Help! Plane does not recover from dive

Boiii

Member
Hello,
today I tried to do a roll with my DIY plane( I have done some in the past.). When the plane was upside down it pitched down 90 degrees and crash in to the ground. I am assuming that it stalled because the speed was too low, but I don't know what caused it to not recover. I can think only two possible causes for this 1) The elevator area is too small ~20% 2) I did not cut the throtle( I did not touch it, it eas about 60%)

thank you in advance,
Martins
 

Piotrsko

Master member
I would first suspect a radio glitch since it appears to have been successful in the past. Possibly a null point by pointing the antenna directly at the plane. Second would be a cheap elevator servo. You don't mention moving any other controls during the down part, but I assume you tried to bend the right stick.
 

Boiii

Member
This was the 2nd time something like this happenned, the first time I stalled and never recovered and crashed in ground nose first
 

quorneng

Master member
How easily a plane rolls depends more on the power of the ailerons compared to the wing span than anything.
You will be creating extra drag during the roll and loosing lift so more power and or speed may be required.
If the roll is slow lots of power gain some speed then go into a gentle climb and roll. Then the nose can drop a bit during the roll before the plane is even pointing at the ground as shown by this slow light pusher .
 

quorneng

Master member
If you don't get into a nose down position you don't have to recover. ;)
Without a picture of the plane it is hard to determine if its basic aerodynamics were at fault.
Unlike a conventional stall in an inverted stall the plane is almost bound to rotate to full vertical before it even begins to restore the correct airflow over the wing. Apply any recovery action before it has sufficient air speed and it simply may not.
How high were you at the stall?
 

Boiii

Member
I was about 10 -15m high when it happened. It hit the ground at I would saz 2x the cruising speed. But I can' t get out of my head that I did not chop the throtle
1645416548220.png
 

Boiii

Member
If you don't get into a nose down position you don't have to recover. ;)
Without a picture of the plane it is hard to determine if its basic aerodynamics were at fault.
Unlike a conventional stall in an inverted stall the plane is almost bound to rotate to full vertical before it even begins to restore the correct airflow over the wing. Apply any recovery action before it has sufficient air speed and it simply may not.
How high were you at the stall?
The tail is the same height as wing, maybe that is at fault?
 

Piotrsko

Master member
Looking at the picture, some issues with design: CG location too forward, not enough elevator authority, flat bottom wing or not enough wing area to avoid high speed stall inverted. Not visible: wing loading.

I've been flying forever, and I still forget throttle in a crisis. That doesn't stop the crash, only makes the hole deeper.
 

Boiii

Member
Looking at the picture, some issues with design: CG location too forward, not enough elevator authority, flat bottom wing or not enough wing area to avoid high speed stall inverted. Not visible: wing loading.

I've been flying forever, and I still forget throttle in a crisis. That doesn't stop the crash, only makes the hole deeper.
what do you mean by"CG location too forward "?
 

SSgt Duramax

Junior Member
It could be any and all of the above. I am voting for the elevator being too small.

I've had a few crashes like that.

I have one of the $20 hobby king radios. It was a waste of $20. It randomly browns out and plants the plane into the ground. Right now it is on a 100g EPP plane, so at least when it wrecks nothing gets hurt.

One was in my chipmunk, I had it flying great, then my elevator control horn popped out and the plane was nose heavy. Straight into the ground, you could hear the thud, it was bad.

Just yesterday I was flying my wing. My right elevon was acting janky, but I tossed it anyway. About 50 feet away from me I realized I had little to no roll authority, and very little pitch. All I could do was chop throttle, and wait for it to go down. Turns out one of my servos had loosened up during a rough landing and my right elevon came unhooked. Good thing that plane is EPP.
 

Bifi.baarlo

Well-known member
All the Flite test planes with non symmetrical wing need a lot of stick movement on the elevator when you fly inverted, when you roll them upsitedown and the elevator stick is in the middle, they nosedive, especially when you don't have enough speed, wich you said you had. In my opinion it was pilot error.
 

Flightspeed

Convicted Necroposter
All the Flite test planes with non symmetrical wing need a lot of stick movement on the elevator when you fly inverted, when you roll them upsitedown and the elevator stick is in the middle, they nosedive, especially when you don't have enough speed, wich you said you had. In my opinion it was pilot error.
I broke a few planes learning to fly inverted🤣 it’s definitely a learning curve
 

Tench745

Master member
Hard to say for sure what caused the crash, but I can say that elevator looks too small to me. Typically the horizontal tail (h-stab and elevator together) is around 20% of the wing area and the elevator makes up about 1/3 of the horizontal tail's area.
 

quorneng

Master member
I would say the elevator is on the small side but how effective it is very much depends on the CofG position which is also coupled to the longitudinal wing to tail incidence (decalage).
If the decalage is big the tail plane is pushing the tail down so the CofG has to be set forward. The plane will have very positive pitch stability but applying up elevator will not have a huge effect.
If the decalage in modest the tail plane is not pushing down so hard, the CofG is thus set closer to the wing CofP so the same size elevator becomes much more powerful. The plane will become much more sensitive.
In a DIY plane this sort of thing requires trial and error testing.
A DIY design with a modest elevator?
NewFuseFin.JPG

With a small tail plane & elevator clean fuselage aerodynamics can help. The small elevator only needs a small servo so in the above plane it is buried in the fin under the tail plane.
 
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SSgt Duramax

Junior Member
All the Flite test planes with non symmetrical wing need a lot of stick movement on the elevator when you fly inverted, when you roll them upsitedown and the elevator stick is in the middle, they nosedive, especially when you don't have enough speed, wich you said you had. In my opinion it was pilot error.
Exactly. My FT P40 requires about 50% forward stick and tons of aileron shennanigans to fly inverted. Its natural tendency is to either nose dive like you say, or try to self right on the roll axis.