Hello, I purchased a plane from a hobby swap meet a few months ago. The seller built it, and covered the wings, installed servos, pushrods etc. I covered the fuse,stab,rudder,elev., turtle deck (all balsa) installed a never run-in OS 90FSR Which I ran in with help from fellow club members, and it is running GREAT!! -ANYWAY...I installed my new Futaba 6EXA
Radio and servos, I now need to know CG for this plane. I have no plans or instructions, I don't know what brand of kit. It looks like a 300 Extra or that type of aerobatic plane.The wings measure32-5/8"L and are 15-3/4" at the root and 8-3/4" at the tip. The overall length from rudder to cowl is 55". the Fuse is 7-1/4" wide. I hope this isn't too much info. I need help determining CG and beginning control throws movement. Since I have my first computer radio, I can use low rates or whatever I need to do. I just want to avoid a tail-heavy,super sensitive beast on the maiden flight!!! Thanks in advance for all input!

A good STARTING POINT for the CG is 30% of the chord OR you can use the main spar (usually this is ABOUT right also ). I suggest making the plane SLIGHTLY nose heavy for the maiden flight - a sluggish plane is much better than a squirrely one on a maiden.

Since you are describing a fast, aerobatic plane, a reasonable STARTING POINT for throws is:

Ailerons and Elevator - Low rate 1/4" High rate 1/2"
Rudder - Low rate - 3/4" High rate 1"

These rates are to be used for the maiden flight at a REASONABLE SPEED, NOT FULL BORE.

Once you have completed the maiden flight you can adjust from there.

I prefer to use a conservative approach for something like this.

Thanks, Campy,
One question-"slightly noseheavy" does this mean that at 30% of the chord, -the reference CG point- if the planes nose drops slightly it's ok? And also add no weight to the tail, move batteries etc, to compensate. I don't know why but I need some help understanding nose/tail heavy in relation to the desired CG point. If a plane is slightly nose-heavy is the cg point slightly forward of the desired CG or slightly aft of the desired CG? (In this case, 30%) Thanks again!

If the nose points down slightly, then it's a little nose heavy. Desirable over a bit tail heavy.

If it's a little nose heavy, then the CG is is slightly forward of the "described" location. I say "described" because moving the CG further back may NOT be "desirable".
Dennis-

If 30% the target then the CG is only at 30% if it is it sits level when balanced at the point. For that airplane it is a safer bet for a first flight particularly in you are not familiar with that type airplane to set the true cg at 25 to 28%.

If the airplane is a single or double tapered wing then measure out half way from the fuselage side to the wingtip and measure from the leading edge to the trailing edge at this point. This will be roughly your Mean Aerodynamis Chord. Now divide this measurement by four and the result is the distance the target CG will be aft of the leading edge at this mid span point when shooting for 25% MAC.

John

Here's a graphic that may help. You can determine the CG of pretty much any plane by using it.

Keep in mind, that ideally you want to start out by balancing at 25% of the "Mean Aerodynamic Chord" or "MAC". This graphic will show you how to get the MAC, and from there the CG.

Spend a couple minutes looking it over, and pay attention to "root" and "tip" cords.
Dennis-

Thanks to all I'll print the graphic and go from there.

OK,
I found the 25% MAC and tried the balance at 25-28% CG This plane is VERY tail-hevy at that spot. At around 30%(slightly fore of the spar tube) it is still tail-heavy, but less. If I need nose weight, where would be a good place to put it? I have a large plywood engine mounting block under the cowl, should I add lead to the top surface? Add a small (2 oz.) Heavy Spinner, and aluminum Spinner after that? Where do most of ya'all add your nose-weight?

This cannot be, 30% CG is more 'Tailheavy' Than 25% if the airplane is truly balanced at that point and sittling level when supported at that point. Move the CG (balance point) toward the tail and it becomes tail heavy, move the CG toward the nose and it will become nose heavy.

John

What I am saying is this. I lift the plane at 25% back of the LE Let's say 3-1/8" the Tail drops severely. If I move the lifting point back let's say 3-1/2" the tail drops less. That seems to make sense to me if the tail is heavier than the nose, the farther aft of the LE you lift, the less the tail will drop. -What does this mean-? I may not be using the terms correctly, does "tailheavy" mean the target CG is closer to the tail? and because it has to be moved closer to the tail, the plane is tailheavy? Or does it mean that at the target CG the tail drops more? Should I shoot for 28% point for CG and add keep adding nose-weight until it balances at that point?

[quote]ORIGINAL: raynman

does tailheavy mean the target CG is closer to the tail?
[/ore?quote]


That is the correct answer although 'actual CG' may be a better term. 25% is always closer to the nose than than 30% which leads me to beleve your calculations of where the 25 and 30% points really are is confused somehow.

Here is another way to look at it: Actual CG is the point that when supported at that point thye airplane sets perfectly level, This point may not be at the point that is indicated in plans or manuals for that airplane This is the target CG. To move the actual CG to the Target CG we may need to move something in the airplane like a heavy battery or add lead to the nose or tail to get the airplane to balance level when supported at that target CG .

If you are able to move things around such as the battery mount location, this is always preferable. The further from the CG point you add weight, the less you will have to use. The heavy spinner is a good place to start, then add more if needed to get to the 28% MAC for initial flights. Make the weight easily removable because I think you will want to mave the CG aft slowly as you get used to the plane. Remember that this initial CG is a static one to get you going. After that you will determine the dynamic CG that the plane flies best with and that is a personal preference. It should take a slight amount of elevator to maintain altitude when in inverted flight. Another way to check is to roll 90 degrees and let go of the sticks. If the nose pitches down, you are nose heavy. My planes end up being at 30% to 32% MAC.

OK, I added a solid brass spinner(nut), and 6 oz "stick-on" weight (more like, stick on, and drill/screw in) anyway, after all this weight approx 9 oz. the plane balances around 32% CG. How much weight should it take? (for a 25-28% CG) Should I keep adding until it's right? It just seems like alot of weight to me.(let's just say more than I've had to add ever) but most of my experience so far is 40-46 size ARF's... Just looking for some feedback. Thanks, in advance for all the help. My goal is to get this bird flying by Sat 1/15/05 And not one and only one maiden flight if you know what I mean!!

Hi!
9oz..let's see that's around 260g! Uuh! Heavy!
What you should do is to remove all that lead and start all over again carving out as much wood you can from that heavy fuselage, rear part of it!
Also move the servos and batteries as far forward as possible.
Have you any pictures to show us....it would help us to help you!

During my 30 years in this hobby I have learned a very useful thing ...light airplanes fly much better and hold up/lasts much longer then heavy ones.
I never put lead weights in any of my airplanes to balance them, I rather try everything possible to obtain balance by using my Dremel drum sander or tungsten carbide routers to remove as much wood I can ...and believe me there is a lot of unnecessary wood in every model airplane...!
Ok I admit I use some lead...and that is for axial balancing where I put in some 5-10g ocasionally in the wingtip.
Regards!
Jan K
Sweden

That is a LOT of weight! Are you sure its that much? Most of the stick on weights are in 1/4 oz increments. Anyway, yes you are ruining performance by adding that much. Move your battery as far forward as you can along with receiver. Do you have elevator servos in the tail? If so, you might look at moving the location of those (a real PITA). If all this fails to get you around 28% MAC, then I would look at using a larger engine. If you have to add weight, it might as well be power. By the way, the 32" is for one wing right?
Good luck.

I'm a little worried that with the way you are asking all these question and the questions that you are asking is this plane a little over your head and is there not someone in your area that has building experience that can help i would hate to see you put all that time into this plane just to see it crash

I will continue to ask questions, and look for advice in this forum. Yes, the one wing is 32-5/8" I am not worried that the plane is over my head, as a matter of fact, that's where I like it....But, u r rite- I do not have the building experience alot of fliers may have. This plane was built, and servo's/ p-rods all installed when I bought it. I covered it entirely(except wings) and mounted the engine/mount. As far as stick-on weights are concerned, I used a whole package. (Great Planes 6oz) And a brass prop nut. I think the real question is with the CG for this model. With the weight I added It does balance right around the aluminum wing tube. I don't own a Digital Camera, so I can't post a pic. I do Intend on taking this plane to the club and having some more experienced builders look it over for balance, before I fly it. If there are any concerns, I will address them before my maiden. I will keep posting including the maiden voyage of this bird. P.S. There are no elevator servos in the fuse. They are up front just behind wing tube. And the battery is as far forward as I can move it, just behind the fuel tank.

raynman:
Perhaps I can help you if you can provide me with a bit more information about your airplane.
I can tell you where the 30% cg balance point at the root chord if I know the sweep angles of the wing leading and trailing edges. From that info, I can calculate the wing area and find the 30% MAC and project that back to the root chord.
You never said how much your plane weighs. From that I can tell you the wing loading which is an indication of the flyability of the airplane. For that size plane, my guess is that the addition of 10 oz weight or more probably won't hurt a thing.
So get this info to me and I will give you my comments.
feihu

feihu-RCU,

The plane weighs 10 lbs. The sweep angle is hard to tell but only the trailing edge is "swept"The leading edge is straight...The root is 15-3/4 and the tip is 8-3/4. So, the sweep is the difference between 15-3/4 and 8-3/4 over a 32-5/8" span of each trailing edge on each wing....correct? By the way, the weather finally cleared up here in the Bay area, I had the plane out and looked over by some of the "experts",at the club and then.. I did maiden the plane today..but I had a dead stick after some minor trim adjustments. The plane flew for around 1 minute and then dead stick landed into the tall tuley's. After a swamp walk, I found her resting on a soft green clearing. Only small amount of mud on a wingtip, NO DAMAGE!!! So I am ready for another try. THANK-YOU for any input you can give. I Appreciate it!! ;-) Raynman.

raynman:
First look over the attachment and see if I got your wing planform right.
the fuselage is 7 1/4" wide, root chord 15.75", tip chord 8.75" which means a wingspan of 72.5". There is no sweep to the LE, and the TE is swept forward.
If all that is correct, then the wing area is 6.17 sq ft and the wing loading is 26 oz/sq ft.
The balance point is 4" aft of the leading edge of the wing at the fuselage, put your finger at this point and make the airplane balance level. Balance upside-down if necessary.
Since you had a short flight already, you probably already have a feel for the control throws.
Good luck, and good flyin'
feihu[img][/img]

THANK-YOU Fiehu !!
I am checking now to see fuselage width to confirm your drawing. I will post you back!!
Yes, I got a feel for some of the throws, but very little on such a short flight... the guy who helped me on the maiden said that the higher of the dual rates would be ok for the maiden. He thought the elevator needed more up/down throw though. I may need to increase the throw of the ailerons and elevator. I'll keep posting. Thanks Agian!!

raynman:

By the way, the wing loading of 26 oz/sq ft is in the normal range for your size airplane - not too heavy, not too light - so you're OK here.

Regarding control throws, I suggest you set them up by angles to determine the measured inches of deflection. To set up deflection by inches without regard to control surface width could be very misleading; for example, a 1/2" deflection measured on a 1 1/2" control surface width would be about 20 degrees, whereas a 1/2" deflectiion on a 3" control surface width would be less than 10 degrees.

My suggestion would be 20 degrees for aileron and elevator, and 25 degrees for rudder at high rates. Low rates would be ABOUT 1/3 lower, like 13 for aileron and elevator , and 16 for rudder. To get the deflection in inches, you would have to draw the angle and measure the deflection at whatever the control width is. ( Hope I'm not confusing you)

feihu

feihu,
Your drawing is almost spot-on!! My fuse is actually 6-3/4 at the leading edge of the wing.
At the 4" spot aft of the leading edge the plane "almost" balances I might need 1-2 oz. more weight in the nose. As far as angles of control throws I will use you measurements and re-adjust accoring to angle, not inches. I'll get a protractor and get the proper angle.
Thanks again for your posts!! I'll be back!!