Having trouble with scratchbuilds, most won't even glide properly

[quorneng]

Well done! That is a proper glide.
Where is the balance point as a percentage of the wing chord. Around 25%?
In the last video it is just beginning to display a slight tendency to slow down and pitch nose down. Ideally the glide should be if anything a bit the other way. It should descend at a constant rate and just before it lands start to level out.
In any case when under power it will pay to have it slightly nose heavy compared to gliding.
So long as you keep the same balance when all the bits are back you know it can fly but it will require a slightly faster launch.
Adding power does impart some more forces on the plane than when in a glide so there can be benefit in not using full power even if it results in just an extended glide.
Just remember to close the throttle just before it touches down.

 

[Jessek1486]

Well done! That is a proper glide.
Where is the balance point as a percentage of the wing chord. Around 25%?
In the last video it is just beginning to display a slight tendency to slow down and pitch nose down. Ideally the glide should be if anything a bit the other way. It should descend at a constant rate and just before it lands start to level out.
In any case when under power it will pay to have it slightly nose heavy compared to gliding.
So long as you keep the same balance when all the bits are back you know it can fly but it will require a slightly faster launch.
Adding power does impart some more forces on the plane than when in a glide so there can be benefit in not using full power even if it results in just an extended glide.
Just remember to close the throttle just before it touches down.

So the balance point was dead center between 25% and 33%. I have a little device, the block of wood with two vertical 1/4 inch dowels to measure it. I found using my fingers to be ambiguous, since they do not come to a defined reproducible point, plus you can't really see unless you lift it over your head.

Unless I'm wrong, I dunno how others do it, unless it's by "feel" of the plane.

Why when under power is it better to be nose heavy? Not arguing, just wanna know the rationale.

I imagine since I'll be adding 6-7 oz of components, it won't glide as well, so it'll more or less require power.

Oh I did notice the forces imparted, such as torque roll. That plus the stall would result in a spin.

 

[Tench745]

A lil update: I took her to the park, and she glided beautifully.

That's what we love to see here!

 

[quorneng]

Weight does not alter the glide angle only the speed it goes down it. The glide will travel the same distance particularly if you leave the prop off.
The reason for having slightly nose heavy when under power is any excess thrust will cause the plane to speed up beyond its normal flying speed. The natural stability aerodynamics will make the plane climb so it returns to its normal flying speed.
The problem is depending on how much excess thrust there is this may cause the automatic stability to over react causing the plane to slow down so much it stalls, the nose and wing will drop and still under power it will then descent rapidly towards the ground. The pilot panics and puts in a major up elevator input which will cause a rapid climb. Its all to easy to effectively loose control before sooner or later the ground meets the plane. Slightly nose heavy will make the power on pitch up a little less dramatic giving the pilot time to sort things out calmly. If the plane appears to be climbing too rapidly close the throttle a bit until the climb is gently and constant. Easy to say harder to do neatly.
It is possible to limit the power on pitch up by adding a few degrees "down thrust" to the motor so a portion of the thrust tends to pull the nose down is the plane speed up.
With a plan design it will usually give the recommended down and side thrust angle (or washers under the motor mounts). These figures will have been obtained by trial and error flying by the designer. With own design scratch building you have to do this for yourself!

 

[Merv]

...Why when under power is it better to be nose heavy?...

Stability

There is a old saying: a nose heavy plane flys poorly, a tail heavy plane flys once.

When I have a new plane, I always set the CG at 25% of the MAC, a flight stable place to start. When I get the plane trimed out, I will slooowly move the CG aft until I get the amount of instability I'm looking for. I enjoy flying flips & flops, with just a bit of instability they can be done at will. But if my plane turns into a flying squirrel, I know I've gone too far. I'll move the CG forward a bit. Most of my planes will do what I want with the CG around 30% MAC. But it's always a bit of experimenting between CG & throws.

 

[Jessek1486]

Weight does not alter the glide angle only the speed it goes down it. The glide will travel the same distance particularly if you leave the prop off.
The reason for having slightly nose heavy when under power is any excess thrust will cause the plane to speed up beyond its normal flying speed. The natural stability aerodynamics will make the plane climb so it returns to its normal flying speed.
The problem is depending on how much excess thrust there is this may cause the automatic stability to over react causing the plane to slow down so much it stalls, the nose and wing will drop and still under power it will then descent rapidly towards the ground. The pilot panics and puts in a major up elevator input which will cause a rapid climb. Its all to easy to effectively loose control before sooner or later the ground meets the plane. Slightly nose heavy will make the power on pitch up a little less dramatic giving the pilot time to sort things out calmly. If the plane appears to be climbing too rapidly close the throttle a bit until the climb is gently and constant. Easy to say harder to do neatly.
It is possible to limit the power on pitch up by adding a few degrees "down thrust" to the motor so a portion of the thrust tends to pull the nose down is the plane speed up.
With a plan design it will usually give the recommended down and side thrust angle (or washers under the motor mounts). These figures will have been obtained by trial and error flying by the designer. With own design scratch building you have to do this for yourself!

Ahh, so a trim issue essentially. I get it now. Also helps explain why a plane I bought, a UMX radian kept porposing so badly. It was uncontrollable, it'd just go up, porpoise until it hit the ground violently.

How does one accurately measure 1-5 degrees of angle for a motor? I always wanted to know that. Eyeballing it isn't reproducible, so there must be a way.

 

[Jessek1486]

Stability

There is a old saying: a nose heavy plane flys poorly, a tail heavy plane flys once.

When I have a new plane, I always set the CG at 25% of the MAC, a flight stable place to start. When I get the plane trimed out, I will slooowly move the CG aft until I get the amount of instability I'm looking for. I enjoy flying flips & flops, with just a bit of instability they can be done at will. But if my plane turns into a flying squirrel, I know I've gone too far. I'll move the CG forward a bit. Most of my planes will do what I want with the CG around 30% MAC. But it's always a bit of experimenting between CG & throws.

You really helped explain why more experienced people like an aft CG. It gives more acrobatic ability. Less stability can be desirable for some, is what I'm taking.

 

[Shurik-1960]

In the last 2 videos, your model slides almost perfectly with a slight tilt to the right (trim the rudder if there are no ailerons). With such a slide, do it in the park, smoothly add gas until the model flies. If the model pulls down when gas is supplied, make the engine tilt up, if left or right, we change the engine tilt by adding washers under the engine legs.

 

[Merv]

...How does one accurately measure 1-5 degrees of angle for a motor?...

I use a protractor to set a carptenters angle guage. Then use the it to cut my power pod. 2 degrees down & 2 degrees right works pretty good.

I like using power pods, they tend to absorb the impact with the ground. It's much easier to cut a new power pod than rebuild the fuse.

 

[Tench745]

Ahh, so a trim issue essentially. I get it now. Also helps explain why a plane I bought, a UMX radian kept porposing so badly. It was uncontrollable, it'd just go up, porpoise until it hit the ground violently.

How does one accurately measure 1-5 degrees of angle for a motor? I always wanted to know that. Eyeballing it isn't reproducible, so there must be a way.

I don't usually measure. If I'm drawing up my own plans I use a CAD program and can just tell it what angle I want.
Alternatives:
-Do some math and measure out the two legs of a triangle that gives you that angle. If you measure down 2" and over about 3/16" that should be approximately 5 degrees. (Sine, Cosine, and Tangent are useful things to know when mucking about with angles.)

-Just shim under the motor mount until you get the behavior you want when adding power.

 

[Jessek1486]

So... I actually got it to fly! Controllably, and for more than 2-3 secs. Twice!

I quickly realized I was not at all prepared for it to work, and did not actually know what to do. I then began to panic a bit, stalled it, was high enough to recover and mostly safely land. A little damage, but not bad enough to stop me from trying again!

I truly could not believe that it actually flew.

Another thing I learned, is that fixed wing controls are a bit delayed relative to rotary wing. It is very easy to over control and begin fighting yourself.

 

[Foamforce]

Love it. 😀 I laughed like that when I finally got my first plane flying.

 

[Jessek1486]

Totally involuntary, I really did not expect it to fly. It goes off camera for a sec, but that's when I realized I was flying towards trees/houses and needed to use the flappy thing in the back to avoid that(rudder! That's what it's called). My brain turned to goo.

CG was just past 25%, so I intentionally flew it nose heavy like was suggested here. I gave it a good firm toss slightly down, like instructed by posters here, and the "feel" learned from glide testing. How hard to toss and when to release, all that muscle memory stuff.

As to why it yaws left immediately after takeoff, that is from torque roll. There were two failed launches before this, where I adjusted my rudder trim right.

I'm assuming the plane lacks enough rudder authority at slow speeds to fully counter torque roll, as I did not have an issue with doing a 180 turn under power. Hmm, I just realized that now.

 

[Mr NCT]

So... I actually got it to fly! Controllably, and for more than 2-3 secs. Twice!

I quickly realized I was not at all prepared for it to work, and did not actually know what to do. I then began to panic a bit, stalled it, was high enough to recover and mostly safely land. A little damage, but not bad enough to stop me from trying again!

I truly could not believe that it actually flew.

Another thing I learned, is that fixed wing controls are a bit delayed relative to rotary wing. It is very easy to over control and begin fighting yourself.

Way to go! Dogged persistence and pig-headedness win every time.

 

[Jessek1486]

Way to go! Dogged persistence and pig-headedness win every time.

Oh I ain't giving up until I build a plane that flies right. Then I'll build another one, and another one...

Ultimately I want a plane I can thermal. That's my goal.

 

[Mr NCT]

Oh I ain't giving up until I build a plane that flies right. Then I'll build another one, and another one...

Ultimately I want a plane I can thermal. That's my goal.

You might try an explorer with a longer wing. Mine glides forever. Saves props, too.
Free plans are here: https://forum.flitetest.com/index.php?threads/sp0nz-plans-index.17136/

 

[Jessek1486]

You might try an explorer with a longer wing. Mine glides forever. Saves props, too.
Free plans are here: https://forum.flitetest.com/index.php?threads/sp0nz-plans-index.17136/

Woah, that's very similar to what I was trying to design, or at least the design I have in my head 😂

Haven't been able to make reality match my imagination yet. The foam hasn't complied, but it will eventually, after being beaten into compliance.

How do others keep their square fuselages from racking so much? I've added triangular braces, but they tend to get in the way. It works for now but I imagine there's a more convenient solution others have found.

 

[FlyerInStyle]

Woah, that's very similar to what I was trying to design, or at least the design I have in my head 😂

Haven't been able to make reality match my imagination yet. The foam hasn't complied, but it will eventually, after being beaten into compliance.

How do others keep their square fuselages from racking so much? I've added triangular braces, but they tend to get in the way. It works for now but I imagine there's a more convenient solution others have found.

Take a look at experimental airlines on YouTube, he does a lot of square fuselages. Also I have found that if it isn’t just a straight tube but closed up on one end, mostly the tail, then it is much stronger.

 

[Jessek1486]

Take a look at experimental airlines on YouTube, he does a lot of square fuselages. Also I have found that if it isn’t just a straight tube but closed up on one end, mostly the tail, then it is much stronger.

No that's exactly who I got it from, but he doesn't seem to do anything for preventing racking. I even saw a comment where someone asked about bracing, and he said he didn't think it was needed.

However, mine does. Even just sitting the plane on a table, the weight of the plane itself causes it to go out of square and be kinda floppy before I added my triangles.

I guess my braces are sufficient for the time being. I was just looking for better solutions or if anyone else had suggestions.

My braces easily remove the paper on the inside of the tube, negating their benefit.

 

[Merv]

... Then I'll build another one, and another one ...

Remember the FT moto: Build, Fly, Crash, Repeat