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Mar 31, 2020, 10:14 AM
crazy innovative engineer
Thread OP
Help!

Any idea what could be wrong with this airplane?


Hello recently I tried my new home built 3D printed airplane. Itís behaviour was just so weird and it went down in matter of seconds.
It weights about 1400g and itís wingspan is 145cm.
Has anyone experienced the same? Can you help me please?
Here is the link of the captured flight:
1 (0 min 4 sec)
Last edited by Adam 1; Mar 31, 2020 at 10:40 AM.
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Mar 31, 2020, 10:55 AM
A man with too many toys
Looks like the angle of the V-tail looks wrong. The standard is 110 degrees between the two sides. Yours might not have enough tail volume for proper stability - Wrong V angle makes that worse.

Did you do any calculations to make sure that the cg is correct. For V-tail airplanes you need to make sure your calculations are correct.

.
Mar 31, 2020, 10:58 AM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
Can you show us a video of the elevator trim , and the amount of elevator travel ?
Mar 31, 2020, 11:02 AM
crazy innovative engineer
Thread OP
I am not very sure if the cg is correct, I tried to calculate it but my calculations may be slightly imperfect.
The V tail angle is 110
Mar 31, 2020, 11:04 AM
crazy innovative engineer
Thread OP
V tail detail
2 (0 min 9 sec)
Last edited by Adam 1; Mar 31, 2020 at 11:10 AM.
Mar 31, 2020, 11:37 AM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
You may not have enough elevator travel . You can increase the travel by moving the pushrod connection to an outer hole on the servo arm , and/or moving the pushrod connection to an inner hole on the control horn .

Also , make sure the elevator direction of travel is correct : when you pull the transmitter stick towards you ( stick down ) , the elevators should go up . When you push the transmitter stick away from you ( stick up ) , the elevators should go down .
Mar 31, 2020, 12:22 PM
crazy innovative engineer
Thread OP
I agree that Increasing the elevator travel is probably be necessary. However it was maybe more pilots mistake. The airplane was climbing at very high angle at low speed and then the pilot stopped climbing and tried to accelerate the plane, however it was not high enough to curve the “acceleration fall” and hit the ground. This aeroplane has a weaker motor than it should have, so next time I may try to climb higher and then accelerate.
Mar 31, 2020, 12:51 PM
AndyKunz's Avatar
What prop is on it? It sounded like it was screaming, but not going anywhere. Typical under-propped situation.

Andy
Mar 31, 2020, 12:55 PM
crazy innovative engineer
Thread OP
Dlapro cyclone, prop for drones with high rpm drone engine
Mar 31, 2020, 01:05 PM
A man with too many toys
Quote:
Originally Posted by Adam 1
Dlapro cyclone, prop for drones with high rpm drone engine
1. 3-blades props are less efficient than 2 blades props.

2. APC props are more efficient than other brands. Find an APC 2-bladed prop that is 1 or 2" larger diameter and more pitch. Drone props tend be too small for airplanes. Exactly what is the diameter and pitch of your prop - that way I can recommend a prop to try.

3. It could be that the power system is too small for your airplane and 3D printed airplanes tend to be much heavier than other construction methods. Since you have one drone motor your airplane should be 1/3 to 1/4 the weight of the drone that the motor was used on.

.
Mar 31, 2020, 01:06 PM
B for Bruce
BMatthews's Avatar
At 145cm and 1400gms we're not talking about a very light wing loading. So it's going to need a good turn of speed or at the very least a greater degree of acceleration. And what I'm seeing in the picture is a model which simply did not have enough speed to fly. And didn't accelerate fast enough to get the speed needed before it hit the dirt.

In the first video look at your throw. You take a few steps that are more a fast walk than a run and then actually stop on the last step as you push the model upwards instead of directly out. So you gain nothing from your "running" because you stopped as you threw the model.

But you also threw it upwards with the model fairly flat instead of along the line of flight. As a result the actual forward speed of the model was very low. I have a feeling that the wing simply never got going fast enough in the right direction to come out of a fully stalled condition.

You or a helper need to give the model a far stronger throw and put all the power and velocity into driving the model along the line the nose is pointed at release to get a better chance of flying from a hand launch. Forget about running. Too many folks can't smoothly run and give a full power throw all at the same time. Olympic javelin competitors practice for years to get it right. And use your strong hand. You launched left handed. Are you in fact left handed? Nothing you can do on the controls will save the model if it's not up to flying speed.

Speaking of practicing that is something you/we/your helper can do. Get a meter length piece of building lumber about the same cross section as your model and weight the nose so it sort of balances like your model and at the 1400gms total weight. Use that to practice giving the model a good strong throw.

Get your body side on with your strong arm and leg to the rear in a martial arts like stance with feet fairly wide apart. Now use your back leg, upper torso and arm back and all at once use all three to throw that stick like a javelin. You should be able to throw it at least 10 meters like this. And for sure 8 meters would be a minimum. Have your camera buddy video these and work on your style so the stick is aligned with the direction of the throw quite closely.

With that all good you're ready for another trial which should be a LOT more successful.


What motor and prop is it on there? Any idea of the power it provides? It's obviously a racing quad motor and prop. But I see from the pictures that you've got a big flat rear of the fuselage pod blocking part of the prop. Also in general pusher props are not as effective due to the disturbed air coming off the fuselage and wing. Using a motor that can spin a bigger prop can help with that a bit but it's still an issue. You might benefit from making a higher mount for the motor so you can use a larger diameter two bladed prop instead of a small three blade. But try the stronger launch practice first.
Mar 31, 2020, 01:08 PM
aka: A.Roger Wilfong
gnofliwr's Avatar
The pusher prop at the rear edge of the wing explains the scream.

The prop looks to be a low pitch quad prop - ment more for stalled thrust than actual speed. Try anothe inch on the pitch. If you're changing the prop pitch or diameter, check the amp draw with a Whatt meter.

In the video, it looked like a down command pulled the plane out of the initial climb, but it rotated into a nose down angle and the nose didn't come back up - maybe CG or elevator authority, or slow/dumb thumbs.

- Roger
Mar 31, 2020, 01:33 PM
crazy innovative engineer
Thread OP
Thank you so much for your ideas.
I totally agree that my throw was terrible, It would be better and stronger with my right hand, but I rather than that, I decided to control the elevator at the initial point of the flight.

The weather might be great tomorrow so I may practice the throw and actually throw it better just in a small degree of climb, and hope it flies.

The thing I am still concerned about is the tail efficiency. I managed to use the V tail to not get that much interference from the prop, however I still think that the distance between the cg and the tail should be greater for more stability. Sadly I am not able to push the tail more to the back, because it would change the cg and I cannot balance it by adding extra weight in the front.
Mar 31, 2020, 02:23 PM
An itch?. Scratch build.
eflightray's Avatar
Have you used a CG calculator such as ---

https://rcplanes.online/cg_calc.htm

Use the plan view of the stab for dimensions, don't flatten it out.

Regarding power, you should feel the model wants to really pull out of your hand to fly well..

.
Mar 31, 2020, 02:25 PM
WJH
WJH
B777F Captain
WJH's Avatar
Quote:
Originally Posted by Adam 1
Thank you so much for your ideas.
I totally agree that my throw was terrible, It would be better and stronger with my right hand, but I rather than that, I decided to control the elevator at the initial point of the flight.

The weather might be great tomorrow so I may practice the throw and actually throw it better just in a small degree of climb, and hope it flies.

The thing I am still concerned about is the tail efficiency. I managed to use the V tail to not get that much interference from the prop, however I still think that the distance between the cg and the tail should be greater for more stability. Sadly I am not able to push the tail more to the back, because it would change the cg and I cannot balance it by adding extra weight in the front.
Don't change CG by adding weight, do it by shifting weight. CG generally falls around 1/3rd the width of the wing, behind the leading edge. Balance the CG around there.
Is this your first time flying fixed wing?


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