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

[Jessek1486]

Hey all, so I've read about powered off glide testing, but have found very little in the way of troubleshooting a poor result. That part seems to be left for interpretation.

Anyway, I seem to be having a strange issue with my planes gliding in such a way that they eat dirt.

It's not as simple as them just stalling, then going nose down, as that'd be simple, right? CG too far back.

No, this is more like a 30 degree angle trajectory.

I even spent 30 mins at the park(after crashing in <2secs of powered flight) throwing my plane but with the CG in all different positions, just trying to see if something would get it to glide. I had it very far back, very far forward and all inbetween.

My only thought is, the plane is too heavy to glide at speeds which I can comfortably throw it?

I can barely get my planes to glide, so powered flight is a nogo until I resolve this.

I've even been playing around with little bamboo skewer gliders that I kinda improv, just to understand various different configs and their modes of failure. None of these do what my actual RC plane builds do.

Perhaps, I have unrealistic expectations of what a successful unpowered glide is supposed to be, in my mind that is a graceful glide similar to a paper airplane that slowly coasts until the ground gently meets it, and friction slows it to a rest.

Is a near 30-40 degree angle nose first landing successful? There's almost no info on this topic.

 

[Tench745]

You are correct in your assumption of what a successful power-off glide should look like.
Glide tests can be done a number of ways. The standard is to throw the plane, level or slightly nose-up with enough force to get it to fly. I will usually conduct these glide tests with the receiver powered on and the transmitter in hand so I can add some up or down elevator to save the nose if the CG is too far off.

Sometimes, if the plane is nice, and I'm not confident it would survive a bad glide test, I have held the plane about shoulder level, just sitting in my hands, and then run as fast as I can and let the plane go. With a good CG it should glide relatively level and slowly descend back into my hands as I run. If it starts to go poorly, I'm still right there to catch the plane.

If I'm understanding you correctly, your plane pitches down and arcs towards the ground?
What are the planes like (size, weight, design, etc)
Do you have any pictures or video that can help us troubleshoot?

 

[Jessek1486]

You are correct in your assumption of what a successful power-off glide should look like.
Glide tests can be done a number of ways. The standard is to throw the plane, level or slightly nose-up with enough force to get it to fly. I will usually conduct these glide tests with the receiver powered on and the transmitter in hand so I can add some up or down elevator to save the nose if the CG is too far off.

Sometimes, if the plane is nice, and I'm not confident it would survive a bad glide test, I have held the plane about shoulder level, just sitting in my hands, and then run as fast as I can and let the plane go. With a good CG it should glide relatively level and slowly descend back into my hands as I run. If it starts to go poorly, I'm still right there to catch the plane.

If I'm understanding you correctly, your plane pitches down and arcs towards the ground?
What are the planes like (size, weight, design, etc)
Do you have any pictures or video that can help us troubleshoot?

I really appreciate your thorough reading of my post, and your helpful demeanor.

I like the idea of a running test, will consider that in the future.

I don't have any videos of an unpowered glide, only crashes. Essentially, it'll "glide", then the bottom/front corner slams into the ground as it descends. It's not at all graceful, and often provokes a visceral groan from me, as it's like watching a car get rear ended. It's extremely abrupt and violent, often throwing components off...

I've experimented with various designs but anything from 25"-30" fuselage, 30" wing span at 5" chord to my largest at 42" by 6" chord. I realized I needed larger wings...

Mostly based on the Noob Tube design, since it was simple looking and didn't have lots of complex geometry to cut/glue together. Only reason I've avoided the tiny trainer, it's all curved and seems difficult to cut.

The confusing thing to me is that I literally moved the battery around to various different placements, including what I thought was too far forward/back and all inbetween JUST to see what'd happen, and basically no significant change. Just slamming the front corner into the ground, violently each time.

 

[bearit]

from the looks like they might be tail heavy where do you have your cg at?

 

[leaded50]

I really appreciate your thorough reading of my post, and your helpful demeanor.

I like the idea of a running test, will consider that in the future.

I don't have any videos of an unpowered glide, only crashes. Essentially, it'll "glide", then the bottom/front corner slams into the ground as it descends. It's not at all graceful, and often provokes a visceral groan from me, as it's like watching a car get rear ended. It's extremely abrupt and violent, often throwing components off...

I've experimented with various designs but anything from 25"-30" fuselage, 30" wing span at 5" chord to my largest at 42" by 6" chord. I realized I needed larger wings...

Mostly based on the Noob Tube design, since it was simple looking and didn't have lots of complex geometry to cut/glue together. Only reason I've avoided the tiny trainer, it's all curved and seems difficult to cut.

The confusing thing to me is that I literally moved the battery around to various different placements, including what I thought was too far forward/back and all inbetween JUST to see what'd happen, and basically no significant change. Just slamming the front corner into the ground, violently each time.

Dont just check your plane at CG forward/backward.... check the balance sideways too!

 

[luvmy40]

What are you building with? Adams Readi board is the go to. The Elmer's or other foam boards from Wally World and the like are way too heavy for these planes.

 

[quorneng]

jessek1486
To be much help I would need to know the wing span and its 'ready to fly' weight. From this info it would be possible to get a rough idea of the wing loading and an indication how fast it will need to fly.

In the first picture the general configuration of the plane looks reasonable BUT you have two 9g servo stuck on the tail and a 40g? motor right out front. Be aware that the aerodynamic "stability" forces to correct the flight path are really quite small.
I fear the inertia from the widely placed servos and motor would mean that the aerodynamic effects would simply not have time in a hand launch to make any impression on the flight path, which sounds rather like what you have found so far.

My suggestion would be to remove all the weight, Servos, battery and motor just leaving the wing, fuselage and tail . Tape the control surfaces to neutral and add nose ballast to give a CG at 25% chord and then try a glide. The aerodynamics will work just the same at a light weight but at a much lower speed. When it glides well you can add all the weight back and provided you keep the same CG it will fly but faster. It is possible it maybe beyond that you can achieve with a hand launch.

As suggested weight is the key factor in any plane. In smaller sizes it is even more important.

I hope this helps

 

[Draftman1]

I never have done the glide test, equip it properly and get the cg correct, it should fly well

 

[Merv]

...to my largest at 42" by 6" chord....

I don't do glide testing, I prefer math.

So let's do a little math. 42x6=252 square inches. There 144 square inches in a square foot. So 252/144=1.75 square feet. A good wing loading for this size is 10-12 oz per square foot. If your plane all up weight is around 17-21 oz, the wing is the correct size. That is including the battery & everything else needed to fly.

Adjust the battery until it balances at 25% of the wing area, also called mean average cord. In your case, 1.5 inches from the leading edge. If the CG is at 30% MAC or greater, in your case, 1.8 inches from the LE, your plane will be difficult to fly.

 

[Shurik-1960]

The video shows how I threw the model with force. To get into the alignment of 25% of the wing chord, 2 2200 mA 3S batteries had to be installed. During the flight, a strong roll to the left is visible, which was corrected by trimming the ailerons to the right. I throw a light model easily and it plans slowly, I always THROW a heavy model strongly-with a slow throw, a heavy model is prone to stalling. These are the basics of setting up the model after pre-centering (CG-25%).
The following video is about finding the optimal alignment of a light model before installing an engine with a regulator.

 

[Jessek1486]

Okay so lots of replies and I couldn't get to this during the day, but I have videos!

I see some don't do glide testing, however I am doing this diagnostically, as powered flying results the same way and it's annoying to have the ground rise each and every time into the mouth of my plane. I can just throw it, saving a prop.

So the AUW is 460g or 16oz. Adams Readiboard. Motor itself is 70g. Without battery in these throw tests, plane missile weighs 368g or 13.69oz.

Wing chord: 6" 42" span. I did add some polyhedral, approx 5 degrees 8" from wing root(I believe that's the term, measuring out from where wing attaches to fuselage).

I did tosses with/without battery. The balanced pics are with battery. The lines are 1.5" and 2".

I also taped the control surfaces flat. Nothing is turned on, battery isn't even connected, just there in one video for weight. I didn't check balance after removing it, but it honestly doesn't matter, plane is on a death mission anyway...

One from my perspective

It violently hits the ground, at the front bottom corner, and it's very jolting. Knocking off the battery every time, and breaking other shit. I just can't think this is normal. According to this thread, all of my specs are about right...

Yeah any insight would be great.

 

[Jessek1486]

One more video to show how violently the ground comes up to meet the nose... And sure, maybe I'm not throwing it absolutely perfectly, but there's just no way this thing can fly and there's obviously something I'm missing.

 

[Jessek1486]

I don't do glide testing, I prefer math.

So let's do a little math. 42x6=252 square inches. There 144 square inches in a square foot. So 252/144=1.75 square feet. A good wing loading for this size is 10-12 oz per square foot. If your plane all up weight is around 17-21 oz, the wing is the correct size. That is including the battery & everything else needed to fly.

Adjust the battery until it balances at 25% of the wing area, also called mean average cord. In your case, 1.5 inches from the leading edge. If the CG is at 30% MAC or greater, in your case, 1.8 inches from the LE, your plane will be difficult to fly.

I have. All of that is correct, but it flies like a brick. I just don't know what I am missing here...

 

[Cnat]

I think the glide test looks okay? Ive never done one. I just make sure the CG is close and send it. The glide test should always be done at flight weight. Without a battery your CG is going to be way off. Id just fly the thing and see how it feels. To me it looks like it glides well when the speed is up and level, and nose dives as it slows down which is normal. If anything just move the battery back a bit. On the plans there should be a CG mark or at least a measurement back from the leading edge where the CG should be. As long as its close Id just send it. Forward CG is more favorable than Aft CG due to a number of things.

If those lines in the photos are the CG limits id move the battery back a bit or remove weight from the nose. It looks like its balancing on the forward side of the correct CG.

Anyway id just fly it and see how it is. Thats more fun than crashing it with a glide test, and i think you will be surprised. Good luck!

 

[Jessek1486]

I think the glide test looks okay? Ive never done one. I just make sure the CG is close and send it. The glide test should always be done at flight weight. Without a battery your CG is going to be way off. Id just fly the thing and see how it feels. To me it looks like it glides well when the speed is up and level, and nose dives as it slows down which is normal. If anything just move the battery back a bit. On the plans there should be a CG mark or at least a measurement back from the leading edge where the CG should be. As long as its close Id just send it. Forward CG is more favorable than Aft CG due to a number of things.

If those lines in the photos are the CG limits id move the battery back a bit or remove weight from the nose. It looks like its balancing on the forward side of the correct CG.

Anyway id just fly it and see how it is. Thats more fun than crashing it with a glide test, and i think you will be surprised. Good luck!

Right, but the glide test is being done because this is what it does in powered flight.

I took a video with and without the battery...

The plane violently nose dives into the ground, if this is how it's supposed to fly, then I may need to make it out of metal to surive that. It's violently jolting the plane.

When it is powered, it does this but does not survive. That is not more fun than crashing it with a glide test(but you had said it looks okay?), and is actually getting very discouraging to have to bring home a bag of parts...

I do not understand, and feel as if you perhaps did not fully read what I had posted.

 

[Jessek1486]

Maybe I'm crazy, but looking up "glide test" on youtube and they are very graceful landings.

Perhaps I'm missing something, but mine does not look/feel/sound like that.

This is becoming more frustrating.

 

[Merv]

I have. All of that is correct, but it flies like a brick. I just don't know what I am missing

It's hard to tell from the video, it looks like you are not throwing it hard enough & the plane is stalling out, that is going too slow to produce lift. If you want a glide test, find a place with a lot more room & throw the plane a bit harder. Like you are trying to throw a baseball 10 yards or so.

If you have done the math, I would not bother with a glide test. I would go for a powered flight.

 

[Jessek1486]

So how much does the plane weight, battery included, ready to fly?

460g or 16oz.

 

[Tench745]

Those videos are very helpful. Here's what I'm seeing:
You are throwing the plane with enough force for it to fly, so that's good.

The second video was close to what I would call a good glide, right until the end.
The third video looks like the plane may be on the verge of a stall: the nose stays high and the plane kind-of mushes towards the ground, then the nose begins to drop and the plane doesn't recover.

I think you're fighting a trim issue rather than a CG issue. Most wings need a little bit of a positive angle of attack to generate sufficient lift. In your pictures it looks like the one plane has a symmetrical airfoil. Symmetrical airfoils need slightly more angle of attack than a flat-bottomed airfoil to generate lift.

So here's what I think is happening: When you throw it, the plane is at a good angle of attack and starts to glide, but because the horizontal stabilizer is in line with the wing chord, it lets the nose drop, eliminating the AOA necessary to generate lift, and the plane begins to lawn-dart.

Here's my solution: Either add a shim under the leading edge of the wing to give the wing some incidence (I'd guess 1-2 layers of foamboard), or add in a little up-elevator trim until you get a predictable flight path. Once it flies predictably you can continue to tweak trim and CG to get a good glide.

 

[Merv]

460g or 16oz.

Read my prior post again. I completely changed it after watching the video & reading the post