Loop problem MPX skycat: Advice please

[davidleitch]

Hi I have about 50-60 flights up on a MPX Skycat. This is a 480 sq inch, 60 0z standard sports plane epp foam model that I am running on a Jeti Phasor 30/3 with 10 x 2400 (3000) and a 10x 6 prop.

This generates about 72 watts/lb at the prop from about 270 output watts at around 40 amps

I've been fond of the the plane and learnt heaps from it. It rolls fairly well, flys inverted with little down elevator (so I think the CG is about right) and does good snap rolls. It will or used to do an outside loop and and normal loop. Slows down quite well for landing despite 19oz wing loading.

Now it won't loop at all, I get it up to the top and it just falls out.

I think there is enough power, because it will just about do an outside loop. Climbs about 45 degrees. Maybe the 10x6 doesnt give enough thrust?
I think the CG is in the ballpark because it needs little down elevator to fly inverted.

What else could be the problem? Not enough elevator throw, declage. Its true I've crashed it a few times, but its never looped that well.

I like the look of the plane but if it don't loop well it doesn't have much of a future as my aerobatic trainer.

I've tried stiffening the fuselage and elevator, but not much difference.

[mkirsch1]

What do you mean by "falls out?"

72 W/lb should be enough to loop. I have a plane with about half that much power, and it will ROG from smooth grass and loop from a shallow dive.

Decalage is a term associated with biplanes IIRC; it has to do with the difference in incidence between the top and bottom wing. You want plain old incidence :) However, even a plane with a wing that's waaaay off will still loop. A couple of degrees one way or the other really doesn't have much effect on the flying characteristics of the airplane.

If the airplane falls out by suddenly rolling to the left, you're yanking too much up elevator. All planes have their limits in this respect; they will all snap out of a loop if too much up elevator is used. In fact, all airplanes will do a high-speed stall like that if too much elevator is used, period. Try reducing the amount of elevator you use when the airplane goes inverted.

If the airplane gets vertical then seems to run out of steam, your use of (excessive) elevator might also be the culprit. The elevator controls pitch AND airspeed; it does not "make the airplane go up and down." Assuming you go into the loop going full-out, the sudden change in attitude from yanking full back on the elevator kills all your airspeed. The airplane is still moving horizontally, even though you have the nose pointed up.

Just like in a car, the faster you're going, the less control input you need. Most people believe that loops are accomplished by simply flying as fast as you can, then yanking full up elevator. This couldn't be farther from the truth.

[Bill Glover]

Yes, all good suggestions and info.

If it is screwing or flicking out (rather than just mushing & dropping) the other thing to check is that you're not carrying crossed trims (for example left rudder and right aileron). If the model has been repaired a few times this may not be obvious i.e. the whole fin or even rear fus may be out of line, rather than the rudder being deflected releative to the fin. If you compensate for this with aileron trim the model will be OK straight & level but at low speed and high power (like at the top of a loop) the rudder effect increases and aileron decreases ... which will really mess up the manoueuvre!

Also check the tailplane is still level compared to the wing.

Try flying the model away from you and smoothly pulling straight up on full throttle, as if for a stall turn. Does it climb straight (assuming you started wings level ;) )? If not, adjust the rudder trim as necessary. You may then need to alter the aileron trim to fly hands off in level flight, but it will loop more accurately.

[Dsegal]

> Decalage is a term associated with biplanes IIRC <

I always thought that decalage applies to all aircraft with a horizontal tail. It refers to the angular difference between the wing incidence and the tail incidence. BTW what does "IIRC" mean?

Dave Segal

[davidleitch]

Well I dont know about declage, but with a more gentle use of elevator Skycat is looping, although not comfortably.

[Bill Glover]

Originally posted by davidleitch
Well I dont know about declage

Decalage is one of the terms used to describe a biplane setup and does indeed refer to the difference in incidence between the top and bottom wing. The other attributes are the gap (vertical separation between wings), and stagger (horizontal separation).

It's true though that decalage is quite commonly (mis)used to refer to the difference in incidence between wing and tailplane (also sometimes called 'longitudinal dihedral').

[Bill Glover]

Originally posted by Dsegal
BTW what does "IIRC" mean?

Dave Segal


If I Remember Correctly.

HTH :)

[YZYgold]

Mine is on a Axi 8 cells 12x8 prop loops are ok But Mine has a balooning problem especially on full throttle

[wperko]

Aloha,

I put my Sky Cat together, using 8-sub C cells 3300MaH NiMH's ... I was driving around on the ground to get used to the ground handling and just making turns on a paved runway the left wheel curled-up under the plane ...

When I went to see why the plane looked odd and didn't drive on the ground I found the landing gear practically as soft as butter bending the wheel back to a normal stance ...

Did I get a defective landing gear arm or is this just how cheap Multiplex is?

My local dealer just say's I've got the plane over weighted at 4lbs.

I'm still new to R/C ...
http://4.3.109.150/AudioVideo/Video/WalterPerko/theElectricRed-SIG-Kadet-Senior/vElectricRedSIGKadetSenior-FMLPs.avi

http://www.brainless.org/ndmp/ThisIsAmerica.htm

[BMatthews]

Decalage is one of the terms used to describe a biplane setup and does indeed refer to the difference in incidence between the top and bottom wing. The other attributes are the gap (vertical separation between wings), and stagger (horizontal separation).

It's true though that decalage is quite commonly (mis)used to refer to the difference in incidence between wing and tailplane (also sometimes called 'longitudinal dihedral').

Really? It doesn't surprise me that we've come to use it wrongly. I've never heard of any other term for the wing to stab angle. Even all the books I've ever read on aerodynamics use it that way so here again we've all been had. But such is the life of words and their meanings these days.

Nice bit of trivia Bill.

[vintage1]

This came up in modelling science before, and I did some googling: Alhough the first usage of degalage was AOI betwen the wings of a biplane, its very commonly used as an alternative to 'longitudinal dihedral'

The term comes from the French word for time lag or delay, and presumably means the difference between one wing stalling and the onset of a stall in another. As such I see no reason not to apply it to either case.

Dihedral refers to the angle between two meeting planes, and seems a less elegant term, since the planes in question - wing and tailpane - do NOT meet. Cf polyhedral etc etc.

A selectiu9on of googled sirtes found it was so commonly used (decalage) tpo represent teh main to tail plane incidences, as to be mpre or less de facto 'correct'

There are a hard core of sites insisting that it only applies to biplanes etc, but they are in a slight minority.

There are more sites talking about its use as a psychological term.

I think I found a reference to 'rhythm de decalage' meaning more or less syncopation...;)

[Bill Glover]

It's always weird when a thread from several years back suddenly springs to life again!

Since a biplane has both measurements (the difference in incidence between the top and bottom wings, and the relative angle of the tailplane) how can it be "correct" for both to be called the same thing?!

[vintage1]

The same way that 'chord' applies to both main and tail planes?
Or 'angle of incidence'

Decalage is just a short way of saying 'difference in angle of incidence' that's all.

[Bill Glover]

The same way that 'chord' applies to both main and tail planes?
Or 'angle of incidence'


Well, no. Chord and angle of incidence (and span, area, section, etc.) apply to any surface ... so are usually qualified (e.g. tailplane incidence, fin area).

Decalage refers to two surfaces. As I said above, if you consider a biplane and think that the term can properly refer to any two horizontal surfaces you fancy then it's completely ambiguous (and therefore pointless).

PS Methanol burning IC engines are commonly referred to as "gas" ... is that correct just because lots of people say it?!

[vintage1]

The point is that a lot of respected sources on aerodynamics also use the term to apply to what we are talking about. Its easier to type than 'longitudinal dihedral' which as I pointed out has no real meaning either.

As far as alcohol engines go, take your pick from :-

Glowplug ignition
Glo
Gas
Nitro
Slimer

Language is there to communicate, and provided its unambiguous and has got the point across clearly, and hasn't broken too many grammatical taboos. its enough. It is not ambigous to talk about decalage in a monoplane: There can be only one thing being referred to.

More than 70% of sites I googled with 'decalage aerodynamic' were using it interchangeably with 'longitudinal dihedral'. he other 30% made it clear that they were in the biplane/multiplane game, and took a purist view.

If language were static we would still be talking about 'nice edges' on razors, not nice surprises at Christmas.

[Bill Glover]

It is not ambigous to talk about decalage in a monoplane: There can be only one thing being referred to.

How about a monoplane with an airfoil surface on the undercarriage then (e.g. Fokker DVIII)?! :D

Yes, I know, that's technically a sesquiplane. Or perhaps some 'respected sources' have started using that to mean something completely different too? ;)

Traditionally, incidence angles of wing and tail (on monplanes or anything else ;)) are given relative to a datum line, and not each other. Decalage is a specific term to describe a particular setup that occurs on a biplane where one wing is rigged at a different angle to the other. If you decide to hijack that term and use it for something completely different then what do you call the original? What next? Let's call the angle that the main wheels of a tricycle u/c are raked back from the CG "stagger"! Then how do we describe the vertical alignment of the wings on a biplane? :rolleyes:

PS I thought that "gas" was short for "gasoline"? Calling a motor that runs on methanol a "gas" one does seem a bit ambiguous to me, when there are other model engines that do run on gas/gasoline!