Hello. I’m new to this and would greatly appreciate some help analyzing my flight log to see if we can figure out what happened.
Upon launch, the plane flew for about 1 second, before crabbing right, then rolling hard right and hitting the ground.
Please let me know what additional info you need. I’m on a different computer, so I don’t have the FW version number handy, but it was current as of a couple months ago.
Roll input is slightly to the right. At A there is enough speed for the aileron to take effect and the aircraft begins to roll (red) to the right. At B you notice this and apply full rudder to the right, causing the aircraft to tip completely.
Other than what we can deduce from the log, I’m not sure exactly what happened with the stick inputs at takeoff as I was the one throwing the plane and my friend was piloting.
During preflight, control surfaces moved in the correct directions in both FBWA and manual.
I should’ve mentioned this is a relatively heavy plane - I’ll have to double check, but AUW was in the neighborhood of 4-5kg. Despite full throttle input, it’s been suggested that the launch was too slow and that a stall occurred. Is there anything in the log that could confirm that?
You are right. Assuming that the aileron direction was correct, a tip stall is even very likely. The aircraft then tends to tip to the side on which the ailerons go down (A reason for differential ailerons: greater deflection being upwards) . Sorry I didn’t think of that right away.
The log does not speak against it at all. However, it would have been helpful if you had posted the movie here instead of only emailing it as a PN afterwards. Please provide the available information as completely as possible!
Any ideas as far as what might’ve caused a tip stall? The amp draw seems slightly lower than others, but only by around 10-12% (compared to very similar builds). Not sure how the takeoff speed compares.
This is very helpful! Using UAV Log Viewer, I can see that there was a decent amount of unintentional roll input to the left when pitching up. How likely of a culprit would you say this is compared to other ideas you might have?
If anyone has any ideas on what could’ve cause this possible tip stall, please chime in. We really need to get it airworthy asap and appreciate all the help we can get.
Is control surface deflection alone enough to cause a tip stall on takeoff? I think flight speed was in the neighborhood of 9 to 11m/s prior to the dropped wing. The plane is 1600mm and weighs 4kg+. CG was neutral (or very close to it).
I had already written that you were probably too slow. Please post the link to the youtube video (which you strangely only sent to me as a PN) of the accidental take-off here, as you can clearly see that the glider sinks with too high an angle of attack. Before that, you could see the windsock with absolutely no wind.
Thanks. I’m not seeing where you said it was probably too slow. Maybe your reply didn’t go through?
The other takeoff videos I’ve studied of this plane start out in roughly the same speed range (9-11m/s). When this happened, we wondered if maybe the ESCs weren’t outputting full power, despite full stick input. Perhaps not calibrated correctly.
If the speed were the same next time, but we had some headwind and launched in Takeoff mode, do you think it’d be ok? Re AoA, does the throw need to be more level, or were the stick inputs the sole issue there?
As far as the video, like I said, I’m just respecting my friend’s privacy. I could edit it and re-upload.
If anyone has any input on what they think happened, please let me know. Really need to get this airworthy in the next couple days, if possible. Thanks!
As far as I know, CG was neutral. AUW was below max, at around 4kg (max is around 5kg, maybe more).
I’m trying to get better motor thrust specs - I found 2 different data sheets, but it looks like between 1.1-1.6kg per motor, if using 11x3.7 props (we had 10x7 on there for this flight).
Significant up elevator was applied immediately (along w/ unintentional left roll).
This was my first time throwing such a large plane. After seeing the phone angle video, it seems like the throw had pretty good heft, but I don’t know if it should’ve been more level (pitch wise). There was essentially zero headwind.
We’ll double check that left/right is set correctly re differential thrust.
Not a bad idea at all actually, maybe even a longboard with tight trucks! Thinking along the same lines, we attempted a caddy before this, but it had issues and wouldn’t track straight.
Based on other takeoff videos of even heavier loaded, but nearly identical builds of this plane (example1, example2), it should take off easily right out of the thrower’s hands. I just want to make sure we’re not overlooking something before trying again. For all I know Takeoff mode and a little headwind is all we need.
From my experience it looks that the speed at the throw was not enough to hold altitude and also the speed not rises fast enough. In such situation it is not the best idea to put elevator up. This increases the angle of attak possible followed by a stall. To increase speed elevator must be neutral or very low down. If than still the speed is to low to have enough lift trust for the plane, you directly smooth land in front of the starting place.
CG is best little forward as neutral but never backward.of center of lift.
I believe we pulled around 37a on takeoff (if I recall, there was a 43a spike, but I don’t have the log handy to look at the timing). 37a is roughly 10-12% below what others say they pull on takeoff for the same plane (with the same or similar build). I don’t know if that difference is enough to account for what we experienced.
One of the motor charts showed 1100g of thrust, but with 11x3.7 props (versus our 10x7). I don’t know what the real world thrust is. Our initial feeling was that it was underpowered at full throttle. Perhaps due to needing to re-calibrate the ESCs, or something else. Is there anything in the log data that could shed light on a possible power issue?
Pretty close, we have 710kv (2814) motors. I was told these would work for both 4s and 6s and that the original owner was using them on 4s. It seems like most are running higher KV motors (at least 770) on 4s though, so I think you’re touching on something we might need to address. They seem low for 4s and high for 6s.
I’ve been looking at the Tmotor MN3110 470KV motors for 6s, which a number of people have been running with this plane successfully.
Edit: the chart for the MN3110 470KV motors says they have 890g at full throttle w/ 10x3.3 props - since others are using these successfully (and with much heavier loaded planes), would you agree this could mean the ~1100g thrust of the current motors might not be the culprit?
As also @Rolf and @menschel assumed I also think the main problem is to less motor thrust at the moment of throw. If the motor thrust is such limited best is to throw the plane not with nose up but with nose slightly down and as much speed as possible. I know this is not easy for such a plane. You throw the plane with two hands. I think this is due to the wide hull which makes a spear-like one hand grip impossible.
And yes elevator and ailerons should be neutral at the moment of throw.
How did you checked the CG.
Did the up elevator and left roll deflections at takeoff cause the speed to be reduced enough that it stalled? i.e. if control surfaces were neutral (or very close to neutral) during and immediately after the throw, would there have been enough speed to takeoff successfully?
Were the up elevator and left roll deflections and the resulting angle of attack the cause?
Did the throw itself cause a problem (e.g. nose too high up, as @Juergen touches on)? I gave it a pretty solid toss and there are many takeoff videos of this plane where it seems to takeoff easily and climb immediately (even when loaded w/ an extra 1kg of batteries) right out of the thrower’s hands, even without taking steps forward. That said, it was my 1st time hand launching a bigger/heavier plane.
How much did the lack of headwind contribute to the problem?
If the Tx was at full throttle at takeoff and the log confirms AP was receiving it, what are the things we need to check to see if the motors were actually receiving and outputting full throttle? Is there anything relevant in the log data that can help?
With it pulling around 37a at takeoff and with similar builds known to be pulling around 10-12% more, is this enough of a difference to help us understand what’s going on? I don’t know if others with this plane are taking off around the same 37a, I just know that the videos I’ve seen are in the low 40a range, but up to low 50a range when fully loaded.
Note that the flight speed was in the neighborhood of 9 to 11m/s prior to the dropped wing and videos of similar builds start around the same speed at launch, but quickly increase from there.
One of menschel’s suggestions was to check if the motors are set as left/right correctly. I just finished re-gluing the plane today, I will check this once it’s back on the bench. In the meantime, I did check that the correct props (CW/CCW) are on the correct motors re direction of rotation.
Some combination of factors mentioned above?
I’m not disregarding the potential relevance of the motors or props, it’s just that this plane flew successfully a half dozen or so times with this combo prior to me buying it. So, if thrust was the problem, it seems safe to say the root cause is somewhere upstream.
CG was setup carefully and checked based on the documented CG location for this plane. I talked to my friend earlier and he confirmed that CG was slightly nose heavy.
I really appreciate everyone that’s been helping. This plane will be used for Search and Rescue, so thank you very much for your input.
Edit: elevation at the test site is around 1340 meters, temps around 30c.
is your video a slow motion or realtime.
It take about 15sec between launch and crash. On 9m/s this is round about 100+ meter.
But it looks much less at the video.
Also you compare your launch with others but you also states that the other increase quickly the speed. This don’t happen on your video.
So I think both the speed at launch and the motor thrust are to low an this caused the crash.
The 1st video (phone angle) is slow motion. The 2nd video (head mounted) plays in realtime, then one more time in slow motion. If it helps, I can make the 1st video realtime too.
I’m just comparing speeds at the moment of launch. One of my questions is about whether or not the significant control surface deflections (and/or the resulting angle of attack) played a part in preventing the speed to increase adequately.
IMHO, without addressing that, we aren’t in a position to say confidently that the speed or thrust at the moment of launch was insufficient.
