New member and Skyray 35 question

[Mike Morrow]

Hello,
I have flown a little R/C, glow about 20 yrs ago and now all electric. Just getting started flying C/L. I've built and flown an Akromaster and just finished a Skyray 35; both electric. My question concerns the Skyray. Following Brett Buck's mods, the CG he recommends is considerably farther forward than the plans. Why is that? Thanks in advance for answering a newbie's questions.

Mike

[Brett Buck]

Hello,
I have flown a little R/C, glow about 20 yrs ago and now all electric. Just getting started flying C/L. I've built and flown an Akromaster and just finished a Skyray 35; both electric. My question concerns the Skyray. Following Brett Buck's mods, the CG he recommends is considerably farther forward than the plans. Why is that? Thanks in advance for answering a newbie's questions.

   It has to be stable enough to fly. The CG shown on the plans is on the edge of unstable. The CG doesn't affect the turn nearly to the degree it does with a flapped airplane. You have far more than enough control authority around zero even with the CG this far forward, and it barely affects the control loads. So there is no harm to putting it very far forward.

     The effect in this case is to give you a little bit of control feel with no real harm otherwise. The turn is limited by the length of the tail, and running out of torque as the pitch rate goes up. To combat this, you need to make the controls move a lot (and I currently have a wider elevator than stock). This makes it twitchy around neutral, too twitchy for reliable flying. You can adjust this nonlinear characteristic by playing the CG and control rate against each other:

   Aft CG + slow controls = faster around neutral, falling off with deflection
   forward CG +fast controls = slower around neutral, speeding up with deflection.

   On a a conventional flapped stunt plane in the year 2019, the former is how most of them are trimmed, this to get positive starts/stops for the maneuvers. The (unfortunate) "exponential bellcrank" and "exponential handles" are intended to act like the latter through mechanical advantage changes. But it's relatively simple to create that situation with just CG and control rate adjustments, as in this case.  And you don't have to start to rely on mechanical trickery that you will have to figure out how to stop using later.

 Even in this case, its still on the twitchy side in level flight. This is really the weakness of the airplane from a competitive standpoint, it won't fly straight lines very well. In the unlikely event I ever build another one, I will shorten the tail by about an inch or an inch and a half and make it larger to achieve the same tail volume, which will reduce that tendency, but probably require a more aft CG to give a linear control response.

      Ted Fancher and I have experimented around a lot with these sorts of no-flap airplanes, and unlike full stunt planes, having the CG at about 15% of the mean chord is a pretty consistently workable starting point. You won't ever get or want the 25+% CGs you generally want with full stunt planes.

     Brett

[Mike Morrow]

Brett,
Thank you for the explanation.

Mike

[Brett Buck]

The farther the CG is forward the more stable it will fly for learning control line. You don't want to do unwanted aerobatics on your first go round.

   I have flown 550-point flights with it that way, so it's not just for cruising around level. It is still *extremely maneuverable* in this configuration, and does square 8's with dead-nuts reliability. I would never suggest someone pile on a bunch of nose weight to keep them from the ill effects of over-controlling, that's what handle spacing is for, and in any case, if you fight over-controlling in a beginner with reduced control speed, all never learn not to do it.


Trainers should be "honest" with their control responses, not dulled-down to reduce the carnage. My big airplane would make a great trainer - if it didn't take a year to build it. That's why I like the Skyray/20FP  - it does almost exactly what you tell it to do, good or bad, a lot like a full-stunt airplane, except you can build it in a couple of  evenings and not even have to know what you are doing to get "honest" performance.

    Brett

[Lyle Spiegel]

Great discussion, but one item I didn't see clearly mentioned was an actual recommendation for where the balance point should be setup. I'm well beyond beginner stage so am interested on what is a good starting CG for flying pattern maneuvers. I use 4 inch handle spacing and fly it on either 56 ft or 60 ft lines.

I also have a Ukie40 and would like to know what id a good CG.
Thank you.

[Dave Hull]

Lyle,

Actually, Brett did give you his recommendation. The Skyray has a non-tapered wing with a 9" chord, so a CG at 15% should be 1.35" back from the leading edge.

For a tapered wing, you need to find the mean aerodynamic chord, and then at this "rib location" measure the chord, and apply the 15%. If you look up MAC you will see easy ways to graphically locate this position. Again, the straight-wing, constant chord Skyray makes graphing unnecessary.

Dave

[Brent Williams]

The Skyray is one of those planes in this hobby that has been so thoroughly flogged, tested, re-tested, and has been proven to work if you just follow the recipe. 
As Brett Buck says, "these are solved problems."  Given his level of ability and time on the airframe...just start out with his recommendations and you'll do well.

https://stunthanger.com/smf/open-forum/adjustable-leadouts-on-my-new-skyray/

http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=198043&mesg_id=198043

 

Not too many details! I suggest building right as it comes out of the box, with two exceptions - get some 2-wheel landing gear, and put the fin on with no rudder offset and no airfoil at all. For the landing gear, I have used both the landing gear SIG sells for the Phazer, and a Hallco fuselage-bottom R/C gear, cut in half, and re-bent to go up along the fuse sides. The rudder and fin offset should be self-explanatory, but measure it very carefully. Otherwise, build as shown, including Monokote on the wings, and minimal finish on the fuse.

Run the engine exactly as it comes out of the box, with the stock muffler, not modified in any way! This includes the stock needle/spraybar, stock .259 venturi, stock muffler baffle, stock muffler outlet hole, no changes at all. Use a 9-4 APC prop. Use either a 3 oz tank no more than 1.5" wide with uniflow open to the air, or a Sullivan 4 oz. clunk tank set up for suction, with muffler pressure into the vent, anbd with the wide side against the fuse. In the winter, or at sea level, or if you crash a lot, use a Bru-Line "fine" air filter. I use whatever 10% fuel I happen to have around - don't use all-castor. SIG, Powermaster, etc, it doesn't care much. More nitro is not necessary but it doesn't hurt.
The engine setting with the uniflow tank should be around 11,300 rpm+- a few hundred. With a suction clunk tank, you will have to experiment a bit. It needs to be *just barely* into a 2-stroke in level flight at the beginning of the flight. Too rich and it's just too slow at the beginning. Too lean and it sags off at the end. It's a pretty narrow range of acceptable.

Balance the airplane at 12-13% of the chord - about 1" - 1 1/8" from the leading edge. The centerline of my leadout guide is 2.25" from the LE. This is about right for a 34 oz airplane on .015x60ft lines at about 4.8 second laps. The position on my airplane is fixed - I just put it were Dr. Soule's "LineII" program said to. You can put in adjustable leadouts, but don't put the leadouts anywhere but where program says unless you really know what you are doing.
This will fly pretty good as long as you get it straight. Get the wings level with warps and tipweight, then go to it.

I also recommend that you use the plans to clone a second and third airplane. Make the wings out of balsa - 3/32" ribs, full at every station, with 1/2"x1/8 medium balsa spars set vertically at the high point. Everything else is fine as it is. What this does is save about 5-6 oz. Surprisingly, without other mods, it doesn't fly significantly better (plenty of power even at 36 oz), but it's much more durable. The stock wings tend to self-destruct due to excess weight. When the nose hits, the tips just keep going, and the structure just disintegrates, even on grass. The lighter wings will just flex and then return to shape with no damage. On pavement, it's broken in any case.

The stock airplane will end up about 35-36 oz. The balsa-wing version will be around 30. The tail length is just about right for the stock wing, but the lighter airplane would probably benefit from a shorter but larger tail. It's rate-limited - it has plenty of lift to turn tighter, but the maximum pitch rate is too limited. My first significant mod will be to shorten the tail about 1.5" and enlarge it to get the same tail volume. This is just an experiment that I haven't tried. It's plenty good enough as is - I got a 540-something with it in P-40 one time, only to be beaten by David F. flying it to a 547 with no appearance points. Story of my life!

Brett

[garry saccone]

I was gifted  a Sig Skyray 35 (retirement present !), have not built for 40+ years.  Would like to build this as an electric powered plane.  I am looking for a conversion kit or suggestions on remodeling the front end to accommodate an electric motor/battery, etc...
Thank you

[Tim Wescott]

I was gifted  a Sig Skyray 35 (retirement present !), have not built for 40+ years.  Would like to build this as an electric powered plane.  I am looking for a conversion kit or suggestions on remodeling the front end to accommodate an electric motor/battery, etc...
Thank you

First, see this thread.  If you can figure out how to do searches on StuntHanger that's a good place to start, because just about everything has been discussed.  For electrocution of all sorts of planes, there's a thread in Getting All Amp'd Up called List Your Set Up; that's a good guide for ESC, batteries, motor and prop, although it usually doesn't show details of the motor mounting.

Second, welcome to StuntHanger.

Third, for the most part, when you have a new topic to introduce it's nice that you start a new thread.  You can do this by going to the appropriate forum (like the Open Forum or Getting All Amp'd Up) and clicking on the "New Topic" button.  We will help you out.

Somewhere buried in all the conversation there's a thread on mounting motors on aluminum brackets so that when you post-hole the airplane you're no more likely to damage the motor than you would the engine on a typical slime setup.  If you mount the motor on an electric as firmly as you mount engines, you will bend it up, because they're lighten than engines.  On the other hand, they don't vibrate much, so if you make a mount that's nice and rigid but will bend or break on impact, then you save your motor.  If you smack straight down into pavement you'll still trash the motor - but then, the same thing applies to your engine when flying slime.

[Mike Morrow]

Hi Garry,
Sorry about not getting back to you sooner. Here's what I used. I also used a Brodak battery box/motor mount but the aluminum angle mount worked the same and was cheaper.

Suppo 2814/8
40A ESC
4s 2200 LIPO
APC 10x5

Mike Morrow