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2m kfm2 airfoil trainer plane built

#1
Hi
Does anyone know how i can get some dimensions to build a 2m wingspan trainer using kfm2 airfoil with a total weight of 1000g?
How large does my wing chord need to be?
Tailplane size / area?
Fuselage?
Control surfaces...
Motor sizes?
Etc.
All help will be much appreciated
 

quorneng

Well-known member
#2
AJ114
The simplest solution would be to find a 2m span trainer and copy its dimension. The difference in performance between a kfm2 airfoil and a normal one will not be that significant. How you build it and the motor/prop/battery combination is rather more likely to determine how it will fly.
 
#3
I have found some resources that the fuselage needs to be 75% of the wing span...
What might happen if i slap on a smaller fuselage to the longer wing?
The horizontal and vertical stabilizer area is said to be 25% of the wings area...
Its going to look funny if i put it on a smaller fuselage... would this create a problem for me?
Dimensions::
material are 5mm depron
Fuselage length is 90cm x 7.5cm wide
Kfm2 airfoil span 2m x 20cm chord
Tail area is 25% of airfoil
Planning to put on a 3550 lipo 3s
2200kv brushless motor with a 3 blade 5050 prop

Any ideas?
 

JasonK

Well-known member
#4
Run your items through this to find your CG-> https://www.ecalc.ch/cgcalc.php, I don't think that finds your stability, just your NP/suggested CG point.

So, your tail is working on a lever arm, so if your reducing your fuselage (length from the wing to the tail), you are reducing the effectiveness of the tail, so you will either have less authority or need a larger control surface.

a few things to look at:
https://www.flitetest.com/articles/easy-aircraft-design
https://www.flitetest.com/articles/propeller-static-dynamic-thrust-calculation
https://forum.flitetest.com/index.php?threads/determining-control-surface-area.62991/#post-565944
 

quorneng

Well-known member
#6
There really is no hard and fast rule about plane geometry. It rather depends on what you want the plane to do and what you make it from.
Model gliders tend to have fuselages around 1/2 the wing span where as many fast or aerobatic designs have a wing span the same as the fuselage length. 75% is a reasonable average.
A JasonK points out the rear of the fuselage acts as lever. The stability of the plane is effected by the effectiveness of this lever. The longer the rear fuselage the smaller the tail plane (or fin) needs to be to generate the same lever action.
The material the rear fuselage is made of is likely to limit how long the fuselage can be without it becoming too heavy. A carbon fibre tube for example can be very slender and still be rigid enough.

Aerodynamics and plane stability are very complex subjects and there are a great many variables at work so following the proportions of an existing design is always a good starting point but deviate too far from it and the plane may become difficult or even impossible to fly.

A 5 x 5 prop sound very small for a 1 kg plane. With a 2 m span and a 10:1 aspect ratio made of 5 mm Depron it is unlikely to be structurally capable of flying fast so you would be better with a low Kv motor (1000 or even less) turning a bigger (10 x 3.8?) prop slower. It would also be considerably more efficient at converting the Watts into usable thrust.
I presume your big battery is to obtain endurance.
This layout might give you some ideas.
03Jun17.JPG

60" (1520 mm) span. A 'built up' (Clarke Y section) wing from 3 mm Depron with a balsa spar. Fuselage 6 mm Depron with a glass fibre tail boom. A 3000 mAh 3s driving a 950 kV motor turning a 9 x 3.8 prop. At 565 g all up it was a slow flyer but could climb vertically at full power. At minimum cruising power it had an endurance of over an hour. It could glide (& thermal) pretty well too.
 
#10
Let hope this flies well... fingers crossed ....
I realised that my tail was angled alittle to the right.... :( i hope thats not going to cause any problems...
 

quorneng

Well-known member
#13
Looks good.
How much does it weigh? Is it 1 kg 'ready to go'?
I hope you have a nice smooth surface to take off from with those rather small wheels.
As I have said that 3 blade 5 x 5 prop is small compared to the size of the plane.
It would be useful to know just how much thrust there is. You really need thrust equal to about 75% of the weight as a minimum to be sure of it having the power to fly.
 
#14
Looks good.
How much does it weigh? Is it 1 kg 'ready to go'?
I hope you have a nice smooth surface to take off from with those rather small wheels.
As I have said that 3 blade 5 x 5 prop is small compared to the size of the plane.
It would be useful to know just how much thrust there is. You really need thrust equal to about 75% of the weight as a minimum to be sure of it having the power to fly.
1kg ready to go
Measured thrust at 800grams
Waiting for wind to calm to maiden.
I put expo on tx to -50% aileron and -40% for elev what do u think
 
#15
I tried uploading video of maiden but wont allow me coz its mp4.
This is how it went
It was windy... 20kph
Took off str8 away.. engine definitely too small
Ac kept pitching up aggressively with power on logitudinal stability great
Lateral stability a problem. Always needed to pitch down .
I tilted motor down but didnt do much... cg was around 27 28 percent...
Controls were all sufficient and effective with expo set to -50 for aileron and -45 for elevator...
 
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quorneng

Well-known member
#17
Without actually seeing how it flies it would be hard to advise what might be wrong. Any chance you could convert the video or download it to YouTube as it will then automatically be converted to an acceptable format.
It may be useful to see how it glides. From a hand launch It should maintain a constant attitude and speed as well as travelling quite some distance, say 30 ft (10 m) without touching the controls at all.
 

Ketchup

4s mini mustang
#19
What video format is supported in the forum?
Just upload to YouTube and use the “insert media” thing to post it here. Also, power on climb and power off nose dive? It seems like your thrust angle is too high up. You should normally have a little bit of down and right thrust angle. Also, the plane should glide fine without any throttle. Finally, about what you said with your motor not having enough power. If you decide to change the motor, get a similarly sized motor with a much lower kv that can run larger props, this will give you less pitch speed, but much more thrust and probably efficiency.
Another quick question here... What do you mean by longitudinal and lateral stability? Is longitudinal along the length of the plane and lateral is along the wing? Usually in aviation we use the pitch, roll, and yaw axes. Elevator is for pitch, ailerons are for roll, and rudder is for yaw.
 
#20
Just upload to YouTube and use the “insert media” thing to post it here. Also, power on climb and power off nose dive? It seems like your thrust angle is too high up. You should normally have a little bit of down and right thrust angle. Also, the plane should glide fine without any throttle. Finally, about what you said with your motor not having enough power. If you decide to change the motor, get a similarly sized motor with a much lower kv that can run larger props, this will give you less pitch speed, but much more thrust and probably efficiency.
Another quick question here... What do you mean by longitudinal and lateral stability? Is longitudinal along the length of the plane and lateral is along the wing? Usually in aviation we use the pitch, roll, and yaw axes. Elevator is for pitch, ailerons are for roll, and rudder is for yaw.
Will work on the youtube. I dnt have an account just yet.
I did adjust thrust angle maybe it needs more pitching down...
The glide was bad... perhaps due to the heavy headwind kept it pitching down then again my tRims were all over the place because i trimmed when I had power on.
2300kv with that prop gave me 800g of thrust with the propeller that came with it it was two bladed and a tad longer... gave me less thrust when i checked it on the scale. For that reason i went with that 3 blade 5050 .
In regards to stability yes i did state them incorrectly. Its been a while since i read some theory and unfortunately i did get them the other way around. Longitudinal stability which is about the lateral axis i.e pitching moments. And lateral stability is about the longitudinal axis- rolling moments. My problem is rather with the lateral stability. (Private pilot theory by bob tait)
So here we have either my trims were set bad because my thrust angle perhaps...
maybe cg should of been set to 20-25%chord....
Somthing worth noting is i did have the tailplane slapped onto my fuselage at -2deg angle of incidence and wing was set to +2degrees angle of incidence.... ??
What do you think? Need your help!