CG and control throws for a GP Spirit 2M

[belouder]

I'm re-habbing a (very) old Great Planes Spirit 2M to teach my young son to fly. I never marked the CG location and I'm changing a bunch of stuff to get the weight down. Does anyone have a CG location and possibly control throws? This was kit built, not the ARF.

Thx
BK

[Gary Binnie]

The manual is available as a pdf download from the Great Planes website:

http://www.greatplanes.com/parts/index.html

Happy landings

Gary

[Bark065]

Just took some measurements on mine. I have the ARF. Up and down elevator 3/8". Rudder slightly over 1 inch. The tow hook is in the forward position. I started with a C.G. right in the middle of the wing spar. I recently moved it so it now balances right on the back edge of the spar. Moving it back made it much easier to detect thermals. Also improved the turning. Launches are straight as an arrow off an upstart or winch. I don't have to touch the controls at all unless there is a slight crosswind. Then just a touch of rudder one way or the other puts it right back on course. I've had some really good flights with this plane in the 4 or so months I've owned it. Hope this helps.

[lincoln]

Suggest as much throw on the rudder as you can get, perhaps 35 or 40 degrees. Also, fin is too small and you might want to enlarge it a little. Or maybe a lot. Maybe extend the rudder a little too. And if there's an easy way to lighten the wing tips, do it.

Alternatively, if the tail group isn't too heavy, stretch the fuselage a bit.

The Spirit can be a bit squirelly, and I think it's because the fin is too small and/or the rear fuselage is too short. Not sure about the stab and elevator.

[Soar_dude]

The Aft edge of the spar is a good starting point. the suggested CG in the planes and the manual is too conservative. the middle blind nut is the sweet spot for launches. the rudder is a bit on the small side. Also check Decalage.
Here is a link to Charles River Radio Controllers.
http://www.charlesriverrc.org/articles/kitmods/dickwilliamson_gpspiritmods.htm
this will give all the good info on the Spirits.
Soar Dude

[rdeis]

How about washout? The upper tips on mine are sheeted from the LE back to the spar. Looks like CRRC reccomends no washout when this sheeting is done?

Anyone tried turbulators? They made a huge difference on my Eflight Ascent, but I don't remember using them on my previous sailplanes. It's been a while...

[Soar_dude]

How about washout? The upper tips on mine are sheeted from the LE back to the spar. Looks like CRRC reccomends no washout when this sheeting is done?

Anyone tried turbulators? They made a huge difference on my Eflight Ascent, but I don't remember using them on my previous sailplanes. It's been a while...

For teaching a little washout will make it easier to fly. If you are just flying it don't use any washout it will have better flight performance. Forget Turbulators they are only a fix for a poor airfoil/wing design.

Soar Dude

[rdeis]

Forget Turbulators they are only a fix for a poor airfoil/wing design.

Strongly disagree-- but even if I didn't I've heard enough complaints about the Spirit's design recently to make the question worth asking. :) (Especially given the age of the airfoil and planform design.)

What do you mean by "easier to fly" for the washout? Either it thermal turns and handles well at slow landing speeds without wash, or it doesn't. There's no sense in squeezing a few % off of the straight-line drag if the penalty is stalling out of turns at inconvenient times.

[Ricardo RW]

What do you mean by "easier to fly" for the washout? Either it thermal turns and handles well at slow landing speeds without wash, or it doesn't. There's no sense in squeezing a few % off of the straight-line drag if the penalty is stalling out of turns at inconvenient times.

I agree. Looking other designs, almost all do have some wash-out.

[Soar_dude]

Strongly disagree-- but even if I didn't I've heard enough complaints about the Spirit's design recently to make the question worth asking. :) (Especially given the age of the airfoil and planform design.)

What do you mean by "easier to fly" for the washout? Either it thermal turns and handles well at slow landing speeds without wash, or it doesn't. There's no sense in squeezing a few % off of the straight-line drag if the penalty is stalling out of turns at inconvenient times.


The biggest complaint about the Spirit design is tip stall in the thermal turn. The fix for this is to sheet the outboard panels. Since the wing is already covered this is not a option a little extra washout in the wing will delay the stall. The drawback is that the wing will be creating more drag. Why do you think a J-3 cub flies so well it has washout in the wings. If I remember the story correctly Steve McQueen owned a J-3 But he was not happy with the speed and range. He had his mechanic remove the washout in the wing. The result was that it flew faster and could fly farther, but on the negative side is that it was more prone to dropping a wing in a stall and going into a spin. The main thing when teaching someone to fly gliders is have a stable glider that has no bad tendencies. If you have a trainer that loves to tip stall at slow speed turn your are gonna lose the plane and student because he or she feels that they do not have the talent to fly. Now of course there are always exceptions to the rule. Straight line flying is good for getting somewhere but most of time we are in a thermal making thermal turns so it is best to optimize for this type of flying.

Soar Dude

[rdeis]

I'm not arguing about the tradeoff of tippiness vs washout's increased drag. That's well established.

My idea of "performance" just includes the specific flight envelopes we use when soaring. If it's too tippy to perform well in that envelope, then adding wash doesn't merely make it "easier to fly", it's a _requirement_ for reasonable handling characteristics.

Unlike other trim that makes the bird more stable (such as a forward CG) you wouldn't remove required washout as your skill improved.

But either way, you say the answer is "no" with sheeted tips. Mine are, so that's what I need to know. Thanks!

[Soar_dude]

But either way, you say the answer is "no" with sheeted tips. Mine are, so that's what I need to know. Thanks!

I did not say that this what I said:

The fix for this is to sheet the outboard panels. Since the wing is already covered this is not a option a little extra washout in the wing will delay the stall.

I have a Spirit 100 my advanced wing leading edge is sheeted, my 120" wing leading edge is sheeted, and I made the mistake of not sheeting my sport wing I put a bunch of half ribs in to get less sag between the full ribs bad idea but it stll flies so when it comes time to re-cover that wing I will take out the half ribs and sheet like I should have in the beginning. Putting more washout in the wing will make it easier to fly in turn. Also to much of good thing can turn bad if you go overboard with washout you can make the outboard wing create negative lift another words the outboard wings are trying to pull the wings down.

My idea of "performance" just includes the specific flight envelopes we use when soaring.

When you are trying to learn how to fly performane is not a big priority. learning how to control the plane and getting it down in one piece is the number one priority. You are not gonna let someone that is just learning to fly hand them a Transmitter that is controling a high buck glass slipper and expect them to fly it as well as you do?


Soar Dude

[rdeis]

Well I've read a few other sources that say it's not necessary when the tips are sheeted. I also went and RTFM in the link Gary posted above. It says no washout even on an unmodified wing, though it seems that most disagree with that assessment. I'll find out this weekend unless the snow gets here early...

You are not gonna let someone that is just learning to fly hand them a Transmitter that is controling a high buck glass slipper and expect them to fly it as well as you do?


No, but I'm also not going to add washout to a wing that doesn't need it to prevent the new pilot from tip stalling. CG, control throws, expo, things like that sure- but not wash out.

If it's _that_ easy to tip in a thermal turn, it needs wash no matter who's flying. If it's not, then it doesn't. That's all I'm saying.

[rdeis]

Wohoo! And it even signals lift! <grin>

It's a bit heavy, though.. 34oz despite the fact that I put a V tail on it so save weight. I think the wing's previous owner went to great lengths to bullet-proof the spar against winching...

No wash on anything, CG is right at the back edge of the spar. Using 2500mAh AA NiMH camera batteries I didn't even need lead! (have to get some smaller ones to lighten up the ship a little...) She's well behaved and flying great.

I'll be tuning for a while yet to get the control response and flight mode trims all dialed in, but for now I'm just really happy to be soaring again.

I had the CG just aft of the spar for a few flights and it was not so well behaved- very, very pitchy. I'll measure my throws and take some pictures after this cold snap goes thorugh.