wingspan to fuse length ratio

[gigelus2k3]

how does it affect the handling capabilities?

I'm building a model based on a plane that had 11.7m/7.3m wingspan to fuse length (including the long nose). In the 3-view (http://www.aviastar.org/air/romania/iar-16.php) it didn't look too bad, but now that both the wings and the fuse are done, it just seems wrong.

I could cheat a little and lengthen the fuse beyond the scale looks, but... what should I expect from a low-wing plane with very wide wingspan and stubby fuse, handling wise? In case it matters, I'm doing a 1/12 scale model (i.e. almost 40" wingspan).

Thanks,

Serban

[Tom Harper]

Serban,

The moment arm looks OK, but the stab area is small for a model.

It's probably OK for scale performance. I doubt that the full scale airplane was aerobatic.

If you are concerned about simply flying, make a glider with a stick fuselage and flat plate sheet balsa surfaces. Raise the TE of the stab a bit. Use clay for balance. If you can get a stable glide you are safe.

[Brandano]

Hem it was a fighter, I expect it WOULD be fully acrobatic. Probably it was a twitchy plane, with the CG well far back.

[eflightray]

Nice looking plane, but I hope that short nose isn't going to give you a problem getting the model to balance, (CG position).
I had a lot of problems with a very short nosed long fuselage biplane trying to get all the weight up front and keep the tail as light as possible.

[richard hanson]

Try this ----------
draw a line from rear of rudder -- to each wing tip
IF---the angle is acute -under 90 degrees -you have a good aerobatic setup -
IF the angle is open (over 90 degrees ) you may not have the best setup
when the model speed slows to minimums - the relatively longer fuselage has more control over wing alignment
No formulas required - It is a generalization but the concept is correct.
Your model is really not bad -and with proper wing loading -could be pretty darn good aerobatic setup.
The short nose setup was due of course to engine weight of the full scale craft- but this is of little worry .--except for balance.

[gigelus2k3]

Richard,

It looks like the angle in question is about 95*... What bad effect would this have?

Regarding the proper balance, I'll find out in a day or two when I'll glue in the tail. I think I'm OK, there is a lot of space to push the battery forward.

Thanks,

Serban

[richard hanson]

Really not all that bad -especially for a scale subject - a slight increase in all tail surfaces may /may not be desireable for you - the lighter you can make the model - the better scale sizes will perform
Why?
the model always flies at lower angles of attack There is NO cure for an overweight model
no airfoils no trick thrust - nothing - weight means more than anything else.

[vintage1]

The short answer is that there is no answer. Planes can fly from zero tail length up to 1:1 or more. The shorter it is the more forward the CG needs to be, and in general the more responsive it will be to elevator input. What counts a bit more is the ratio of fin to tailplane area. Too little and the plane will wag its tail when balanced properly in the pitch plane.


Just go ahead and build it as is: You MUST do a CG calc. though. Tentatively, I'd say that no further aft than 20% of root chord is the correct place.

Given the likely elevator sensitivity, I'd go even further forward for early flights.

[BMatthews]

The wing to tail length to stabilizer ratios are well within a normal range and the model will fly just fine even though it IS a little on the small tail size side of things. If it looks short then that is only because of the nose length. In reality it's no worse than a P40 or Spitfire for the wing, tail length, stabilizer size issue.

You could go and plug in the numbers at http://www.geistware.com/rcmodeling/cg_super_calc.htm and find out where the balance point needs to be for the suggested stability amount.

The real battle here will be keeping the tail and rear of the wing light enough to avoid a huge amount of nose weight. Nothing you do to keep the rear of this model light will be in vain. Extreme measures are going to be just right or the nose will soon be sporting a rather sizable hunk of lead.

What a great looking airplane! All the best with the project.

[MCarlton]

I've often wondered this, so I'll ask it here, as it seems appropriate for the discussion.

If you look at a wing, like the one under discussion, which has a straight LE and tapered TE, then it seems to me that the wing is, in fact, slightly forward swept, thinking about where the max camber line would be?

If that is the case (I really don't know) then the tail moment doesn't look as bad as it might, because the eye sees the moment arm from the TE of the wing/fuselage fillet, NOT the moment arm between the MAC of the wing and that of the stabiliser.

I'm probably totally wrong, so I shall crawl back under my stone :)

Matt

[richard hanson]

I've often wondered this, so I'll ask it here, as it seems appropriate for the discussion.

If you look at a wing, like the one under discussion, which has a straight LE and tapered TE, then it seems to me that the wing is, in fact, slightly forward swept, thinking about where the max camber line would be?

If that is the case (I really don't know) then the tail moment doesn't look as bad as it might, because the eye sees the moment arm from the TE of the wing/fuselage fillet, NOT the moment arm between the MAC of the wing and that of the stabiliser.

I'm probably totally wrong, so I shall crawl back under my stone :)

Matt
Theory-- lots of opinions - -I look at the effective center of gravity for a particular wing layout - then the length from that to the dingus which is used to stabilize the craft .
In this case the length is about "normal" but the tailgroup is on the smaller side
designers all have some criteria -good or bad - they like to follow and in THIS case - as a model - a 10% increse in the horizontal stabilizer would be a GOOD thing for most model flying. Yer eye is just fine

[BMatthews]

I've often wondered this, so I'll ask it here, as it seems appropriate for the discussion.

If you look at a wing, like the one under discussion, which has a straight LE and tapered TE, then it seems to me that the wing is, in fact, slightly forward swept, thinking about where the max camber line would be?

If that is the case (I really don't know) then the tail moment doesn't look as bad as it might, because the eye sees the moment arm from the TE of the wing/fuselage fillet, NOT the moment arm between the MAC of the wing and that of the stabiliser.

I'm probably totally wrong, so I shall crawl back under my stone :)

Matt

No need to crawl away at all. You're 100% right on all counts. The wing is techically swept forward slightly since the standard for wing angle sweep is to look at the 25% chord line. Also the MAC (mean aerodynamic chord) will be well forward and fairly narrow to boot given the strong taper of the wing. which means the tail moment isn't at all short by any normal standards.

By the way, the strong amount of taper in the wing suggests that any model of this airplane should have an honest 2.5 to 3 degrees of washout in the wing to help avoid any tip stalling tendencies when flown fairly slowly. Even with that you would not want to use a lot of aileron all of a sudden when flying slowly and close to the ground.

[gigelus2k3]

Even with that you would not want to use a lot of aileron all of a sudden when flying slowly and close to the ground. I wonder why :rolleyes:

Seriously speaking, I already increased by 10% the vertical fin and intend to do the same with the horizontal stab. Quick question: do I increase just the width or should I scale up both dimensions?

I'll definitely add a little washout, it makes the life so much easier; anyway, I don't intend to do prolonged inverted flights.

The CoG calculator indicates that the optimal position is at 12-15% behind TE. I may get lucky and be able to balance there the plane without adding lead.

Cheers,

Serban

[gigelus2k3]

Hello, it's me again.

Well, I tried to maiden it this week (twice). Both times, it veered hard to the left right after take-off. Cartwheeled both times, without any significant damage (the first time a wing tip snapped clean, it was easy to put it back).

I am at a loss understanding what could be wrong. The tail is true to the wings and all the deflection surfaces are in neutral position. There was no wind whatsoever. The CG was set to 22-25% of the wing chord.

After the second crash, I checked the WOT behavior while holding the plane and I found no throttle-related issues. The motor has a few degrees of down and right tilt. The plane is perfectly balanced laterally.

What could be the problem? I have never had before a properly-aligned plane doing this.

Help!

Serban

[Brandano]

Cartwheel or ground loop? If the main gear is far forward when compared to the CG the plane might be prone to ground looping. Also, since this is a tail dragger the initial movement of the nose as the tail lifts off the runway will be pitching down, which results in the plane veering to the left due to the prop's gyroscopic precession, especially if you have a scale prop, as it looks very large in those pics! Have you tried any test glides, and if so how did it behave in those?

[gigelus2k3]

I'm taking off from a rough baseball diamond. The model picks up speed while going more or less in a straight line and, when I'm about to hit the grass, I mildly pull on the elevator. The plane barely lifts off the ground when it starts turning to the left, more than what I can correct with the ailerons.

It's true, my ailerons are rather small, but I was thinking that the only downside is less roll authority.

I don't think it's stalling. The wing loading is pretty low (~10.5oz/sqft) and the model had quite some speed when gently taking off.

Serban

[Megowcoupe]

Serban

Do you have any washout in the wings? Are you using the rudder at all? The ailerons may be ineffective at the high angle of attack and slow airspeed on takeoff whereas the rudder will be far more effective.

How big is your prop? A larger prop reduces the yaw stability in a tractor design- you may need a larger vertical surface to compensate. The full scale airplane looked pretty sketchy in terms of vertical surface.

Sam

[Texas Buzzard]

Tweeking a design ?????

--------------------------------------------------------------------------------

For what it's worth, remember that this didn't cost you anything.

This comes from years of picking the brains of several excellent scratch builders when we were grounded by the wind. In short it is ........

Draw a line on a side view of the fuse from the hinge line of the elevator to the rear of the prop. Decide the best position for the CG and mark it.(Temporary)

Then measure dist. from prop to the CG and from the CG to the elevator hing line. Divide the "tail moment" by the "nose moment". If the result is between 2.1 and 2.6 you are very close to having a stable plane in the pitch mode. A value of closer to 3.0 will work but then you decrease the area of the vert. and hor. tail a bit.

Once you find the position of the CG then place the wing so that about 25% of the chord is forward of the CG. This will be a starting place. Some want a longer "tail moment" for stability at slower speeds.

The area of the entire elevator/stabilizer will be close to 20% of the wing area. The fin/rudder will be about 6 to 7% of the wing area. This will get you in the air. Remember, this didn't cost you a thing. :rolleyes: