Help! Cub is unpredictable

[ISuperfan]

So i scratch built the FT Simple Cub. Attempted to fly it on 3 separate occasions and it was just unpredictable to fly. My dad, who has been flying for decades, couldn't even determinate what was wrong. it was like trying to ride a bull in the sky. It would not turn with ailerons, no bank and yank. i could make a rudder turn, but it still seemed very unpredictable. it would also climb and stall seemingly random. this was my first build and sitting on the bench, the plane looked air worthy. we are not sure if it was an issue with the throw of the control surfaces, or the center of gravity being off. Sadly, the last attempt stuck it about 30 feet in a tree, irretrievable. I am just waiting for the rain to weaken the foam board enough to let it drop from the tree to retrieve the electronics. I know no one will be able to give a definitive answer and now that the plane is stuck in a tree and has had multiple rain storms, we will never be able to truly determine what the exact issue is with my build. I am just hoping that someone has experienced similar issues and can give me some insight as i go into my next build, hoping to avoid what may have caused this issue.

 

[JTarmstr]

So i scratch built the FT Simple Cub. Attempted to fly it on 3 separate occasions and it was just unpredictable to fly. My dad, who has been flying for decades, couldn't even determinate what was wrong. it was like trying to ride a bull in the sky. It would not turn with ailerons, no bank and yank. i could make a rudder turn, but it still seemed very unpredictable. it would also climb and stall seemingly random. this was my first build and sitting on the bench, the plane looked air worthy. we are not sure if it was an issue with the throw of the control surfaces, or the center of gravity being off. Sadly, the last attempt stuck it about 30 feet in a tree, irretrievable. I am just waiting for the rain to weaken the foam board enough to let it drop from the tree to retrieve the electronics. I know no one will be able to give a definitive answer and now that the plane is stuck in a tree and has had multiple rain storms, we will never be able to truly determine what the exact issue is with my build. I am just hoping that someone has experienced similar issues and can give me some insight as i go into my next build, hoping to avoid what may have caused this issue.

Was the battery secured? It might have been sliding arround causing random CG changes. Just a guess.

 

[ISuperfan]

Was the battery secured? It might have been sliding arround causing random CG changes. Just a guess.

That is a legitimate question, but i feel that we did address that issue adequately in our search for the cause of the problem. i am confident in saying that the battery was well secured. thank you though.

 

[JTarmstr]

That is a legitimate question, but i feel that we did address that issue adequately in our search for the cause of the problem. i am confident in saying that the battery was well secured. thank you though.

ok, sorry, I have limited piloting experience so I can only theorize on the aerodynamics and engineering side of things.

 

[rockyboy]

Most unpredictable planes I've dealt with (including a couple FT Cubs at Flite Fest) usually turned out to be either tail heavy, not having a parallel wing and stabilizer, or there was control surface funny business. By funny business I mean control rods that flexed under load when being pushed, or hinges that were not secured and when in flight let the control surface move independently of the servo, or control surfaces that were slightly warped which limited movement responsiveness in one direction, etc.

Hopefully some of these ideas will help for next time

 

[PsyBorg]

I would also suspect tail heavy. But I think that maybe in tandom with thrust angle being off.

It is easy to get that wrong with scratch built power pods if you cut the slots on the wrong side of the foam. Instead of a down and right thrust angle you get up and left which would cause all kinds of interesting wierdness depending on throttle inputs.

 

[ISuperfan]

ok, sorry, I have limited piloting experience so I can only theorize on the aerodynamics and engineering side of things.

no worries, i appreciate all input.

 

[ISuperfan]

I would also suspect tail heavy. But I think that maybe in tandom with thrust angle being off.

It is easy to get that wrong with scratch built power pods if you cut the slots on the wrong side of the foam. Instead of a down and right thrust angle you get up and left which would cause all kinds of interesting wierdness depending on throttle inputs.

hmmm...thrust angle, that is something i do not think we looked at. Thank You

 

[Gazoo]

I have had issues with my cub wanting to nose down or up at random. I found that the trailing edge of the wing can buckle when the rubber bands are attached to hold the wing on. if it does buckle, it will lift the back side of the wing and change the angle if incidence. It will want to dive and then the wing will again be flat on the fuselage and it will want to climb.

This also could explain the uncontrollable aileron input as well. A bit of a wobbly wing.

A couple of Popsicle sticks glued into place before the wing is folded will help. Otherwise, there are many fixes by others that work too.

 

[ISuperfan]

I have had issues with my cub wanting to nose down or up at random. I found that the trailing edge of the wing can buckle when the rubber bands are attached to hold the wing on. if it does buckle, it will lift the back side of the wing and change the angle if incidence. It will want to dive and then the wing will again be flat on the fuselage and it will want to climb.

This also could explain the uncontrollable aileron input as well. A bit of a wobbly wing.

A couple of Popsicle sticks glued into place before the wing is folded will help. Otherwise, there are many fixes by others that work too.

this is something we noticed too and suspect might have been a contributing factor... i think my solution on the next one i build will be screw down wings instead of rubber bands. thanks

 

[PsyBorg]

This was mentioned in another FT video way back in the day. The fix was two bands across the wing like normal and either two more in an X pattern or.

Go from one point across the the far corner, under the skewer and back up over the wing. Then do the opposite on the other side. This gives that sidewards hourglass shape either way you do it.

 

[Hai-Lee]

My cub did display somewhat similar characteristics during the first few flights and was only calmed by increasing the wing incidence by around 1/16th of an inch. This was simply achieved by using a Popsicle stick under the LE. It increased the lift available greatly.

I found that the wing was effectively stalling, (suddenly losing lift), as I tried to fly around at any speed other than flat out. In wing gusts it was really dancing around in the sky in a very unpleasant manner. The only thing that saved my cub from a similar fate to that your cub suffered was that I was deliberately nose heavy during the first few flights and at stall the plane would nose down and recover speed, well this and landing hot!

Since increasing the wing incidence my cub can putt around at very low throttle lands like a STOL plane and even can take off with as little as a one metre ground run from stationary. It still doesn't like a lot of wind but handles it far better now than it did originally.

Please note that there must not be any gap under the flat bottom of the wing where is sits on the fuselage at the trailing edge portion, EVER!

Just what worked for me!

Have fun!

 

[basslord1124]

this is something we noticed too and suspect might have been a contributing factor... i think my solution on the next one i build will be screw down wings instead of rubber bands. thanks

I think this has been a design flaw from the early days. Any FT high winger where the wing is held on by rubber bands and has the ability for ailerons is a good indicator. If not sure, just look at the wing to see...the LE will extend past the flat underside of the wing. I found this out early on when I built, flew, and crashed my Storch. The crash was actually what showed me the issue as the wing buckled at that point.

What I did was take some scrap foam and either some popsicle sticks/wood dowels and insert them on the under side of the wing in that section. On my Storch, I even reinforced the top by gluing popsicle sticks in a criss-cross fashion. Helps make the area stiff, plus it helps with any rubber band rash.

Once I noticed the Simple Cub had that design, I reinforced its wing as well.

 

[ISuperfan]

I think this has been a design flaw from the early days. Any FT high winger where the wing is held on by rubber bands and has the ability for ailerons is a good indicator. If not sure, just look at the wing to see...the LE will extend past the flat underside of the wing. I found this out early on when I built, flew, and crashed my Storch. The crash was actually what showed me the issue as the wing buckled at that point.

What I did was take some scrap foam and either some popsicle sticks/wood dowels and insert them on the under side of the wing in that section. On my Storch, I even reinforced the top by gluing popsicle sticks in a criss-cross fashion. Helps make the area stiff, plus it helps with any rubber band rash.

Once I noticed the Simple Cub had that design, I reinforced its wing as well.

I am having a difficult time visualization what you are describing. can you provide a picture please?

 

[basslord1124]

I am having a difficult time visualization what you are describing. can you provide a picture please?

I was thinking as I was typing that, that a pic might be better. lol. Oh well. Once I get home, I'll post a pic for ya.

 

[JGplanes]

I would also suspect tail heavy. But I think that maybe in tandom with thrust angle being off.

It is easy to get that wrong with scratch built power pods if you cut the slots on the wrong side of the foam. Instead of a down and right thrust angle you get up and left which would cause all kinds of interesting wierdness depending on throttle inputs.

What’s everyone’s thoughts on p-factor with the cub? With a large motor (C pack) It always seems to throw my first flights off until I get it dialed in and trims set. Once they’re set it flys awesome.

BTW - I have Cub problems of my own. 😜

 

[Hai-Lee]

What’s everyone’s thoughts on p-factor with the cub? With a large motor (C pack) It always seems to throw my first flights off until I get it dialed in and trims set. Once they’re set it flys awesome.

BTW - I have Cub problems of my own. 😜

The previously mentioned wing incidence problem can and does give a plane that can leap into the air quite quickly BUT when you try to level it out the lift rapidly decreases and the "P" factor takes over causing a roll and crash.

As I determined that it was not ever going to be a high speed aerobatic creation I fitted a SF prop which not only reduces the roll on take off but also gives a large amount of thrust across the fuselage, wing centre, and tail feathers so that I can lift the tail and turn the plane before the wheels even start turning, (in the same direction).

For my first flight I was forced to maintain a nose high attitude to maintain altitude and it seemed to want to dive from level flight unless the throttle was full. I tried trimming but this was only moderately successful. After much messing around, including checking for warping and other build errors - which did not exist), I increased the wing incidence and all became like I expected it should be.

Currently I can take off from stationary quite quickly by just applying a little right rudder and then slam open the throttle. Once the tail rises I just add up elevator and it lifts off immediately and I am airborne. Sometimes I forget to let go of the rudder and I get a roll to the right but the fingers quickly catch it. All of this is done in about a second and less than a metre of ground run.

Mid you I also use 3 inch wheels and tyres as I use a grass runway!

Just my setup!

have fun!

 

[JGplanes]

Interesting. Good info! That’s a very similar set up to my normal cub. I have 3 inch wheels also, but I would need 10 inch wheels to overcome the grass where I usually fly.

 

[basslord1124]

I am having a difficult time visualization what you are describing. can you provide a picture please?

All right, the yellow Cub wing photo is on the bottom side of the wing. I circled the foam piece that I added to the wing. Underneath that piece are 2 popsicle sticks going into the wing, probably hitting close to the spar. All that is glued in of course. This causes this section to be more in contact with a fuselage and would help prevent that area from bending from the stress of the rubber bands.

Very similar to this post here on the Storch thread:
https://forum.flitetest.com/index.php?threads/ft-storch-owners-thread.22674/post-323405

The other pic is the top of the wing on my Storch. This type of thing is optional, but it does make it more strong. When I had my crash with the Storch, that section buckled and it did cause some creasing of the paper on top of the wing. Which is why I glued the popsicle sticks on there. I also though the criss cross design sort of fit the Storch look too. The bottom part of the Storch wing looks just like what I have done with the Cub with the scrap foam and popsicle sticks.

If you look more on the Storch thread too (around the post that I shared), you will see others having to mod the wing in a similar fashion to keep it from buckling from the rubber bands.

Hope this helps. If you are running ailerons on your Cub, I would highly recommend this.

 

[Gazoo]

This is a picture of mine after the first flight and before I added reinforcement.