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Mar 11, 2018, 01:01 AM
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Discussion

I think I've made an idiot-mistake but I'm not sure what it is (crash diagnosis)


Hi Guys,

I've had two crashes in two days and would love some help. I'm new to the forum and looking for some guidance from experienced flyers.

I've spent around 3 months flying a 1200mm Super Cub RTF as my first plane and I've come really proficient with it. I'm able to fly it in most wind conditions, I can land it at my feet, and I can fly laps of my park about 6ft off the ground. I've also logged many hours on a simulator.

A few days ago my second plane arrived on the courier. I had bought a Nano Talon, a PNP, belly-landing, V-tail, foam, pusher. I love the design of the plane and looked forward to flying it.

I ignored the on-board gyro ( I've never needed one before, and gyro's are for amateurs not pro-flyers like me right? ), and connected my X8R directly to the servos. After a bit of fiddling I had all control surfaces reacting correctly to my inputs. My v-tail rudder and elevator mix resulted in only partial throws, i.e. full up-elevator resulted in only half throw on the two elevator control surfaces. I decided to drop rudder out of the mix and just use the v-tail for elevator, effectively making my plane a three channel bank-and-yank instead. I now had full throws on the elevators

What I didn't have was any documentation of what the throw limits should be on the plane, or any experience in setting up planes other than basic checks that the surfaces move in the correct directions. I also did not know what expos should be used. So I ended up with the following on my maiden flight:
  • No throw limits/weighting
  • No expo
  • No gyro
My plan was to just try to feel the plane's characteristics in flight and to then put it down and make changes / dial in expo etc. My maiden was hair raising. The first thing I noticed when I chucked the plane in the air was that it was fast! On 1/4 throttle it was about twice as fast as my Super Cub at 3/4 throttle. The next thing I noticed was that it wanted to pitch down constantly. I was too busy to adjust trim (and it's not something I'm used to doing). I was flying the plane but it felt twitchy and scary. It was stressful rather than enjoyable. After 2min of flying time - and never actually feeling like I had proper control of the plane due to its pitch-down attitude, I crashed it into a light-mast on the edge of the playing-field. I glued it back together that day and vowed to try again tomorrow.

The following day I moved the CG back by around 5mm-10mm and tried again. I took off and the plane still wanted to pitch-down constantly. This time I was ready and hit some trim, but I trimmed too far and it started to pitch up into a steep climb. The next part happened very fast, I killed the trim, I levelled out and appeared to regain some airspeed but without warning the plane promptly entered a death-spiral which I immediately knew in my heart was unrecoverable for someone like me. I tried to fight it but failed and the plane flew nose-in straight into the ground.

Questions:
1) How do people normally handle setup of throws and expo on an un-documented plane? Is it normal to allow full servo travel and to set no expo for a maiden? If not, what do you do?
2) Is it always just the CG that determines whether a plane has pitch-down tendencies or are there other factors? i.e., if my plane always want to pitch down do I just move the CG back and try again or are other steps required?
3) Could my spin have been created by a control surface that travelled too far and caused the plane to depart from normal flight?
4) What do you do when you plane starts spiralling towards the ground? I had never experienced that before and I think I just panic-mashed up-elevator all the way down to the ground. I probably had a good 3 seconds to do something smart if I knew what to do, and I'd love to know what to do in the future

Thanks for your help
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Mar 11, 2018, 01:32 AM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
My advice is : if you get another Nano Talon , set it up the way it's meant to be set up ..... and use the 6 axis gyro ..... you can always turn it off later .

FYI , v-tail & elevon mixing are purposely set up with low throws . Here's why : if you were to input and hold full up elevator , the left side of the v-tail would go up and stay up partially . Then while holding full up elevator you also input and hold right rudder , the left side v-tail control surface would then go FULLY up ..... to the full extent of the servo .
Mar 11, 2018, 02:08 AM
Registered User
Thanks for that regards the v-tail throws. I figured that would be the case. You can't have full deflection for one input ( elevator for instance ) and then still be able to apply rudder

The hard part regards "set it up the way it's meant to be setup" is that it had no documentation with it. As you say, probably should have kept the gyro though. Could always switch it on and off.

Would you normally use expo on a maiden with a small fast plane, or stick with straight-ratio? Is it even something I need to worry about, or does it not really make a difference?

I'm also really interested in what to do in a consistently-pitch-down plane? Just shift the CG?
Last edited by my_fly_sky; Mar 11, 2018 at 02:16 AM.
Mar 11, 2018, 02:40 AM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
Quote:
Originally Posted by my_fly_sky
Would you normally use expo on a maiden with a small fast plane, or stick with straight-ratio? Is it even something I need to worry about, or does it not really make a difference?

I'm also really interested in what to do in a consistently-pitch-down plane? Just shift the CG?
Expo is a personal preference thing , some people like it & use it .... some don't . If you feel expo will help you fly the plane , then use it . For a plane that's meant to fly fast & as if it's on rails , you don't want sudden/extreme control surface movements .

A good indicator of proper CG is how it flies after you shut the throttle down . If it glides gradually down , the CG is OK . If it pitches down sharply with no power , then move the battery rearward .

If it only pitches down sharply when throttle is applied , then you need to adjust the thrust line .
Last edited by balsa or carbon; Mar 11, 2018 at 02:45 AM.
Mar 11, 2018, 02:48 AM
Registered User
Grup's Avatar
See unboxing/flying video at this link:

RC Reviews: ZOHD Nano Talon FPV Plane (20 min 38 sec)


It does not address expo etc but does talk a bit about setting up stabilization. Maybe you could send an inquiry via youTube to the poster of the video

Regards
Mar 11, 2018, 03:07 AM
Registered User
Thanks for the help Grup and Balsa_or_Carbon.

I'm going to hook up the gyro and see how I get on. It should give me the best of both worlds in the sense that I can turn it on or off as I like.

I may also put some limits on the throws, the ailerons look pretty extreme at full deflection compared to other planes I've seen.

I've glued it back together once more and it will fly again. It's a pretty resilient plane considering I've stacked it twice at speed
Mar 11, 2018, 03:30 AM
Registered User
There are other factors than c of g position that can cause a plane to pitch down, such as incorrect elevator trim, mis-aligned motor, and inaccurate wing and tailplane alignment.

My first reaction to a model that's a handful to fly is to move the c of g slightly forward, NOT backwards. Rearward c of g can make a model very twitchy.
Mar 11, 2018, 04:06 AM
Registered User
Thanks abenn. I have actually experienced that on my Super Cub. If it becomes too tail heavy then it wallows and waggles through the air, acting like it is about to stall at any moment and fall to the side ( which I suppose it is due to AoA ).

I will have a good look at tailplane alignment and thrust angle. I didn't actually fly it long enough to discern if the pitch issue was changed by throttle.

The elevators were neutral and in-line with the rest of the tail plane when I set it up, so I assume that was the correct position for normal flight.
Mar 11, 2018, 04:13 AM
Registered User
IMO this looks to be an extremely unstable design aerodynamically: WAY too much fuselage area fwd of the CL & CG rendering the empennage barely useable, "tricky" at BEST (would be at least somewhat better with INVERTED V-tail!)

".... an accident waiting to happen" (without AS and/or lots of experience with unstable designs)
Mar 11, 2018, 07:51 AM
I'm not the pivot man
You should move up to a turbo-fan swept wing jet.
Mar 11, 2018, 07:52 AM
WJH
WJH
B747-400 First Officer
WJH's Avatar
Very short moment from CG to the tail, did you experience that it was hard to keep it level?
Mar 11, 2018, 02:11 PM
Registered User
@FusterCluck, you are right, the way it flew did indeed make it seem like I had bitten off more than I could chew

@WJH, it was very responsive to elevator but it didn't seem "hyper"-pitch-sensitive. I feel that I could live with it if I use low rates and maybe some expo ( which I'm planning to try )

I've noticed that the CG mark on the wing does seem very far forward on the wing. Wing chord is 17cm and the CG mark is 3.5cm from the front of the wing, giving 20% of wing chord. It also does not align with the fattest part of the wing chord. I don't know enough about lift on wings where the leading edge sweeps back to know whether this is normal or not? Does this have anything to do with compensating for the long fuselage at the front?
Mar 11, 2018, 04:59 PM
Registered User
It should have a SHORT fuselage in the front and a LONG fuselage in the rear, else it is like trying to shoot an arrow feather-end first ....

this is an extremely stupid and ill-conceived design
Mar 11, 2018, 05:23 PM
I'm not the pivot man
Quote:
Originally Posted by xlcrlee
It should have a SHORT fuselage in the front and a LONG fuselage in the rear, else it is like trying to shoot an arrow feather-end first ....

this is an extremely stupid and ill-conceived design
I expect that is one of the reasons for the flight controller / stabilization, to account for the design. I think though the OP turned it off unless I misread. With that short coupled tail it likely flies like a Gee Bee. Which can be scary at times.
Mar 11, 2018, 05:26 PM
I'm not the pivot man
Quote:
Originally Posted by my_fly_sky
@FusterCluck, you are right, the way it flew did indeed make it seem like I had bitten off more than I could chew
as long as you recognize that. Most of us have done something silly in this hobby at one time or another. Your best path I would reason is to turn on the flight stabilization and use it. The design of this plane does not afford the same flight characteristics of your Super Cub.


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