anton k asked 3 months ago
I ran T2 analysis in flow5 for a simple plank with NURBS fuse and got the following graphs.
Then I actually build this plank with electric power. The wing is 1.5m by .35m, AUW is 2.2 kg. The motor is SunnySky x2216 950Kv, 10×5 APC prop. The plane isn’t as smooth as the 3d model, but close enough. the airfoil is quite accurate, rigidity is just fine, etc.
When I fly the model the current draw ranges from 3 amps (barely hangs in the air at 10m/s) to 20 amps (25 m/s). At 4S the electrical power range is from 40W to 300W, and goes up with speed.
I now realize that this plank is poorly designed and probably the motor+prop combo isn’t quite right but still, this is for learning purposes. Couple of questions:
- Can I assume that Drag x V is mechanical required power to maintain level flight (the chart at the bottom)?
- Is it fair to say that this design is most efficient at around 25 m/s where the curve Drag x V (required power) vs V (the chart at the bottom) has minimum? And the cruise speed should be around 30 m/s where the tangent from the origin touches the curve?
- According to the analysis going from 10 m/s (15 deg angle of attack) to 25 m/s (2.5 deg angle of attack) the the required power actually goes down (given #1 is true). Is this realistic?
- If #3 is about right then what might be the reason that I’m seeing the opposite in the air? I.e. the electric power goes from 40W to 300W as I accelerate from 10 m/s to 25 m/s. While the theoretical required power goes down from 40W to 25W. Can it be the motor/prop choice? I realize that the electrical power system has an efficiency figure, but if that is the explanation then at full throttle the efficiency is just 10%?
- CL/CD peaks at 40 in the analysis. How realistic is this figure? I though that it’s hard go go above 10-15 in real world for e-powered model planes.
Thanks!
techwinder Staff answered 3 months ago
Hello,
- Yes; given the assumptions made in flow5, it is also equal to m.g.Vz which can be plotted in the polar graphs
- I suppose that it depends on the definition of efficient, but if your goal is to minimize power consumption, the answer is yes.
- In a T2 polar as the aoa increases, the velocity and viscous drag move in opposite directions so that the calculated Drag(N) exhibits a minimum
- I can’t tell – my experience is essentially with pure gliders. If you are piloting manually, it can be pretty difficult to increase the power while maintaining horizontal flight, so that you are never too sure of what the actual flight conditions are.
- Likely unrealistic, because even with the inclusion of viscous airfoil drag and fuselage drag, there are many other contributors to drag which are not accounted for in flow5.
André
anton k
replied 3 months ago Thanks for the response.
Re #5 How do I enable viscous analysis properly? When I check the checkbox “Viscous Analysis” it just doesn’t converge:
Main wing Span position -730.26 mm, Re = 252311, Cl = 0.30 is outside the flight envelope
Main wing Span position -690.79 mm, Re = 252311, Cl = 0.51 is outside the flight envelope
Main wing Span position -651.32 mm, Re = 252311, Cl = 0.65 is outside the flight envelope
Main wing Span position -611.84 mm, Re = 252311, Cl = 0.75 is outside the flight envelope
Main wing Span position -572.37 mm, Re = 252311, Cl = 0.83 is outside the flight envelope
Main wing Span position -532.89 mm, Re = 252311, Cl = 0.89 is outside the flight envelope
……….
It works in the same way as in xflr5.
I made a tutorial video on the matter in the flow5 playlist
In the foil analysis menu, use the blue arrow to exporf the foils and launch xflr5.
Create the polar mesh in xflr5
Use the red arrow to import the viscous data back in flow5.
This document explains the error messages:
http://www.xflr5.tech/docs/Point_Out_Of_Flight_Envelope.pdf
anton k answered 3 months ago
I just realized that viscosity impacts the analysis a great deal. Here are the graphs:
Here we can see that the least drag power is at around 15 m/s which is much closer to the reality. Then it doubles at around 30 m/s, so I would expect that at 30 m/s the amp drawn by the motor prop should double. But the motor/prop combo I have can theoretically produce thrust at speeds up to 100 km/h: 950kv x 15 volts x 5 inches x 60 minutes ~= 100 km/h which is roughly 27 m/s.
That probably explains why I’m seeing such high ratio between electrical power and drag power – the motor is just saturating at max throttle.
That said if I replace the motor/prop combo to lower kv/higher pitch, do you think this can help to reduce electric power consumption?