riadh

I am putting together a new ARF Thunderbolt 72 inch wing span. Fuslage is fibreglass apart from the firewall and bulkhead and other two or three locations along the back of the fuse. They are all made of ply wood. Wing is balsa and ply with two dowel locators on the leading edge. The model is quiet old and has no markings of brand or other information. The problem I am facing is that the bulkhead is slightly offset to the left in reference to the back wall of the saddle where the wing trailing edge is bolted to the fuse. This offset is about 1.5 cm to the left. My question is if I go centre with the wing dowels on the bulk head the wing will be square with the firewall/engine combination but the trailing edge will be offset by 1.5 cm to the left leaving the wing left side longer than the wing right side by that much. if I move the dowels locators on the bulk head to the right the trailing edge will be centre and square with the tail stabilizer and rudder combo but not square with wing and the fire wall. What do you think I should go for ; centre leading edge on bulkhead or centring the trailing edge of the wing ? Thanks.

jaka

Hi!
Remove the bulkhead! Use a Dremel with a router bit, this takes just a few seconds and then glue in a new bulk head! Easy as that!

raptureboy

More important that the wing and the stab are square to each other. If the stab is attached already then measure from the tips of the wing to the tips of the stab and adjust the wing so it measures equally. If the stab is not attachedlocate the center of the fuselage and measure the distance from wing tip to this point and then adjust the wing until it measures the same from both sides. Now set the stab so it measures the same from the wing. This sounds like you have one of the old CMP arfs or Nitro Planes versions. Pictures always help.

riadh

Thanks for the suggestion Jaka .If I remove the existing bulkhead I would need to remove the firewall too and then I imagine I would need to force the fibre glass front structure to sit in the new location. Probably the stiff fibreglass would crack and it would become a bigger job fixing it.
Raptureboy ,the stab is not attached .So squaring the wing with stab is the way to go even though the leading edge is not with the firewall and the engine ?

JohnBuckner

My Feedback: (1)

I agree squaring the wing with the stab is the way to go but from your first post it was just a little difficult to understand exactly what you were trying to describe. But from your last sentence above could it be that what you are describing is a firewall that is canted to the right to provide right thrust? This is done often in some warbirds.

John

riadh

No John the fire wall and bulkhead are square but both are as if shifted by about 1.5 cm to the right compared to the aft fuse where you would normaly bolt the back of the wing to the fuse.The fire wall does have the right canter to suit engine thrust.





sorry just corrected name.

riadh

A friend maidened the plane for me .The C.G was way backward and the the save was more a work of a miracle and skills! The plane was balanced according to instructions in the manual. Fixed the retracts ,relocated the retract separate battery which was placed under wing trailing edge bolts up front under engine.The plane took off great and controls managed the plane flight pretty good.However, the plane would travel straight with a crab like attitude ,i.e on a straight path, the nose of the plane is tilted some 15 degrees to the left .Not a nice sight and trimming it would require a major rudder ailerons combo off sets. I suspect that the tail combination is at fault . I pulled out the fin and stabilizer, re glued the stabilizer square to wing and stopped short of gluing the fin as I don't know how to square it perfectly towards the prop cone.Any suggestions here ?The vertical angle of the fin with the stabilizer will be the same on both sides of the fin. Also if it is crabbing to the left when it is flying away from you I would think that I need to move the trailing edge of the fin slightly toward the right back side of the fuse.Your thoughts are highly appreciated.

JohnBuckner

My Feedback: (1)

Hi Riadh I definitely do emphatiize with your situation but the problem is you see no one can really help without at the least seeing detailed photos at the appropriate angles of what we are discussing . Perhaps if you got some photos up that would help.

Concerning your CG. I go throught this with almost everyone who shows up locally and its quite a battle convining most that the so called insturctions in many (not all) of these name and no name china products are simply and dangerously wrong. Any warbird is a mistake to fly further aft than 28% mac and I always have the fellows start out at 25% and I use the very simple (old fashioned) mid span averaging technique to determined that all important mean aerodynamic chord. I would be glad to explain if you are interested the simple methiod if you are interested but since I am a poor typist will hold off on that for now unless you want it.


John

riadh

Thank you John for your comments .I would certainly would like to read your comments on finding out the correct C.G that is when in doubt of figures given in some manuals .Also I would appreciate it if I can be shown a way to establish true fin alignment with the center of the fire wall.Yes I can estimate direction by looking at it from back to front of fuse but would that be enough?

JohnBuckner

My Feedback: (1)

Thanks riadh and my apologies for not getting back sooner.

When I approach an airplane and plan for the CG with no other information avalible especially if what is available is likely in question. The first thing I do is decide where I feel that it would be a conservative (safe) point to start with. I know this sounds a daunting task but not really since the majority of our airplanes is quite easy to develop a safe place to start this is especially so if our airplane is fairly conventional, flying wings, deltas, carnard would be a few of the exceptions.

Now since you are concerned with warbirds then that is an easy one since the vast majority will be best somewhere around 28% mac so for that reason I will normally suggest to start out with a CG of 25%mac. In fact that magic 25% number actually will work well with a lot more than just warbird and I would same agines its a safe starting point for most of our non scale sport airplanes.

Before we go any further let me describe that 'mac' thing. This is mearly a method of expressing the where the fore and aft CG is located in relation to the wing and just positions this point in a measurement from the leading edge of the wing which is normally a positions aft of that leading edge. This measurement is expressed in a percentage aft of the leading edge and that percentage is a percentage of the Chord of the wing. Of course the chord is nothing more than the width of the wing but where things go astray is the not all wings are straight and uniform. Some are tapered in the front or the rear or maybe double tapered front and rear, some may be elliptical and some may be a combination of all the above.

This is why we always use an average or mean chord when doing CG problems such as multiplying wingspan times mean chord to figure out wing areas or just finding the mean chord to decide where to locate the CG.

How do we find this "mean aerodynamic chord", for our purposes we can us 'mid span averaging'. To find the mac we need only to measure the wings actual chord (leading to trailing edge) half way between the wing tip and the fuselage. Its that simple, Lets use your P-47 for example you need to only measure the chord out there half way between the fuselage side and the wingtip. If you want to use 25% mac you simply divide that measurement by four and measure back that sum from the leading edge (out there at the half way point) and this becomes your target CG. You only need to project this point straight inward to the fuselage sides for easy checking.

This is not a precise scientific method but then agine most of what we do is not. Anyway I hope I did not offend by oversimplifiying it is just in the hope that can help others. This simple method will serve well. Some other CG locations that I have had much success with are controlline airplanes fly well around 20%mac Flying wings with no wing sweep and reflexed airfoil or ailerons works well for me at 10% mac. Fling wings with considerable wing sweep seems nice about 15%. For canards or true deltas I have no clue.

The last part of your question I will take a shot at in the morning as I am pooped, Had a fun club meeting tonight.

John