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Feedback Request! Failed Test flight.

Hai-Lee

Old and Bold RC PILOT
#2
Hello!
I have a team at school that competes in an aero design competition every year. This year our aircraft had some trouble as once it was off the ground it yawed hard and crashed. I would love any feedback as to why the plane reacted as it did. If you have any questions please let me know!
Thank you in advance!
To me and only based upon what I saw in the video clip it looked like a serious issue with "P" factor followed by what appears to be a stall.
The nose of the craft is low in drag and the take off was at quite a low speed. As the bird rotated I fear there was a degree of masking of the rudders and so the rudders became more effected by the propeller stream pushing the plane to port.

My approach, But I am not sure what control inputs you applied, is to definitely check the rudder operation for correct direction, increase the fin and rudder area and I would add some serious side thrust on the motor installation before any further attempt and perhaps a touch of down just for luck. In addition make the take off run longer or faster before rotation.

Just my thoughts!

have fun!
 
#3
To me and only based upon what I saw in the video clip it looked like a serious issue with "P" factor followed by what appears to be a stall.
The nose of the craft is low in drag and the take off was at quite a low speed. As the bird rotated I fear there was a degree of masking of the rudders and so the rudders became more effected by the propeller stream pushing the plane to port.

My approach, But I am not sure what control inputs you applied, is to definitely check the rudder operation for correct direction, increase the fin and rudder area and I would add some serious side thrust on the motor installation before any further attempt and perhaps a touch of down just for luck. In addition make the take off run longer or faster before rotation.

Just my thoughts!

have fun!
Thank you for your feedback! As for your last suggestions the competition restraints were that the airplane had to take off within 100 feet. The white line right as the plane takes off is the 100' mark.
 

Hai-Lee

Old and Bold RC PILOT
#4
Thank you for your feedback! As for your last suggestions the competition restraints were that the airplane had to take off within 100 feet. The white line right as the plane takes off is the 100' mark.
OK well all said previously still applies. You have not given the aircraft weight or the motor/battery/prop combination. So I can only guess where to go but I would seriously consider either a larger cell count on the battery OR to fit a larger diameter prop with a lesser pitch, AKA a slow fly prop. The SF prop will have a lower top speed but will develop max thrust for the lower speeds around take off. I addition SF props generate less "P' factor and so the side thrust angle can remain moderate.

Just my thoughts

have fun!
 

speedbirdted

Well-known member
#5
To me, it looks like the plane lacks lateral stability. I'm going to guess this is because the rudders are too small, or the rudder controls might be backwards or the control throws not large enough. Their airflow might also be blocked by the fuselage, though I don't think this is likely. I don't actually think there was a stall here either - usually when an airplane enters a stall condition as a result of unintended yawing, the wing on the inside of the turn drops first. Here it looks like the airplane remained fairly level as it started turning and there was no sudden, sharp loss of altitude that is also common to stalls - it went up, and then went back down at about the same speed.

It also seems that the airplane might not be at fault at all. I see it started to straighten out before it hit the ground - usually, airplanes that enter an uncontrollable yaw go totally sideways before entering any number of weird orientations, all of which end with plummeting to the ground. However, it does look like the pilot applied up elevator, but it didn't help. Maybe the elevators are also too small?

Did you have any significant design criteria that meant you absolutely had to use twin rudders? If not, get rid of them. Just add a big tail fin like most airplanes have...
 
#6
To me, it looks like the plane lacks lateral stability. I'm going to guess this is because the rudders are too small, or the rudder controls might be backwards or the control throws not large enough. Their airflow might also be blocked by the fuselage, though I don't think this is likely. I don't actually think there was a stall here either - usually when an airplane enters a stall condition as a result of unintended yawing, the wing on the inside of the turn drops first. Here it looks like the airplane remained fairly level as it started turning and there was no sudden, sharp loss of altitude that is also common to stalls - it went up, and then went back down at about the same speed.

It also seems that the airplane might not be at fault at all. I see it started to straighten out before it hit the ground - usually, airplanes that enter an uncontrollable yaw go totally sideways before entering any number of weird orientations, all of which end with plummeting to the ground. However, it does look like the pilot applied up elevator, but it didn't help. Maybe the elevators are also too small?

Did you have any significant design criteria that meant you absolutely had to use twin rudders? If not, get rid of them. Just add a big tail fin like most airplanes have...
The design did not require twin tail, it was chosed so that the vertical stabilizers would be out from behind the fuselage. I'll definitely take note of that though. The pilot flying the plane said he had very little control during the flight.

I, too think it's probably something with the tail. Whether it be undersized or not far back enough to provide a big enough moment. Here's the tail:
20200326_110641.jpg
 
#7
OK well all said previously still applies. You have not given the aircraft weight or the motor/battery/prop combination. So I can only guess where to go but I would seriously consider either a larger cell count on the battery OR to fit a larger diameter prop with a lesser pitch, AKA a slow fly prop. The SF prop will have a lower top speed but will develop max thrust for the lower speeds around take off. I addition SF props generate less "P' factor and so the side thrust angle can remain moderate.

Just my thoughts

have fun!
Definitely could have had a better prop/motor combo. As for the battery we are required to have a 6 cell Li-Po with minimum 3000mAh 25c, also with a 1000 Watt power limiter.
 

Piotrsko

Well-known member
#9
What I see is a definite left turn commanded or otherwise. Also a noticeable left yaw on start up . I presume 3 things: the plane wasn't test flown, this is the maiden. 2 the pilot knows how to fly, preferably as a test pilot, and verfied the radio set up as correct. 3 in the post crash photo I see a definate left turn built into the left rudder so was the plane inspected for straight before flight?

Until the left yaw, it appears to be flying stable, the turn is flat so it's controlled somewhat, it's responding to elevator and aileron. If it was me, I would say uncoordinated left thumb syndrome. Imho, it's way overcomplicated ( more servos than I use on 3 planes just on rudder and elevator) so even suspect brown out or LOS event. I agree with @Hai-Lee on torque effect and possible rudder blanking. I don't see the stall.
 
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Merv

Well-known member
#10
I agree with the others, it could be P factor.

Hard to tell from the picture, is the left vertical fin off? It looks like it may have been build with a left turn bias. The 2 vertical fins don't look parallel to me. This appearance could be just from the angle of the camara.
 

Hai-Lee

Old and Bold RC PILOT
#11
Just another thought. From what I can see of the structure there is a lot ow weight there! The vertical fins obviously provided only small corrective forces and yet they are so strong in structure and weight. The real problem could be as simple as a severe weight problem or the weight could be making other problems even worse.

Have fun!
 

FDS

Well-known member
#12
That looks like a tubby bunny!
I think you are right that there doesn’t look to be much moment there, the tail is very close to the wing, that usually makes for a very pitchy ride. A single tall vertical stabiliser with a thinner cross section would probably have been much more effective.
It looked to be climbing slowly with a slight left turn then it stalled, probably the pilot feeding in elevator or a loss of control somewhere? I thought it was going to keep rising gently once it lifted off, that bit looked OK.
What was your weight distribution like?
 

Tench745

Well-known member
#13
So, no one has mentioned it yet as such, but you have a lot of fuselage out ahead of that wing, and the sides are very flat. Basically, the fuselage is acting like a big vertical stabilizer out in front of the plane. As soon as there was airspeed enough a little bit of yaw would build air pressure on the side of the fuselage and push it further off course. Some of those really long racers from the 30's like the Caudron, Goon and Firecracker had big slab sided cowlings way ahead of the wing and they could suffer "rudder lock." Basically, if they yawed far enough the rudder became ineffective and couldn't push the nose back straight ahead.
I surmise that when you couple that with the short tail of your aircraft which limits the effectiveness of the rudder, and the big flat fuselage blanking out the left v-stab.... you get the results seen above.

Edit: Also those are some unnecessarily thick control surfaces and that's a lot of plywood in the structure. Plywood add quite a bit of weight for little extra strength compared to balsa. I don't think it factored into this crash, but it for a competition aircraft I would think weight savings of any kind would generally be beneficial.
 

Hondo76251

Well-known member
#14
I watched it several times and I still cant see a stall. I see pilot error or reversed control surface. It taxis straight, lifts off level, then as soon as the pilot adds control input it goes into a slip then a nose dive, both look to be induced by the controls... intended or not.
 
#17
So, no one has mentioned it yet as such, but you have a lot of fuselage out ahead of that wing, and the sides are very flat. Basically, the fuselage is acting like a big vertical stabilizer out in front of the plane. As soon as there was airspeed enough a little bit of yaw would build air pressure on the side of the fuselage and push it further off course. Some of those really long racers from the 30's like the Caudron, Goon and Firecracker had big slab sided cowlings way ahead of the wing and they could suffer "rudder lock." Basically, if they yawed far enough the rudder became ineffective and couldn't push the nose back straight ahead.
I surmise that when you couple that with the short tail of your aircraft which limits the effectiveness of the rudder, and the big flat fuselage blanking out the left v-stab.... you get the results seen above.

Edit: Also those are some unnecessarily thick control surfaces and that's a lot of plywood in the structure. Plywood add quite a bit of weight for little extra strength compared to balsa. I don't think it factored into this crash, but it for a competition aircraft I would think weight savings of any kind would generally be beneficial.
That definitely seems like it could be the case. As for the structure, it is completely balsa wood. The CG of the aircraft was between 25-30% of the chord of the wing. the overall aircraft weighed about 19 pounds but was also carrying 7 soccer balls at about a pound each (26 pounds loaded).
 

Figure9

Well-known member
#18
That definitely seems like it could be the case. As for the structure, it is completely balsa wood. The CG of the aircraft was between 25-30% of the chord of the wing. the overall aircraft weighed about 19 pounds but was also carrying 7 soccer balls at about a pound each (26 pounds loaded).
How were the soccer balls secured & was it possible for them to contact any control rods or servos?