Need some help regarding max weights for take off with high kv motor

[Nikhil Rathore]

My self Nikhil Rathore and recently i made one new plane that is looking slightly similarly to ft explorer but it has a wooden wing spar and i use a different kind of foam board and together all electronics it is weighting approx 2 kg and the motor i am using is NTM Propdrive 2836 3000kv with 5x5 prop and 60a ESC it is giving 1.3-1.5 kg of thrust. Is it enough for take off if i use a runway.If not then help me what to do?

 

[Konrad]

Why do you think you have a problem? Is it because you are measuring static thrust that is lower than your all up weight?
Now, I wish many of my models had a thrust to weight ratio of 0.65:1 as it would help with the climb.
Please be aware that static thrust from a propeller has little or no bearing on how the model will fly.

Remember a wing makes lift by how fast it flies in the air.

If you want more help we will need the battery voltage, wing area and airfoil type?

 

[Nikhil Rathore]

thank you so much for helping me
the battery is ZOP 4s 30c 4500mah from banggood.com and wing area is 1806cm^2 and foil type is flat bottom.

 

[Konrad]

Wow, now I see your concern!
Why so heavy (2 kg), do you need such a heavy battery? At your weight I get a stall speed of around 55kph
Your power set up looks like it is one for very high speed model, low drag. I get a pitch speed of 270kph. With the 5x5 prop and this motor the prop will be stalled until the model reaches about 90kph.

With that high a wing stall speed and that high a prop stall speed hand launching is out! I'd think you'd need a bungee launch, or a very long smooth take off runway.

Can you change to a 3 cell battery and run a 6x4 prop? This will still leave you with about 270 Watts/KG and the prop will not be stalled. Your wing stall speed will still be rather high. Getting your weight down to 1900g with the battery change from a 3 cell from a 4 cell will lower your stall speed to about 50KPH. Still far too high for a trainer like model.

 

[Nikhil Rathore]

can i use a 4s 3000mah battery because i dont want to fly with 3s and if the avg apm consumption is 35 how much it will give the flight time? and one more thing as you said 6x4 but the company data shows that max prop size is 4.75x4.75?

 

[Nikhil Rathore]

 

[Konrad]

Are you aware of the relationship between the voltage and prop size (load)?

Can you link to the data sheet you're looking at? My data is showing that the motor can handle 56 amps for the time it takes to drain a 2200 mAh 4 cell battery.

4.7x4.7 sounds like the Graupner Cam prop. It is a very nice speed prop.

Dropping down from a 4 cell to 3 cell allows you to then turn a much larger prop. The 3000KV motor you have is not a good match for 4 cells and that heavy draggy model. Running a 6 x 4 APC prop on 3 cells should allow the motor to still draw 50+ amps.

Since a prop is basically a twisted wing they do stall. This means that the air is whipped more than being moved aft. With the 5x5 prop the air hitting the prop blades need to be going aft at about 90kph to allow the air to reattach to the prop's airfoil. You might have heard the guys use the term the prop hooked up. This is what they mean, the prop is now moving are aft to make the model go forward.

If you want to stay running 4 cells I think you will have to move down to a lower KV motor.

 

[Nikhil Rathore]

Are you sure if I shift to 3s and 6x4 prop definitely it will fly? And what amount of battery capacity should I use if avg amp consumption is 35 and I want it to fly for ~10min and a small doubt if with previous setup I manage however to take off will it fly ?

I am asking too many questions please don't hate. THANKS IN ADVANCE

 

[Konrad]

At 35 amps to fly for 10 minutes will take 5800 mAh cells.

My concern is the wing loading 2Kg over 18DM^2 (111g/dm^2). So no I can't guaranty it will fly. In fact I don't think it will. 55km/h is a very high stall speed. But a bigger prop will help get her to flying speed.

 

[Nikhil Rathore]

guide me what should i use for pushrod of servo? and i tail wheel is important

 

[CrossedUpChris]

You need a lower kv motor and a bigger prop. That motor/prop combo on a 4s is designed to send a tiny prop jet model to ballistic speeds. If you're planning on running 4s with that battery and esc you want something like a 3536 - 3542 motor somewhere in the 1000-1250 kv range with something like 9x7 prop. Possibly a bigger prop still with less pitch but that would depend on the specific motor and it's current rating. That would still make for a fast model but you would need the speed to haul that battery around.

I know that is not what you want to hear but I'm afraid that's the truth.

 

[Konrad]

Tail wheels it depends. On light weight models or ones that will have short take off runs I just use a skid. On heavy models (like yours) or ones where I want a smooth graceful take off I use a tail wheel. One thing I try to incorporate in my tail wheel is a fuselage bracket. This takes off a lot of load from the rudder hinges.
https://dubro.com/products/tailwheel-bracket

For push rods I try to make them out the same basic material as my model's structure. This is to control trim changes as a result of thermal expansion with the longer push rods. For my wood models I like 1/4" or larger square balsa. For plastic models I like Nyrods that are well braced. If I can't brace the Nyrods I then often use 3mm stiff carbon with 1.5 mm wire bonded to the carbon rods. For short runs I like solid threaded wire.
https://www.sullivanproducts.com/GoldnRodMainFrame.htm
http://www.parkzone.com/Products/Default.aspx?ProdID=PKZ1218