Recon mission ended badly...

[Dillzio]

Hi all,

I decided to attach my proper Garmin GPS to my cub so that I could see how high and fast the cub goes while it's in flight. The Garmin GPS allows me to log a data point every 3 seconds, and every point shows position, altitude and speed. I also had my GPS tracker on board so that I could find the plane if it happened to fly off on me. This is how the plane looked as a reconnaissance flier:





It turns out that with that much extra weight on board, you don't set the CG where it would normally be. I set my CG to 57mm from the leading edge, a setup that normally makes the plane fly beautifully, but with the GPS on board it turned out to be too nose-heavy.

I hand launched it, but the plane just wanted to nose down. I realised pretty quickly that I needed to move the GPS back a bit, so I tried to pull a 180 and bring it in for a landing. I gave it full left rudder and rolled it to the left then pulled up to bring it round towards me, but the plane was too nose heavy and lost too much altitude. I suppose that with a nose heavy plane, using rudder whilst doing an aileron turn makes the nose drop down TOO low. I saw what was happening, leveled the wings and pulled up, but the cub was too unresponsive - it ended up nosing into the ground.
It broke in an entirely new spot (for me) this time, on the fuselage just behind where the wing attaches. I've got it gluing back together now with two part epoxy so after that sets it should be fine again.

I wouldn't mind trying it again, but next time I'll set the CG further back. In the short time it did fly, I clocked it at 32mph. I've also got some data points from my GPS tracker, but I consider that to be somewhat less reliable. The tracker showed the speed as fast as 77km/h (during a dive headed downwind), but other than that most of the speeds read around 35-45 km/h. The annoying thing about the tracker is that every time I want to get a reading from it, it costs me credit off the GPS sim card, and when I'm flying it, it only has enough signal to give me a speed reading about half the time.

[mebillica]

Ooof!

[killioughtta]

Man, you are one cursed dude. If I had the money, I'd buy this bewitched plane of yours for ten thousand dollars. lol.

I thought nose-heavy planes tend to pull up and tail-heavy planes tend to dive. That's how you judge the CG on a CG dive test, anyway. Art, correct me if I'm wrong, please.

[SCC]

Dec 23, 2010 5:28:31 GMT 1 Dillzio said:

It turns out that with that much extra weight on board, you don't set the CG where it would normally be. I set my CG to 57mm from the leading edge, a setup that normally makes the plane fly beautifully, but with the GPS on board it turned out to be too nose-heavy.

Dill. If your cub is good at 57mm then it should fly OK with the extra weight as long as you keep the 57mm CG.

[Dillzio]

Dec 23, 2010 8:42:04 GMT 1 killioughtta said:

Man, you are one cursed dude. If I had the money, I'd buy this bewitched plane of yours for ten thousand dollars. lol.

I thought nose-heavy planes tend to pull up and tail-heavy planes tend to dive. That's how you judge the CG on a CG dive test, anyway. Art, correct me if I'm wrong, please.

I think you've got it backwards, I think. It seems to me that if it's tail heavy, the tail will get pulled down, the fuselage will point upwards and the plane will want to climb (until it stalls anyway). If it's nose heavy, the nose will get pulled down, the fuselage will point downwards and the plane will want to dive.

Dec 23, 2010 9:50:58 GMT 1 SCC said:

Dec 23, 2010 5:28:31 GMT 1 Dillzio said:

It turns out that with that much extra weight on board, you don't set the CG where it would normally be. I set my CG to 57mm from the leading edge, a setup that normally makes the plane fly beautifully, but with the GPS on board it turned out to be too nose-heavy.

Dill. If your cub is good at 57mm then it should fly OK with the extra weight as long as you keep the 57mm CG.

I agree with you completely Paul - It should have flied OK. Doesn't it pi$$ you right off when our cubs fail to follow the laws of physics?

I think the way it works is that if you have the CG set as far back as it is on a stock cub then it's good for a motor with stock thrust, but much more and the plane just wants to nose up all the time and it makes it too hard to fly fast. When we put powerful brushless motors in the cubs, we have to move the CG forward a bit to accommodate the extra power. After my experience, I'm thinking that the amount you move the CG forward depends on the power to weight ratio, rather than the amount of thrust. The 200g GPS increased the cub's weight by 17% (normal weight is 1.2Kg). Perhaps reducing the power to weight ratio by so much called for a different CG?

Say, now that I think about it, I forgot to attach the GPS the other day and it flew OK with a CG 70mm back from the leading edge! Could it be that my wing has become warped such that the CL has been moved backwards?


Anyway, I like to think of my crashes as a fine-tuning process. Every time it crashes it breaks in the weakest spot, then I fix it up so the cub ends up stronger than it was to start with. Eventually it will just be a flying mass of carbon-fiber, epoxy and about seven layers of profilm.

[SCC]

Dill. When you put more powerful motors in the cub you should change your thrust line, not CG.

[ginginho]

Dec 23, 2010 14:11:59 GMT 1 SCC said:

Dill. When you put more powerful motors in the cub you should change your thrust line, not CG.

Yes indeed. I wonder if you are barking up the wrong tree here. I tried my SC with the cheapo Ebay Camera strapped on top of the wing right above the CoG, as your bit of kit is.
It flew like a pig and seemed much more nose heavy than it normally is, and those things only weigh a few grams.
Perhaps it's due to adding something to this area of the top of the wing that screws up the airflow and hence causes a loss of lift which is perceived as more nose heavy than normally.

Just a thought.....

BTW, the gimps nose is starting to look like Joe Bugner.

[flydiver]

Trimming confusion? Print out this chart:
www.wtp.net/DBEST/trimchrt.html

There's extensive setup help here that is useful
www.geistware.com/rcmodeling/aerobatics/setup.htm

Personally I think it's just too heavy with the extra load and drag to fly right. People expostulate that you power a brick well enough and it will fly. That's right, and it'll fly like a brick. At some point the design of the aircraft is diverged from so much that the entire flight envelope is compromised.

I built a 4mm coroplast plane for sloping in high wind and designed for rough landing areas. Built with fan fold or depron it is a proven design. Built with 3x the weight and I haven't gotten it to fly decent yet in light lift. I'm beginning to suspect I built a pig. It'll 'maybe' fly with a LOT of lift and a LOT of speed...maybe.

Don't know if it's a photo illusion but in your top pic the thrust correction looks pointed left instead of right.

[higgsbosonman]

wouldn't increasing the weight increase the stall speed? all our cubs act non-responsive when they stall, and mine personally drops it's nose right away, acting like it's nose heavy at low speeds. maybe you just couldn't accelerate the plane fast enough. you got airborne, but couldn't sustain it because your plane was in a prepetual stall from pulling up on the stick.
that's my guess.

[Dillzio]

Hmm, lots of discussion going on here.

First of all, by thrust line, do you mean the angle the motor sits on? I use a heads up RC 35-36 motor mount from ebay that has a decent thrust angle built into it. Also, you say that with a more powerful motor you should change the thrust line, not the CG? I thought it was common practice for everyone to move their CGs forwards when they went to a more powerful motor, so their cubs didn't just want to nose up into the air at full throttle? That's the impression from the info SCC and Ging gave me on this thread I made a while back supercubclub.proboards.com/index.cgi?board=sccgeneral&action=display&thread=2508

As for the GPS causing a loss of lift - well it certainly would make sense, but from the position of the GPS I wouldn't think it would cause any loss of lift because it was over the fuselage. Lift is created when the wind has to travel further over the curved top of the wing than it does to travel along the flat bottom of the wing. The air on top is basically forced to thin itself out, reducing the pressure relative to the bottom of the wing, causing lift. As the GPS was over the fuselage where no wind was passing under the wing, I didn't think that part of the wing would produce any lift? With that being said though, I suppose it's possible that it could compromise the airflow either side of it to some extent. I guess another potential cause for the crash could be drag induced by the GPS, but I would have though something sticking up on top of the plane would make it want to nose up, not nose down?

Oh, and higgs, yes, increasing weight will increase the stall speed too. Now that I think about it, my tight turn probably did reduced the plane speed enough to make it stall, resulting in my crash.

In all likelihood, the crash was probably caused by the plane being nose heavy, and having less lift, and having more drag, and having a reduced stall speed.

Oh and yeah fly, the nose is looking a little worse for wear at the moment, but it's nothing that can't be patched with a little film. :-)

[SCC]

Dec 24, 2010 6:30:46 GMT 1 Dillzio said:

Hmm, lots of discussion going on here.

First of all, by thrust line, do you mean the angle the motor sits on? I use a heads up RC 35-36 motor mount from ebay that has a decent thrust angle built into it. Also, you say that with a more powerful motor you should change the thrust line, not the CG? I thought it was common practice for everyone to move their CGs forwards when they went to a more powerful motor, so their cubs didn't just want to nose up into the air at full throttle? That's the impression from the info SCC and Ging gave me on this thread I made a while back supercubclub.proboards.com/index.cgi?board=sccgeneral&action=display&thread=2508

Thrust line is the angle the motor sits at.

Its not common practice to change CG with a more powerful motor. What is common is you have to move your lipo forward to keep the correct CG as the brushless set up is a lot lighter than the Brushed motor and gearbox.

[flydiver]

CG is a function of lift. Unless you screw with the lift you leave the CG alone for the most part.

The motor causes it's own lift pulling the plane which adds the wing lift. As the motor becomes more powerful this effect becomes more exaggerated. To counter the increased motor effect you need to add more down/right thrust correction. You don't mess with the CG, that is assuming the CG is correct.

[toff]

Looking at the photos again, I have to say that she looks like a yesteryear starlet........Good looking at a distance, but you can really see the wrinkles in close-ups!
never mind bumble-bee, what about Madonna, Bardot, or possibly QE4.5 ( and yes, the Queen was a looker (60 years ago or so).....( Hmmm, wonder if I could be hanged for treason for that?)

[Dillzio]

I'm still not too sure about this notion of not needing to move CG forward for a motor with more thrust. Not only from what we talked about in this thread supercubclub.proboards.com/index.cgi?board=sccgeneral&action=display&thread=2508
but my theoretical understanding of the aerodynamics of a high wing plane (I don't think this phenomenon would apply to a mid or low wing plane).

Imaging the supercub is balanced on it's centre of lift, and the rest of the plane swinging/dangling down from this point. If you're particularly conceptual, imaging a wire passing all the way through the wing along the CL, and the cub hanging from the wire. If you apply a large amount of thrust from the motor, the plane is going to want to pivot on the CL and swing upwards. You could see for yourself by balancing it on your fingertips on a point on the wing, then give it a blast of throttle. You'll find that as well as wanting to move forwards, the plane will also want to pivot on your fingertips and swing upwards.

It's really the same principal as a child dangling from a swing, if you push the child forwards, they're going to swing upwards aren't they? I'm hoping to god you're understanding the concept I'm trying to communicate here, cos it's a bit hard to describe. Do any of you get the concept I'm trying to describe here, and why it suggests that you would want to move your CG forwards for a high thrust plane to prevent the cub from "swinging up" too much at full throttle?

Regarding my plane showing it's age, yes, the wrinkles sure are showing now! Periodically, you have to go over the profilm with a iron again, otherwise it becomes loose. Because of the shape of the cub, it's very hard to profilm it without wrinkles. When you're profilming it the first time, all the pieces of film are a bit oversized so you can grip onto the edges and pull it really tight and pull out all the wrinkles. Once you've cut off these overhanging bits however you have nothing to grip onto to pull out the wrinkles, so when you heat it up again the film wrinkles up. Unfortunately, I don't think there's much that can be done about it :-(

[ginginho]

Nope Dillz, the CoG should remain as is, thrust lines are the changes required for more powerful motors. Thrust isn't part of the equation. Consider if you have the same motor but change the prop to a less efficient one (same weight), hence less thrust. Do you really think you should move the CoG? Taking this concept to the other extreme, you'll end up with a plane that will be unable to glide.

Try out the tests in the links Fly provided or search for more online. There are plenty of sites which address setting this up correctly.