abufletcher

Is there anything beside CG issues that could be causing my Seagull Spacewalker II ARF to be wildly uncontrollably in the air? I mean, on its maiden flight it just squirrled all over the air and the best I could do was bring it in for a crash landing. There was some wind that day but that wasn't the main problem. It's now all repaired and ready to fly again -- just as soon as I can figure out what happened. I've done some simulations in REalFlight and it sure looks like a CG problem but the balance was right on at the recommended location (which is at the spar at about the 30% MAC location).

Tall Paul

Next is surface deflection.. amount, rate and exponential.
There's no requirement for ALL the movement all the time.
Setting a low rate at 50% and high at 75% for first flights is good.
Exponential should be set for a soft center. A lot of stick motion to get the surface to move when not much motion is needed, with all of the surface motion at the limit of the stick travel, which should be rarely encountered.
I use -33% EXP on almost all of my planes.
EXP can be either positive or negative. Positive makes the surface VERY responsive around neutral.. a bad thing.
Negative makes it much less responsive around neutral... On Futaba and Hitec transmitters. Those off-brands may do it differently.

joeb102072

My Feedback: (9)

hmmmm maybe you could check for different deflections. By that I mean make sure your left elevator isnt throwing more than the right if they are split. If they are not split make sure there is no slop in them from one side to the other. Also look for the same with your alerons. Spacewalkers are low wing planes right? Are you CG ing the plane inverted if not you should be if its a low wing plane. Also is the fuel tank full or empty? That too might make a difference. And lastly what kind of power source is in it? An under powered plane especially one like that, they dont take to slow speeds too well and that will make it respond as you say yours is. What size is the plane, how much does it weigh and whats the engine size you are using?

Johng

My Feedback: (4)

There are many ways to assemble a wacked-out elevator - and many of them show up emulating a rearward CG. If there is something, anything that decreases the effectiveness of the stabs & elevators - it can seem like the CG is too far back. The effectiveness of the stab and the location of the CG are closely related. If your stab is somehow less effective, the acceptible CG moves forward.

So, check to make sure your servos are up to the job. Not just torque, but make sure the gears are OK. What pushrods are you using? If it's any type of flex-rod like gold-n-rod, you have a prime candidate for poor linkage. What about hinging and gaps. Are the hinges tight. Are the gaps sealed?

Paul makes a good point on surface travel. Too much is a bad thing. Some people think they want more control on a test fllight, because they'll need it if there's trouble. problem is, the travel volume itself is often the cause of trouble. Set a low rate and switch to high if you need it. Not the other way around.

rmh

30%is a LOT on that setup -especially if you have large surface deflections aaaand- if you are not using full servo travel .
this is a common mistake
Ideally the servo travels full rotation --and surface throw is then absolutely all that is necessary.
so if your servo travel is adjustable - adjust for full travel --then adjust linkage for only moderate throw.
low "rate" settings are often the cause

Rotaryphile

I would try pulling the power off and see if the thing will glide stably. If not, move the CG ahead, and then try reduced elevator throw. Almost all the RC models that I have encountered need larger vertical tails. Remember the words of Frank Courtney, WW2 era experimental test pilot: "I have never flown an airplane with enough vertical tail area." A quick and easy way to test for lack of enough vertical tail is to tape a cardboard extension to the rudder, decreasing rudder travel in proportion to the added area, if you already had enough rudder authority, although I have never flown an airplane with what I regarded as excess rudder authority - the more, the better, in my book.

abufletcher

Hmmm...lots of things to think about. Post-crash sims in RF certainly suggest a CG problem (and messing with the throws in RF doesn't do much) but here are the facts about the actual setup of the plane before its fateful maiden:

Wingspan: 62"
Weight: Not sure (5-7 pounds?)
Engine: Magnum 40 2S (I'm interested in scale not aerobatic flight)

Note here: Some people seem to like to make screamers out of these and put in something like a 71 or even 90. Seagull describes this as .40 size plane, though and their suggested engine is a .46. Since I like the WWI planes which were mostly underpowered I thought the 40 would be good training.

Radio: Airtronics VG6000 with no exponential dialed in (I'm new to this)
Throws: Set somewhere between the VERY minimal LOW rates suggested in the manual (for example, elevator 3/8 up and down, ailerons 3/16 up and down) and the higher rates. So I've got about an inch left and right on the rudder. About 5/8 up and down on the elevator and maybe 3/8 up and down on the ailerons. I'll check these again though.

I do have a 52FS I could put in it but I didn't want to have to mess with swapping out the pre-installed engine mount.

nIgHthAwK17

Don't forget to keep in mind that if you are/have programmed exponentials into your radio, that Futaba uses negative expo to make a "soft center", and JR is the opposite (positive expo makes a "soft center" - FYI)

BMatthews

Since it does not sound like it's overpowered in the modern sense and assuming you don't have too much throw and the controls are stiff enough to resist surface movement then it sure sounds like a CG issue to me.

Bear in mind that there's more to the proper CG placement than the basic 30%. If the tail is too small and the tail moment too short for the wing then it'll require a more forward CG position. Also airfoils with a lot of camber will require larger tails with longer moments or more forward CG's. Not knowing the setup of this particular model all I can do is offer possibilities. But if I'm right and it has a thick'ish flat bottomed airfoil then you may want to try the CG a little more forward. Also if it has strip ailerons try reflexing them up about 5 degrees to decamber the wing a little.

Or were you suffering more from tip stalls and near spins or snap rolls? We are all assuming that it was a pitch problem since you're talking about the CG position.

joeb102072

My Feedback: (9)

HMMMM it definatly doesnt sound like you are overpowering but there might be some concern that you are underpowering. You say between 5 and 7 #'s thats a big difference and the type of difference that may aid in making your plane respond the way you say it does. Over correcting on an underpowered plane could definatly react as you say your plane is. Especially if your CG is on. How about lateral CG thats ok?

abufletcher

Thanks for all the suggestions on things to consider. Here's a bit more info. The spec. give the weight of the plane as 6-7 pounds. As I mentioned above I'm currently using a Magnum 40 2S with a 10/6 prop. This is definitely the bottom on the range but others here have said it should be OK.

The problems seemed to be primarily with pitch, it'd climb wildly and then dive wildly, But my (frantic) impressions were muddied somewhat by wind effects (which were either mild or strong) that I seemed to be causing some ballooning. No pronounced tendencies to tip stall (thank goodness).

The Spacewalker is a well proportioned low-wing semi-aerobatic style airplane with a fat, almost symmetrical airfoil and generous control surfaces (see photos below). It's been described as a fairly tame flyer and even OK as a second plane.

I've checked and rechecked the balance and it just can't be more than a small bit off. So maybe CG isn't the issue. At that only really leave throw and exponential settings. So at this point I'm thinking of dialing in 20% expo on the ailerons, rudder, and elevator and limiting the throws to 5/8" up and down on the elevator, 3/4" L and R on the rudder, and 1/4" up and down on ailerons. This is a bit more than the minimal throws listed in the terribly manual which almost everyone says to ignore but still less than the "aerobatic" settings. And of course this time I'll wait for dead calm weather!

Who knows I might even get motivated to put in the 52!

abufletcher

As I said it didn't see windy (or even really breezy) where I was standing on the field but here's the CNN report on the weather the next day:

http://www.cnn.com/2004/WEATHER/08/3...eut/index.html

abufletcher

Not to drag this on forever, but it may be useful for future Seagull Spacewalker II newbies. Further sim experimentation -- this time with greatly exaggerated throws and the radio set for no expo on ailerons or elevator along with slow speed flight (about 1/2 throttle) -- also yielded a "wild ride" similar to that of a badly tail-heavy plane. So given I've checked and rechecked the CG inverted and right-side up, it seems like this second scenario is the most likely cause.

Accordingly, I've set the throws to the recommended minimal throws on low (1) and to the aerobatic throws on high (2) for both aileron and elevator. I've also set the expo at 20% for both aileron and elevator.

Once I fly it again I provide a further follow up report.

BMatthews

Try a few ounces of noseweight to move the CG to around the 27% point for your next flights. Just because the instructions SAY 30% it may not result in a model that's right for what you are used to. Once flying well you can do some low to high throttle or diving tests to determine if the CG and resulting elevator trim is close enough to neutral for your tastes.