I have a 7" ArduCopter setup that I configured and tuned by following the detailed video guide of @andyp1per . The main difference from the guide is that I am using a SpeedyBee F405 flight controller stack, so I do not have ESC telemetry feedback available. So, the notch is throttle based rather than RPM telem based. The propellers are HQ prop 7045 tri-blade.
I have completed the tuning process as thoroughly as possible, and overall the aircraft is flying very well. In AltHold mode, I am seeing a maximum flight speed of approximately 150 km/h. At that speed, vibration levels remain very low (typically below 30 m/s²), so vibration does not appear to be the limiting factor.
I would like to understand whether this is likely the maximum achievable speed for this setup, or if there is still room for improvement through additional tuning or parameter adjustments.
Current observations:
Maximum level-flight speed: ~150 km/h
Vibration levels at top speed: < 30 m/s²
Throttle output (ThO) at maximum speed: ~0.8
The aircraft struggles to maintain the maximum pitch angle of 75° (Angle_max = 7500cdeg)
As I wrote before all technical information of your build are best to know. Specially: what type of motors, parameter of battery, total weight of the drone? Than it is possible to check the performance little more with ECALC
But from your logs I see that all your motors are commanded to the maximum so I assume your drone is at the physical limit.
Quadcopters are bricks as far as aerodynamics is concerned, so drag is an obvious issue. Easy way to measure approximate drag: Climb high, cutoff all power in stabilize, observe terminal velocity. Natually obey all laws during the procedure! …
Well in any case, I usually observe around 25m/s terminal velocity, so if you cruise at that speed, drag is already comparable to the weight of the quadcopter.
I’d look at two factors: do your propellers have enough pitch speed? You don’t have RPM data, but from battery voltage and KV, you can get a good RPM estimate. Then multiply motor frequency with propeller pitch to obtain the propeller pitch speed. If it’s not above your speed by at least 20%, you can likely obtain more performance by switching to a higher pitched propeller or to higher RPM motor option(easiest with higer KV or with higher voltage battery).
The other option to obtain more speed is to improve the aerodynamics. If you observe high speed quadcopters (400km/h+), you’ll notice they look significantly different from the “classic” frame design. You might want to go in that direction if you want to go faster. But in general, I think around 200km/h should be doable even on “classic” frames.
FC/ESC: SpeedyBee F405 Stack with 60A SpeedyBee ESC
Battery: 6S 5000mAh, 75C
AUW: 1450g
From the logs, it appears the motors may already be reaching maximum output, which would suggest the aircraft is close to the physical limit of the current propulsion system. This seems consistent with the fact that vibration levels remain low (<30 m/s²), while the aircraft struggles to maintain the commanded 75° pitch angle at maximum speed.
Given the current setup and a top speed of approximately 150 km/h, what would be the most effective way to gain additional speed?
My initial thoughts are:
Trying higher-pitch 7" props.
Moving to higher-KV motors.
Reducing drag and/or overall weight.
Where would you expect the biggest gains to come from? Is 150 km/h already close to the practical limit for a 2810 900KV + 7045 tri-blade + 6S setup, or is there still meaningful performance left on the table?
Thanks for the detailed explanation—this makes a lot of sense.
I agree that airframe drag is likely a major factor here, especially since the aircraft is already stable and vibration levels are low at high speed. The “thrust vs drag equilibrium” explanation also matches what I’m observing in flight.
The point about prop pitch speed is particularly interesting. I hadn’t fully quantified it yet using KV × voltage × prop pitch, so I’ll calculate that and compare it against my measured top speed to see how much headroom I actually have. That should give a clearer idea of whether the current HQ 7045 setup is fundamentally limiting forward speed.
From the logs, it also looks like I’m hitting a thrust ceiling rather than a tuning limitation, since the vehicle struggles to maintain higher pitch demand instead of showing instability or oscillation.
It can be seen that ThO goes to one briefly when the desired and actual pitch goes to max (75 deg) before falling off.
Appreciate the insight—it helps narrow down the problem a lot.
Why did you go with 900kV motors in the 1st place? My 7" builds, and many others including Andy’s Reference 7" build, are in the 1100 to 1300 kV range.
If you compute the maximum pitch speed, you’ll notice that this is an easy lever to improve. Solutions: increase battery voltage, increase motor KV or increase propeller pitch.
Best would be to increase battery voltage to 8S though might be difficult with the other electronics.
Cheapest would be to increase the propeller pitch which will work, but will reduce hover efficiency.
Your current meter also seems very poorly calibrated, making it hard for me to determine how much headroom you have in regards to the power consumption. BAT.CurrTot suggests you’ve drained 120% of your 5Ah battery but you landed at 23V, so with a lipo, you have over 50% battery remaining. In general I’d expect your motors to be able to handle around 35A each for short bursts so you seem to have a lot of headroom there, but again, hard to be sure given poor current measurements.
Good question. I had selected 900KV motors with efficiency and endurance in mind rather than outright speed. The FPV was intended primarily as a long-range platform, and at the time I wasn’t targeting maximum top speed.
Now that I’m experimenting with the performance envelope, it does seem that the relatively low KV may be one of the limiting factors. As you suggested, moving into the 1100–1300KV range would increase the available prop RPM and pitch speed considerably.
Before changing motors, though, I’m planning to evaluate higher-pitch propellers first, since that should help determine whether the current limitation is primarily RPM-related or drag-related.
Thanks for the suggestions! I would probably try with higher pitch props. Thats the easiest option atm. For higher votage batterie i would have to change a lot of electronics.
Thanks for pointing that out. I set the Battery parameters as per the arducopter documentation on SpeedyBee, so I wasn’t expecting such a large discrepancy.
I ll perform a calibration using an external power supply to verify the current readings.
Arranging a custom made Li-Ion pack in the shape of trianges, 3 cells each triangle, on the top and bottom of a small multirotor is one way to reduce drag. I use hot glue between cells, black silicone glue between cells and carbor fiber top and bottom plates, and wrap the cells in black automotive electrical tape. To take this further, you could shape it into more of a teardop by placing open-cell foam under the tape on the top plate, and fashion a ‘trailing edge’ under the bottom plate. This approach (6s2p li-ion) would increase weight by about 100g but double your mah. Similar with arms of rectangular cross section. I place plastic straws along the upper and lower sides of the arms and wrap them, and the motor wires, in tape to reduce drag. The frames with ‘lattice arms’, like the 10” Helion and AOS UL10 have lower drag at extreme lean angles where it counts most. I doubt that using 6s2p li-ion cells would be a limiting factor. The latest 21700 cells can give up 60A continuously and much for during short bursts, say 90A x 2p = 180A. This is over 4kw.