Zpat

My Feedback: (2)

The airplane pictured weighs 32oz, with a wingspan of 34" and has a wing loading 11.4 oz/ sq ft.
Fuse lenght is 35" w/MVVS .15.

I have a .40 powered model similar to this and it has a wing loading of 13 oz/sq ft and flys very well. It flys very light.

The .15 powered version has a wing loading that is less than the bigger one but it doesn't fly nearly as well. Every time I slow it down, it just falls out, where the bigger one has much less airspeed and floats.
I don't understand how the bigger one flys so much better with a higher wing loading.

Any ideas on how to get the small one to fly just as well as the larger one?

Thanks,
Pat

Mr Cox

You need to put the smaller one on a diet!

The wingloading, mass over wingarea, is missleading when comparing planes of different sizes, instead you should look at the "Cubic wingloading";

Weight (oz) / wing area (inch square)^3/2 x 1728 = ?

This should usually be a number between 6-10. Lower than that and you have a real free-flight floater, higher and you have a speed ship (around 12 is still fine).

Zpat

My Feedback: (2)

The cubic wing loading is 6.83.
Values between 8 and 10 are considered good for aerobatic aircraft.

Mr Cox

It is still simply the wing area...

Zpat

My Feedback: (2)

You are saying to increase the wing span?
And if so, how much do I increase it? What kind of wing loading am I shooting for?

build light

Pat, the numbers don't lie but even so it sounds like you are suffering from an airframe that is heavier than you want.
sounds to me like it needs to go on a diet. Hard to do once complete.

First I would double check the numbers to see if a mistake has been made.

The performance you describe tells us that you have a high stall speed.
Aside from losing some weight from the airframe you might try adding flap funtion to the plane to enable the ship a slower stall speed. If you choose to add flaps watch for low speed snap rolls on approach!

Keep in mind that even if the wing loading numbers are correct, that performance as you go smaller is not linear.

Nice looking plane!

Robert

Mr Cox

What are the cubic wing loadings for the two different planes?

The wing profile also has a large effect on stall speed. To me 32 oz sounds like too much for a 34" wingspan to expect a floater, sounds more like a combat plane to me...

hkbii

Howdy gents. zpat I would check the incedents . check that the h stab and the wing are at or near 0deg. then look at the balance. a noes heavy plane will not float and a tail heavy one is hard to keep leavel. Take the plane up about 150 -200 ft put her into a strait down dive , if she pulls to the canopy then she's noes heavy, if she pulls to the wheels then shes tail heavy. If the air frame is strait and true then look to the balance. hkbii.

Mr Cox

I don't fly any 3D myself, but too me it looks like a very strange wing and wing profile.

From the cubic wingloading number it has a cord of 12 inches? That's a lot and the wing also has a very odd shaped profile, looks like the aileron surfaces are much to large compared to the rest of the wing. Have these been enlarged, or is the whole design homemade or something?

If all this is normal for 3D planes I apologise (and please do correct me if I'm wrong) but to me it just doesn't look right and I wouldn't expect very good qualities when trying to fly off the wing alone...

Zpat

My Feedback: (2)

This is the wing shape.
The root chord is 14" and the tip chord is 9.75".
Pitch is very sensitive. Balance is not an issue.

Mr Cox

To me that's a very odd wingshape...
Have you checked/altered the aileron neutral position?

While set symmetric, a little up or down in both ailerons might alter the lift and stall speed.

combatpigg

My Feedback: (3)

This plane is very similar to one I saw flying with a .074. The video was shown here a few years ago, ending with the pilot hovering it into the back of his pickup truck.

2 pounds is not out of line for a model this size, but it should really weigh no more than 27-28 ozs for dependable 3D performance.

The CG IS AN ISSUE if the plane is being flown too nose heavy. This effectively makes the plane fly heavier. A plane like this should be flown at 30-35% CG. The lighter the plane, the less the CG matters.

Check the wing for warps and for even amounts of control deflection, bad programming and bad servos can imitate an aerodynamic problem.
Make sure the control linkages are not wimping out in either push or pull mode.
This plane looks like the fuselage could twist as you manuever it, it might need flying wires made out of thread.
Make sure the engine has a touch of right thrust....NO LEFT THRUST!
Planes like this fly best with flaps coupled to the elevator, unless you are hovering.

DeviousDave

The airfoil might be on the thick side, small airplanes have to scale their airfoil down a bit more than the ratio you shrunk from your larger size. The thrust just isn't there at low RPM to keep the plane moving forward and a thick airfoil like those on a funfly is like putting on the brakes as soon as you cut power.

BMatthews

Yep, by my figuring it is just over 33 oz. that's about 6 to 8 oz heavier than it should be at this size for 3D like performance with a .15.

The issue is that the wing loading scales with the size. 13 oz/sq foot on a .40 powered model just is no good at all on a .15 model to achieve the same performace. At this size you need to be more in the 8 to 9 oz/sq foot at most to get equal appearing performace.

It's been a while since I saw the usual equations for the cubic wing loading but I think it'll support this. More importantly the scaling factors I saw recently made more sense since they weren't skewed by using a really odd shaped wing such as on this 3D model.

Either way you cut it 33 oz is just a smidge too heavy for a .15 powered 3D model. Adding wing area will help with stuff done from level flight. But if it won't accelerate from a standing start from a hover to decent speed vertical flight then it's too heavy. Adding wing area won't help that at all. A decent 3D model needs to have enough thrust that it's pulling with about 1.5 times the weight of the model or more. More is better in this case.

combatpigg

My Feedback: (3)

That model looks like it should fly just fine, but the Devil is in the details. The servos really take a beating on a .15 sized 3D model. HS .81s are about the lightest I've come across that will hold up to handling the big control surfaces. 4 of these servos adds up to 3 ozs pretty quick. The lightest I've ever been able to make a .15 size 3Der weigh is about 22 ozs, but it was pretty flimsy and always out of trim. Tougher to engineer than a good .40 sized 3Der.

Even the so called .15 sized Penknife from Morris Hobbies needs a piped MVVS .21 to really shine

psb667

Just out of curiosity where is the cg on this thing?

Zpat

My Feedback: (2)

The plans call for the CG to be 3 1/2 " aft of the leading edge. My CG is 4 1/2" aft of the leading edge.
Pitch is very sensitive. I want it to tumble at low speeds but it just falls out due to weight.
I want to use this engine for a 3D plane and this platform seems to be a good beginning point.
I was thinking about shortening the nose moment, lengthening the tail moment, increasing the wingspan and keeping the weight down as much as possible.

The wing dimensions are, wing span 34", root chord 9.5", tip chord 6.5", and this includes ailerons. The fuse is 36" long.
Engine weight is 6.5 oz., with a 3213 carb and a 3241 muffler. I have no weight for the muffler.

The claimed wing area on the plans is 409 sq/in., I calculate 272 sq/in with a wing loading of 17 oz/sq ft.
Are my calculations correct?

Am I on the right track or am I just trying to reinvent the wheel?

eroc144

My Feedback: (1)

There's your main issue - your plane is substantially tail heavy. Move the CG closer to 3.5" and it will fly much better. Also, the "new" dimensions you've provided yield much different numbers: Wingspan: 34", Effective Chord: 8", Area: 272 Sq In, Wt: 32 oz

Wing Loading: 16.9 oz/Sq Ft
Cubic Load: 12.3

Those are warbird values at this scale! So your plane is tail heavy and your wing and cubic loadings are high. Like everyone has said, your plane needs to go on a diet!

EG

Zpat

My Feedback: (2)

I'm sorry for the wrong information.

The wing has a length of 34", root chord 13.5 ", tip chord 9.75" for a total of 395 sq/in. and a wing loading of 12 oz sq/in.

If I stretch the wing to 37". root chord 15" and tip chord 10.75 " I'll have a loading of 10 oz sq/ft. Fuse lenght?

combatpigg

My Feedback: (3)

It looks like 400 squares inches of wing from here. An average chord of about 12 inches means a 4 inch CG wouldn't be unheard of, it'll make the plane fly with the tail low, always looking for a place to hover. The problem is trying to hover 32 ozs. I've played with the 8x3 MAS, but know nothing about your engine. The CVA .15 turns a 8x3 about 19,000 IIRC.
Your overhead view shows some pretty chunky looking pushrods running the length of the sides? If so, I'd convert to pull/pull linkages and ditch the landing gear. Run no larger than a 350 nimh, or I guess Lipos are even lighter? A 2 oz tank VS having a 3 or 4...I think the Hayes tanks are the easiest and lightest, but you could rig a balloon tank in a ultra light plastic container, like blister packaging.
For .15 and under 3D I use cellophane floral wrap put on with 3M77 spray. It's real light and cheap. The framework of your plane looks like typical good ol' 3D frame work...can't see anything obvious that is wasting weight there. That's why it looks to me that this combo should rock!

iflyg450

Great engine for a Sure Shark XV

psb667

This looks to be the same size as a mild aerobatic I have but i cant be sure and I also cant do trig. ya its almost exactly the same.

psb667

my total wing is 294.5 span is 32.7 lnth 31.5 Im getting 2.5 to 3.25 inches for cg at a total weight of 16.9.oz Im not possitive but I would start with 3 from leading edge as my baseline and work out to tune performance. Ill see if I can get this model rebuilt soon so as to get more feedback hope this helps(dont fly in the dusk in treeline)

BMatthews

You can try adding a rib bay to each wing but really the important issue is that the model is just plain too heavy. I'd cut your loses and move on to building a new model with more careful attention being paid to reducing the overall weight down to around 24 or less oz.

Zpat

My Feedback: (2)

Your right.

This is an electric airplane. Glow engines are just too heavy for it.