Help! Maiden Flight Issues

[JasonK]

I just got out and maidened the below plane today and am trying to figure out what to adjust to fix the poor flight characteristics. Initially I had to hold almost 100% back on my elevator to keep my nose up. I tried moving the battery back some and then the elevator became way over responsive. Cutting the throttle didn't seem to make much difference (there is some down thrust angle with it being attached above the wing/CG)

My thought is that my CG is to far forward and when I moved it back, I went to far. Does this sound likely or is there something else I should be checking?

 

[Merv]

My thought is that my CG is to far forward and when I moved it back, I went to far.

Yes, that’s a definite possibility. When I adjust CG, I only make small adjustments, about 1/8 inch at a time. Setting the CG at 25% of the wing cord is a good starting point.

Hard to tell from the picture, it looks like the motor may be higher than it needs to be. The higher the motor the more down thrust it will produce.

 

[Aireal Anarchist]

how much down thrust did you input in your motor mount, and can we see a side view of the motors height....with a pusher motor I keep the motor height as close to level with the wing I can get and add NO thrust angle of ANY kind

 

[JasonK]

Here is a side shot.
I was also just measuring things and might have accidentally put a negative angle of attack on the wing. the front edge of the wing is 1/8" lower then the back edge of the wing.
I don't have a protractor to measure the exact angle.

I have a 6" prop on it. I measured and a 5" prop would either clear the ground with it sitting on its belly or just brush (really close either way).

I was following this guide: https://www.flitetest.com/articles/half-sheet-no-wastethree-channel-trainer, which says it should end up with 4 degrees down thrust. (but given my angle of attack mistake above, it might be different).

 

[JasonK]

Here are the CG marks based on the design. I checked what the CG was based on were I had everything for the first launch and it was about 1/2" in front of the forward mark.
wing cord at the fuselage is 6 1/4", the marks are about 2 5/8" & 2 1/2" from the leading edge.

 

[Aireal Anarchist]

I would remove a ton of that down thrust or drop the motor down lower and get rid of your thrust angle issues and guessing what angle


here is a versa wing I built to test out some long range gear and flight controller before the elctronics gets transfered into the expensive epp wing, notice my prop shaft is only 1 inch above the bottom of the wing and there is NO thrust induced, not saying you have to build this way, but just saying all the thrust issue isnt necessary in a pusher configuration if the motors are installed at a proper height

 

[JasonK]

Thanks! I will take this and see what I want to do next here with my frame.

 

[Aireal Anarchist]

I just went out to my office trailer where I do my hobby work to get you another photo......here is a Long Range FPV rig Im working on, a mashup between the SkyHunter and the FT Explorer..........The "Sky Explorer" it too obviously is a pusher and a similar box tail design, tail feathers and boom in the pic is just scrap rough cut, still in the mock up stage but you can see here again how low I mount the motor and zero thrust angle...my prop shaft is 3/4" above the bottom of the wing

 

[JasonK]

I removed the thrust angle completely, will try again when I get a chance (wind speeds locally tend don't get under 10mph a whole lot and I live in a MOA, so I have a bit of a drive to get to an open field outside the MOA).

 

[Merv]

I agree with @Aireal Anarchist, that’s a lot of thrust angle.

 

[JasonK]

I just did a few toss tests with the CG at the forward mark and that moves very nicely... I can't turn on the motors/TX/RX without a ~25 drive... so I don't think I will be testing it today.

 

[JasonK]

This is after moving the motor mount.. If this doesn't work, I can try cutting a notch in the wing and getting the motor mount in line with the top edge of the fuselage.

 

[Aireal Anarchist]

looks like that will work

 

[JasonK]

The motor mount change seems to have fixed the nose down issue. However, I think my control linkage are to long and loose, making my controls to sloppy. Video when I get home.

 

[JasonK]

Here is the linkage layout, I noticed that I could move the surfaces some without the servos being pushed/pulled on, which I think may be part of the problem.



Second Maiden, sorry about video quality, my phone was set to "20 years ago quality" for videos for some unknown reason. I didn't get any higher as I was having issues getting it stable enough to get higher. I am also noticing that I like ailerons which this doesn't have (but this is unrelated).

 

[Aireal Anarchist]

Im not surprised your control surface authority is off, thats a very very long span for pushrods, Im pretty sure they are flexing, you could cobble up some standoffs from foamboard to glue some coffee stir straws to act as push rod guide tubes.... see my versa wing photo above and what I mean about FB standoffs as supports for the stir straws, I cut some FB triangles to fit the contour of the wing and hold the stir sticks at the appropriate height

 

[JasonK]

yah, when I was building it, I was debating if I should use servo extensions and put them right next to the surface or the long control rods. I need to do some repairs and will either move the servos or work out some sort of guides. Unfortunately we don't drink coffee and I don't have any coffee stir straws laying around (but I should probably get some, I also noticed some flex issues like this on my TT because I didn't have the straws, but the impact wasn't as bad as this).

I have definitely learned a whole bunch about what does and doesn't work from this build, so it has definitely been valuable there, even if the plane itself might end up scrapped for parts by the end of this.

 

[Aireal Anarchist]

yah, when I was building it, I was debating if I should use servo extensions and put them right next to the surface or the long control rods. I need to do some repairs and will either move the servos or work out some sort of guides. Unfortunately we don't drink coffee and I don't have any coffee stir straws laying around (but I should probably get some, I also noticed some flex issues like this on my TT because I didn't have the straws, but the impact wasn't as bad as this).

I have definitely learned a whole bunch about what does and doesn't work from this build, so it has definitely been valuable there, even if the plane itself might end up scrapped for parts by the end of this.

keep going ....patch it , test it and do it again till you learn what it takes to tune it up

if you move the servos to the tail feathers it will change the CG balance again so you need to see if you can re balance it with your battery....extend your battery pod if necessary

 

[JasonK]

My battery has a solid 1-1.5" it can move forward right now and I have some 3cells on order (so I can go forward and eventually heavier). One thing I like about this frame is that it takes hard nose hits better... Eventually the fuselage is going to get warped out of shape if I keep nose smashing this thing, but the same impacts on my other plane would have already required some serious rebuilding. The DTFB method of making airframes is great for learning... not a whole lot of financial risk when it crashes... just time.

 

[dahacker]

I can tell you the fundamental difficulties with a model like this is:

1. It is so short. So getting the center of gravity correct is very difficult.
2. The thrust can be very off the enter of gravity, so getting the thrust angle correct in coordination with the uncertain center of gravity is very difficult.
3. The control surfaces are extremely close to the prop and getting blasted by the rotating prop. This causes much different control authority as you are throttling up and down, as well as funky torque effects.

There is certainly nothing wrong with tinkering with this one and is great fun, but I'd recommend something with a longer body that will just be inherently stable at the first throw. Anything that looks like a Cessna 172 would be fine. Like the Tiny Trainer for example.