@Glidiator sir,

Stab tilt effect cannot be achieved with trim tabs.
Tabs become more effective at higher speeds.
Stab tilt becomes more effective at lower speed (ie, higher angle of attack)

Regards

Quote from: K K Iyer on October 29, 2020, 03:02:29 PM
A CLARIFICATION

STAB TILT

I had not mentioned this so far, as in the initial stages I feel it is more important that the tail parts are perpendicular to each other, than to incorporate more advanced features.

However, since it has been mentioned, let me explain.

Stab tilt means the stab is installed on the fuselage with one tip higher than the other (say 1/8" for this size). It makes the model turn towards the higher tip during glide.

Normally we would use rudder to turn (in a model without ailerons)
This works ok on a chuck or hand launch glider.
In a catapult glider, the launch speed is several times the glide speed.
So the rudder offset required for turn in the glide has excessive effect at launch speed.

What we need is a method to have a turn in the glide without excessive turn at launch.
This achieved with stab tilt.
In the glide, the speed is low, so to remain aloft, the model flies at a high angle of attack.
Actually the model is in a more or less horizonal attitude. But because it is descending, the airflow is not head on, but slightly from below. When this upward airflow hits a tilted stab, it produces a small sideways component. A stab with left tip higher will move to the right, causing the model to turn left.
This is used for turn during glide.

In the launch, the speed is high, so the model needs only a small angle of attack.
The airflow is therefore nearly head on, and the sideways effect of the tilted stab is negligible.

In a nutshell, stab tilt is a way to have a turn in the glide without excessive turn at launch.

Sir I can confirm that lesser dihedral will produce more lift, but it will obviously reduce the self correction capacity of the plane.
Also sir what is the need to have a turn during climb? Can't we have straight climb then turning glide?

@Dreamliner
@Karthick Ashwath
@Freeflight

REDUCTION OF DIHEDRAL

Assume total dihedral reduced from 3" to 2", or from 1.5" to 1" per 4" semi span.

The effects are:
1. Horizontally projected wing area (representing Lift) is increased by 4.6%
2. Vertically projected side area of wing (representing roll stability) is reduced by 33%

In other words, the gain in lift is negligible, but the loss of roll stability is drastic.

@KK
Thanks for the clarification.
Will have to ensure sufficient stab tilt to created turn. Will be evident in horizontal catapult launch.
Anant

Hi everyone,

You may have noticed that Freeflight has been emphasising two things:
1. Stab tilt
2. A right climb, left glide flight pattern

I've already explained how stab tilt works.

Now here's the theory behind getting that right climb, left glide pattern.

1. Put a small wedge/ trim tab under the trailing edge of the left wing, near the tip.

2. This (wash in) increases the left wing tip's angle of attack compared to the right wing.

3. We don't know by how much, but with a small wedge/tab, let us assume it's 1 or 2 degrees.

4. Now let's see the effect at different speeds. The gliding speed of my Lunchbox is about 2-3 m/sec.
The launch speed may be 10 times higher. (Proof in an old thread of mine called 'Estimating Launch Speed' or something similar)

5. At such high speeds, the necessary lift can be generated at very low angles of attack, say 1 degree
(I can give proof, but it's beyond the scope of this thread)

6. In the glide you want maximum duration. Or minimum sink / highest coefficient of lift.
This occurs at an angle of attack close to the stalling angle. Typically at say around 15 degrees.

7. In other words, the model flies at low angle of attack at launch, and at high angle of attack in the glide.

8. This is not apparent due to the fuselage level (or slight nose down) attitude of the model in glide.
The high angle of attack is not due to flying nose high.
It's due to the descending path of the model that the airflow is hitting the wing at an angle from below, not head on.

9. We now have a model with a slight wash in on the left wing.
The left wingtip is therefore at a slightly higher angle of attack than the right wing.

10. The effect of this is different at different speeds. SEE ATTACHED PIC.

11. At high speed (ie, low angle of attack), a small difference in angle of attack leads to little difference in the drag of the left and right wing.
But the lift of the left wing is significantly higher than the right wing.
SO THE MODEL ROLLS RIGHT AT LAUNCH

12. At low speed (ie, high angle of attack), there's hardly any difference in lift, but the drag of the left wing is significantly higher than the right wing.
SO THE MODEL TURNS LEFT IN THE GLIDE

13. Once the model settles into a left circle in glide, there's a second order effect.
The right wing is describing a slightly larger circle.
So it's flying a bit faster than the left wing.
Accordingly generating a bit more lift than the left wing, that could lead to a left spiral.
But this is counteracted by the fact that the left wingtip is flying at a slightly higher angle of attack due to its wash in.

Perhaps now it may be apparent why I'm not in favour of budding aeromodellers jumping head first into RC Models. There's enough learning material in simple 8" span, 8gm chuck gliders!

Please let me know if such detailed theoretical explanations are of interest or not.
Then I'll know whether to continue or not.

Regards to everyone.

Iyer

Very interesting and clear cut explanation! Please enlighten us , the underprivileged (in Aero modelling )

Mr. Iyer,
Thanks for the good explanation. I have learnt from experience, but nice to understand theory behind it.

Please do as before, such as this and dihedral effects on lift and stability etc.

Dear Karthick,
"Also sir what is the need to have a turn during climb? Can't we have straight climb then turning glide?"

Very good questions, others may benefit. In the CLG we strive not to climb in circles. My concept is that I want to get as high as possible to make the glide phase as long as possible, of course, assuming that the transition went well, i.e.. you did not loose height. When climbing in circles, my experience shows that happens with too much incidence, you are loosing all the rubber power climbing in circles. So a maximum height is not attained.

See my sketches carefully in reply# 110 . you are climbing almost vertical. At transition point, the glider nose is slightly (emphasis on slightly) pass the 90 degree angle to the ground. If it goes too back, it is approaching a loop.

You are climbing almost vertically. For a fraction of a second, less than the time you blink your eye, it flops to transition for the glide phase. Now at low speed in glide, the rudder tab is ineffective and the stab tilt is very effective to make the circles.

Why glide in circles? Well, for one reason, you want to keep the glider in your field. But not really, I have almost a 1 mile in field. We want the glider to catch a thermal. Thermals moves in circles, hopefully the glider enters the thermal circle versus going straight and out of the the lift (thermal) that nature provides. All the expert CLG winners know how to "pick the air" i.e. launch in a thermal.

So we choose to put a tiny amount of clay on either wing tips. If the thermals are not small, they are in a bigger circle, so you want a larger glide circle. Put the clay on the right wing. If you get a smaller diameter but strong thermal, you want to glide in smaller circles. So then, you put the clay on the left wing. At every contest day, weather is different. Have to adjust both the CG ( slightly forward on windy days) and glide circle with clay on wing tip as the stab tilt is fixed.

Understanding the trimming is very vital. Every contest day brings a different weather. Need to learn how to take advantage of what nature brings.

Here is an excellent website to bookmark. See how this expert explains: Of course he is talking about hand launch, but CLG is similar process. Study his sketches as to what is desirable, what is not

http://www.f4bscale.co.uk/launch.htm

Excellent elaboration of theory and practice by KK and Free Flight.
Even the most simple paper airplane teaches a lot of aerodynamics.
In fact with respect to very slow forward speeds in indoor FF ( especially rubber power) - very little study has been done of the effects of very low Reynolds numbers
Stab tilt - if it is seen to be insufficient in test flights - not sufficient left turn effect - Twisting tail boom is not permanent - and fuse is pretty stiff - chances of cracking it if one is not careful. So how does one increase it permanently so trim is set once and for all.

Here is one very good video of CLG trimming. He starts with trimming by moving CG back
(remove clay from nose) then goes back to original CG with a little up elevator. He bends, but we go permeant with tabs.

https://www.youtube.com/watch?v=lDsu29TQUDk&feature=emb_rel_end

Dear Glidiator:
"Stab tilt - if it is seen to be insufficient in test flights - not sufficient left turn effect - Twisting tail boom is not permanent - and fuse is pretty stiff - chances of cracking it if one is not careful. So how does one increase it permanently so trim is set once and for all."

great question. I have struggled with this question as there is no way on the field you remove the tail and re fix it. You are not close to being accurate and may affect the incidence (established correct before) too.

So here is my approach. A little less stab tilt is better than a little more. I only build CLG's that have been designed by great experts like in this case. They have gone through a lot of  experiments, before publishing a winning plan. So, I start what the designers recommends, get it as exact as you can.

Now on the field, I use the clay on the wing tip. Left side for smaller circle, right side for make the larger circle.

Last year I competed in AMA nationals, spent the whole day with experts. They use the same clay technique on the wing.

As I said before, every flying day brings a different weather. So cannot be adjusting the stab tilt each time, but start with the recommended one and use wing tip clay.

Let's get back to basics for a moment, and go step by step.

Step 1. Build everything straight. No rudder offset, no stab tilt, no wash in, no wedge, no trim tab.

Step 2. The designer has specified the CG position after trials. Let us balance our model at the specified CG first.

Step 3. Try to get a straight glide slope from say 5 feet high to a point about 30 ft away on the ground.
This approximates the glide slope of the model

Step 4. Launch your test glide a bit nose down, pointing at a point on the ground about 30 ft away.

Step 5. How fast to throw? A bit slower than you'd throw a marble at the same target.

Got a straight flight of 3-4 seconds, neither climbing nor diving, neither turning left nor right?

If it turns a bit left or right, let it be for the present.

If it dives, add an up elevator trim tab. This is a half cm piece of the supplied triangular wedge, or a a similar piece of matchstick.

If it climbs, add a tiny bit of plasticine to the nose and try again.

THE OBJECTIVE IS TO GET A STRAIGHT GLIDE FROM A 5-6 FOOT LAUNCH TO A POINT ABOUT 30 FEET AWAY ON THE GROUND.

Step 6. Now we want it to turn left on glide.
So we twist in a little left rudder.
And it glides nicely in a slow left turn in the glide.
But when catapulted at high speed, it makes it spiral dive to the left...

Perhaps this left rudder is not such a good idea...

Maybe worth looking at reply #204

Quote from: K K Iyer on November 20, 2020, 10:36:29 AM
@Dreamliner
@Karthick Ashwath
@Freeflight

REDUCTION OF DIHEDRAL

Assume total dihedral reduced from 3" to 2", or from 1.5" to 1" per 4" semi span.

The effects are:
1. Horizontally projected wing area (representing Lift) is increased by 4.6%
2. Vertically projected side area of wing (representing roll stability) is reduced by 33%

In other words, the gain in lift is negligible, but the loss of roll stability is drastic.

@Dreamliner
@Karthick Ashwath
@Freeflight

Any response?
Anyone tried to check/ replicate the maths?

Sir,

When one wing flat on the ground, then tip of the other wing is 2" from the floor.

@dreamliner

Now you know what to expect!

Quote from: K K Iyer on November 20, 2020, 10:36:29 AM

REDUCTION OF DIHEDRAL

Assume total dihedral reduced from 3" to 2", or from 1.5" to 1" per 4" semi span.

The effects are:
1. Horizontally projected wing area (representing Lift) is increased by 4.6%
2. Vertically projected side area of wing (representing roll stability) is reduced by 33%

In other words, the gain in lift is negligible, but the loss of roll stability is drastic.

Mr. Iyer,

Agreed with your first statement. not sure how to calculate the vertically projected side area in statement #2 that reduces stability by 33%.

Dreamliner : When one wing flat on the ground, then tip of the other wing is 2" from the floor.

Then part 2 , make sure when you glue it to the fuse, both wings are of equal height from the glider fuse (floor) and that the center line of the wing is exactly on the center line of the fuselage.

Quote from: Free Flight on November 21, 2020, 12:54:38 AM
Agreed with your first statement. not sure how to calculate the vertically projected side area in statement #2 that reduces stability by 33%.

Simple.
In side view chord remains unchanged.
Height reduced from 1.5" to 1", ie area reduced by 33%
(Strictly speaking, it applies rigorously to a rectangular wing...)

Did you verify the math for the lift?

Just looking at flat span , it is 8.5 inches. With dihedral , one wing on floor and other at 3 inches, or 1.5 inch equally, the projected span is exactly 8 inches. So reduced effective wing area due to dihedral.
Basically : Area= Projected span * MAC. 

@Freeflight ,Is this okay ?And  CG is perfect . So what else should be taken care of ? Should I do a trial CLG launch by less pull before the full stretch launch ?

I sought an opinion about this thread from some experienced aeromodellers.

One feedback was:

"There is an overdose of information now on this.
Forget a newbie; an experienced modeller will get scared off"

I agree. Yes, there is something called overdose!

So I've decided to back off. And not post anything more on this thread till after the Contest.

It's high time we started getting some competitive flight videos on the Contest thread...

So see you guys there. Remember the Contest closes with videos posted on RCINDIA till 9pm on 30th.
One Sunday and one weekend left.

Gentlemen, start your engines!
May the best man win...

Was wondering why even Hitesh didnt post a  CLG video yet .

Quote from: K K Iyer on November 21, 2020, 09:10:16 PM
I sought an opinion about this thread from some experienced aeromodellers.

One feedback was:

"There is an overdose of information now on this.
Forget a newbie; an experienced modeller will get scared off"

I agree. Yes, there is something called overdose!

So I've decided to back off. And not post anything more on this thread till after the Contest.

It's high time we started getting some competitive flight videos on the Contest thread...

So see you guys there. Remember the Contest closes with videos posted on RCINDIA till 9pm on 30th.
One Sunday and one weekend left.

Gentlemen, start your engines!
May the best man win...

Can the deadline be extended by one or two week ??
I think more time is required to process the information and put it into action.

Thanks

Hope this works for me .



Perfect gliding is not an easy task ! unlike RC (or even rubber powered !)

@weekendflyer /Glidiator /Dreamliner , could you post your test flights too ?