (as opposed to a pea soup floater - best we can figure the radiation from Maralinga genetically mutated the tasted buds of much of SA's population - the good news is that the condition dissapears after only a generation provided the person leaves SA). However, it was a week day morning and we all knew we had to get off to work reasonably quickly.

The fog lifted enough for a good 100metres visibility so DJ and I put our Tigermoths in the air. The fog was still there so we were tending to keep the planes very close to us, including not letting them climb to high.

After about 3 minutes I found that I had gradually, without realising it, pushed my TM's throttle to full, and that despite this, she was still unable to maintain altitude. Fearing some sort of battery issue, or the oft-warned death of my speed 400 by running it on a 3s (they last for ages - don't stress) I quickly brought her in. After I landed I realised DJ had his bird on the ground - I thought he was just landing, but he was worried he might have finally nuked his battery. We were both wrong. A layer of ice had formed on the props' leading edges, making the prop heavier and also altering its airfoil shape enough so that the motor could not keep the plane in the sky. Some ice had also formed on the leading edge of the wings - wow.

As the fog cleared a little more I also took the Mini Pulse for a spin. It was also affected by the ice, although having so much more power than required for level flight was able to easily keep itself up there. All the same, there was a layer of ice to clear from her prop when she came down.

So, there's a new one for me.

Part 1 - Hovering in Ground Effect

So, this is not a course per se. More a set of notes about my experience of learning to fly RC Helicopters.

The Heli I am trying to learn to fly on is a little GWS Mini-Dragonfly fixed pitch electric heli. Apparently learning on these is tricky as they are quite a bit less stable than larger copters. On the plus side they are of course cheaper. Some essential modifications you should do if you have a little fixed pitch electric heli is get some superskids - they aren't cheap, but your heli will be able to withstand much harsher landings and you will actually have a chance to learn.

Get yourself a 2s LiPo (depending on your heli - 2s was right for mine). Also, think about adding some weight to the head of your heli. Some extra weights on the end of the paddles are a good way to go, just make sure you balance them. Having this extra angular momentum in main rotors will help steady your main rotor speed, a bit like a heavy flywheel stabilises an engine.

So, I started by reading through and doing Radd's School of Rotary Flight Course and suggest you do the same.

I then tried a flight or two at the field on days that were too windy, and also got myself into trouble by trying to punch up (quick high throttle) to get my heli out of ground effect. Problem with punching up as a novice is that you get a broken nose (that's the heli, not you, unless you forgot to duck). This is mainly cause by the time you have finished punching up the heli is basically out of control, and your chances of getting her into a steady hover from there are pretty close to buckley's. So, I would say skip punching up and crashing your heli - here's my theory - if you can hover in ground effect, you can hover anywhere.

What's ground effect? As it applies to helicopters it is effectively the air that you have just pushed down rising back up and providing additional lift. However it's effects on the helicopter are inconsistent and hence the main thing it does is makes the helicopter unstable. Helicopters in ground effect are quite challenging to hover.

Hang on - didn't I just say we were going to hover in ground effect? Am I mad (sorry my writing style appears to have been overly influenced by RADD)? No. In a helicopter, unlike an airplane, being close to the ground is a good thing.

In a helicopter, unlike an airplane, being close to the ground is a good thing.

In a helicopter, unlike an airplane, being close to the ground is a good thing.

Why would I say that? If you've flown fixed wing for a while then you know this little secret - planes basically fly themselves. If you can get the wings back to level, and keep them pointed approximately level you can basiscally recover from anything. And you also know that given enough altitude a trainer almost completely corrects itself. Helis are not like that at all. They get unsettled you have to fix them, and unlike a plane which doesn't tend to get worse (excluding the ground coming closer) a heli that starts to go just keeps on going until either you fix it (effectively impossible given your current skill level) or after transferring a sufficient amount of potential energy into kinetic energy strikes the ground.

In a heli altitude, rather than giving you time to fix things up, gives you time to get into trouble. With a fixed pitch electric heli with superskids if you keep within 2 feet of the ground (so the tops of your legs) you basically cannot damage it provided that as soon as you start to lose her you close the throttle and let her come down as she is.

So, using say a 2.5 x 2.5 metre box (8 ft x 8 ft for US visitors) try and get your heli up and hover in this box, at less than waist high. Remember what you learnt with RADD. Spool up slowly, let the energy come onto the blades, hold the heli steady as she gets ligth on the skids and then very very slowly add power until you are free of the ground. At 2 inches the ground effect is horrendous and keeping the heli steady will be a massive effort, but close that throttle and you are instantly safe. As long as you can hold the heli steady keep on flying her. If she comes above the tops of your legs, or passes outside the box just close the throttle and let her come down.

By keeping her in a tight box, and never letting her get a lot of altitude, and closing the throttle as soon as she gets unsettled we are preventing her from getting so out of control that she will damage herself.

Do that for a battery or two I guess, until you can basically keep her in a one metre square box (3ft x 3ft) for at least 30 seconds.

Now - caues I'm not there yet I can see one possible downside to this approach. You will find, as RADD trained you that hovering at this height will probably need a considerable amount of right cyclic roll as the heli trys to fall off the column of air from ground effect. Just be aware of it, I don't think it will be a problem once we start to go higher.

Prop Unloading - Fact or Fiction?

As I've been progressing through the hobby I've heard of this effect of props unloading where a propellor spinning through moving air will draw less amps than one that is spinning at the same speed in still air (for example where you have the plane tethered by the tail and are testing the amp draw on the lounge room floor).

To be honest I was a bit dubious about prop unloading, and of course when you first encounter it with people telling you its okay to exceed the maximum ratings on equipment because "it will draw less in the air" you tend to be naturally suspicious. After all, is the prop drawing less in the air, or is it just a case that the person is working within the safety limit engineered into the product.

So, I've been doing some research and trying to apply my rudimentary physics knowledge to the problem. As I look around the web I see stuff posted like "as most flyers are aware props unload a little in the air". From my uni days I know what a statement like that means - "I'm pretty sure this is right, but I have no idea where to find a scientific explanation". Well, that's what I wanted to find.

Initially the best my efforts have uncovered is some ball park figures like props with higher diameter versus pitch (ie 3D type props) tend to unload more than props with higher pitch relative to diameter - so an 11x4.7 would unload well, whereas a 5x5 would not unload so much, depending on application.

Those that use the motocalc program have it automatically calculate how much props unload, and those that use data loggers in their aircraft definitely confirm that the prop unloading phenommenon is observable from the data. So, its definitely real - but why?

I'm still left wondering why this happens, and whether a good rule of thumb can be devised for model aviators that is not as coarse as the one currently recommended (amp draw reduces by 20% in the air).

Now - the why. Why does it happen? I've had a few different reasons put to me, and it seems that many of them play a part. The first explanation relates to effeciency. To get a handle on this I had to look into propellors in a little more detail. A propellor actually uses two different scientific principles to work. The first is what you expect - Newton's third law - we push something out the back (air) and as a result the plane moves forward in accordance with conservation of momentum. The second is obvious once someone pointed it out to me (don't you hate that) - Bernoulli's Principle. The propellor is in fact a mini wing and as it spins it creates low pressure in front of the blades, and high pressure behind the blades literally pulling the plane into the low pressure region in the same way that the low pressure above your wings holds your plane in the sky.

Now the effeciency of the propellor, just like the effeciency of the lift from your wing, relates significantly to the angle of attack the blade has with the air. The angle of attack with the air has two components - the fixed pitch of your model aircraft's propellor and the speed at which airstream is striking your propellor. This means the angle of attack of the blade changes as you vary the aircraft's speed.

Now, that explanation covers part of the phenomenon - if the effeciency of the prop varies depending on speed then clearly there will be an optimum speed where you get the most thrust. But more thrust in and of itself does not explain why your motor draws less amps.

The second explanation I've been given relates to the difference in effort between turning the prop through a static medium versus turning the prop through a moving airstream. I must admit this is the one I have the most difficulty seeing as having any relevance. The airstream in front of the propellor is partially moving anyhow, even in a static test (air has to rush in to replace the air that was just ejected out the back). The propellor has to impart force on the air as it passes, otherwise it will not generate thrust according to Newton's Third Law and as best I can see there is no particular reason to believe that imparting force (not lift - force) becomes easier as the airstream moves. After all, even in a static test there are no trully static forces (everything is in motion to some extent or another).

Here's the analogy I considered. Consider a car driving at a constant speed along a road. The road, in motion relative to the car, can be considered our airstream, and our wheels are our propellor. There are a number of forces the car's engine needs to overcome but the two biggies are friction through the drive train/axles, and wind resistance. If we take our foot off the throttle the car doesn't immediatelly stop - instead it gradually begins to lose velocity according to the forces acting against it. However, excepting static friction there is nothing about the car's velocity that makes us need to add less power to maintain speed. The fact that the car would continue to move along the road with no throttle for some distance is merely a reflection of the kinetic energy already invested in the car's body, wheels, drivetrain etc. Yes - it is easier to maintain a velocity than to accelerate, but that is scarcely news to anyone. However, when you are flying at wide open throttle you are trying to accelerate continously. Accepting the angle of attack I don't see any reason to consider that it is magically easier to impart force on the faster velocity airstream.

So, this brings us to the final consideration/explanation. If a prop with a high diameter relative to pitch is observed to unload more, does that give us a hint. Maybe. If you took a look through these notes you are familiar with the idea that a plane can be either pitch speed or thrust limited. Pitch speed limited is like a car in first gear. There is enough power to go faster, but the wheels just can't spin any faster. Thrust limited is where the force of wind resistance and drag equals the thrust produced by the prop and the plane stops accelerating - this is the car flat out on the salt flat - short of redline, but no longer accelerating.

In the pitch speed limited scenario let's say the plane can produce 500 grams of thrust. However, it only needs 250 grams of thrust to overcome drag at it's maximum pitch speed of 40km/hr. The plane can't go any faster in this scenario and it seems entirely plausible that the prop unloads substantially. Simply the motor does not need to add as much energy to the prop to maintain the velocity (just like in the car in first gear you back off the throttle and maintain velocity close to engine redline with much less fuel).

So, where does that leave my understanding - still lacking to be honest. I can grok why:

1. The prop on a pitch speed limited aircraft unloads (which aligns with why large diameter low pitch props unload more).
2. Why the effeciency of propellors changes due to the angle of attack of the blades against the airstream.

What I still can't figure out is why the propellor in a thrust limited plane unloads. The propellor becomes more effecient, but surely that would just give more thrust per watt leading to a higher top speed, but not leading to a reduction in the number of watts consumed at that top speed. And yes, you could maintain a higher speed for less watts, but that is not the same thing as drawing less watts in the air at WOT as on the ground at WOT.

So, for the moment - no rule of thumb - I guess I am hoping for someone that actually understands this stuff to come visit the site and enlighten us all.