Ft Viggen does not fly :(

[Nicodepico]

Help! My flite test viggen does not fly. i bought it from someone without the lipo and the controller/receiver. I got a 3s 5400 mah battery. Maybe it is not the perfect battery becouse of the weight(400g), but i dont have something else. But when i tried Flying with it that did not go so well. The first times i tried taking of from a runway. It got pretty some speed, but i lt just would not fly. Then i tried hand launching it, but that was a failure. It looked to be Flying for a second, but then it started pointing its nose down and crashed. Could someone please tell me what is happening? It has twin 50mm edf and has around 700g of thrust. The plane itself weights around 1300g, so maybe its thrust to weight ratio is not good? Becouse of the heavy battery, maybe its CG is not in the right spot? It is about 2cm in front of where it should be in the plans. Should i find a better place for the battery? And is the GC from the plans in the right spot? And if i repaired the plane, should i hand launch it, or do a normal take-of?
Nico haartsen

 

[Foamforce]

I don’t have a Viggen, or any experience with EDFs, but nobody else has answered. Maybe my answer will inspire others to correct me. 😂

You likely have a few problems, all caused by the battery. The first is that it’s too heavy. If you had enough thrust, it would still fly, but that leads to the second problem. The plans appear to call for a 4s battery, which would provide 33% more voltage than your 3s, so it’s underpowered. 3s seems to work fine with the bigger EDFs, but I think the smaller 50mm will need the extra speed to provide enough thrust. Lastly, due to the extra weight, you’ll have to make a pretty big adjustment to your COG. It sounds like you’re nose heavy right now? If that’s the case, you’ll need to try moving that battery back until it balances properly on the indicated COG mark.

All of that could be solved by getting the recommended battery, which is a 2200-3000mah 4s. However, I’m still concerned about the weight. You say 1300g with that battery? The Viggen supposedly weighs 574g without the battery. If your battery is 400g and your dry weight is 900g, you’re over 50% overweight. That’s probably flyable, but you’ll have to fly fast. If that’s the case, go on the smaller size for the battery, so probably a 2200mah 4s.

Good luck!

 

[Merv]

... so maybe its thrust to weight ratio is not good?

No, your thrust to weight ratio is low. You need more thrust or less weight.
Get the thrust to something like 80-100% of the weight for a good flying plane. If you want speed, the thrust will need to be 150-200% of the weight.

...maybe its CG is not in the right spot? It is about 2cm in front of where it should be in the plans. ...And is the GC from the plans in the right spot?

Flying wings are very sensitive to CG. You need to move things around until it will balance on the CG marks in the plans. The plans are correct.

And if i repaired the plane, should i hand launch it, or do a normal take-of?

It’s a matter of personal choice. The landing gear may be part of the weight problem. But rising from the ground may be easier if you are alone.

For the first few flights, having someone else throw the plane is a good idea. You want them to throw is slightly upwards, 15-20 degrees. You want a good stiff throw, like throwing a baseball 30-40 feet. You need to keep your fingers away from the prop.

Here are 3 different ways to safely throw a flying wing.

 

[LitterBug]

The FT Viggen has a flat wing and needs a nose high angle of attack to fly. Most people recommend a 75mm EDF over a 64mm. Not sure what was done to get two 50s in there but it most likely has made it heavier than intended. One of the guys at the field had one and he said it flew more like a nutball than a standard plane due to the flat wing. (High alpha flyer)

 

[dap35]

Mine flys like a champ. 4S-3300-4400 battery and a 70mm EDF. I agree the 3S 5400 battery is too heavy and not high enough voltage. Check your CG! 50 EDF's don't produce as much thrust as a 70mm, but in a 4S configuration they can do OK.

I also have a FT-Spear that I built with 2x50mm EDFs running on 4S-4400 batteries and it hand launches fine.

Both planes have the Aura5 which makes for great hand launches.