# Maximise range for available battery mass - selection of VTOL cruise speed

Hey guys, need some assistance with understanding how to select a cruise speed for a VTOL (Quadplane) aircraft that will have an MTOW of 32 kg. Construction will be all composite, similar to available airframes like the Mugins, Foxtech airframes, etc.

The main goal is to maximise range given it will be carrying 10kg of battery (4x 6S 22 Ah) dedicated for the forward propulsion.

I’ve noticed most available airframes like the Foxtech Whale 360 andFoxtech AYK-350
have the following specs:

Stall speed: 18-19 m/s
Cruise speed: 22-23 m/s
Endurance: 2.5 to 3 hours
Approx battery mass: 10 kg

The above roughly translates into a range of somewhere between 200-250 km. I understand these are manufacturer’s claimed values, so they are probably calculated in ideal conditions, no wind, sea level, etc.

To adjust for realistic conditions if I’ll assume for now a loss factor of 60%, then the estimated range comes out to 120-150 km, which is a comfortable place for my requirements.

Questions:

1. Is the 120-150 km range bracket achievable for a well-designed composite airframe in the 32 kg MTOW class?
2. Which is a better cruise speed bracket for range: 20-25 m/s or 25-30 m/s?

Thanks!

Well, yeah. Why not? Depends on your design but mainly depends on wing loading and drag.

This again depends on what you are trying to achieve. For a general-purpose design, increasing the gap between the stall speed and maximum speed is a good idea.

Keep in mind that this is a vast under simplification of the numerous elements that go into establishing an efficient cruise speed but…

1.2*Vs (Stall Speed) is usually a good estimate for a cruise speed that allows for the best flight time endurance (hours).

1.5*Vs (Stall Speed) is a good estimate for a cruise speed that allows for the best flight range endurance (miles).

Keep in mind that the two values may require some tweaking to actually achieve the optimal target (i.e. 1.25 instead of 1.2) - but I would never set a cruise speed below 1.2*Vs to ensure that you have a bit of headroom before stalling.

Thanks guys for your inputs, appreciate it!

Yes, I do understand that like all things engineering it always depends on a bunch of factors

I’m trying to better understand the aircraft design approach (especially in the initial stages where there are a lot of unknowns) required when one of the hard design requirements is to meet a guaranteed range number. For example in cargo delivery use cases, where the aircraft has to travel from point A to B.

Correct me if I’m wrong, but as far as I currently know:

1. Stall speed is directly proportional to the wing loading (or the square root).
2. The optimum “happy cruising airspeed” for an airframe would depend on the drag and hence the overall design, which would then dictate the best endurance speed and the best range speed.

Main goal is to achieve a hard range number (and factor in the buffers/headrooms) for a cargo delivery use case, endurance is not primary. My understanding is that more general purpose applications of UAVs has its roots in surveillance so endurance is the metric used more commonly.

If I understand you correctly, if we design for a larger margin between stall and max speed, then it lets us play around with a broader range of cruise speeds later on when the aircraft is built?

Thanks again

Gotcha, thanks a ton for this. I’ll use this as a rule of thumb then.

So I’ve already built and tested a couple of VTOLs at 8 kg & 10.5 kg MTOW. This roughly correlates with my experience on them.

For an estimated stall speed of 12 m/s (calculated, I’ve seen the aircraft go down to 10-11 m/s in FBWA without stalling) I’ve been using the following with good results:
ARSPD_FBW_MIN = 14 m/s
Cruise speed = 16.5 to 17.5 m/s

Still have a bunch of testing to do with finding a good prop for the cruise speeds though.

Understood, this is something I’ve already been following in my setup so far.

The documentation for the parameters says that ARSPD_FBW_MIN should be set 20% higher than level flight stall speed. If I remember correctly, I’d also seen a thread where Tridge (I think) mentioned that the cruise speed should be set at least 3 m/s higher than ARSPD_FBW_MIN for the TECS to work well.

The above is something I’ve been following and have not yet faced any issues with airspeed and/or altitude maintenance during flight.

Given the above, does the following looks like a decent starting point for the speed envelope?

1. Stall: 17 m/s
2. ARSPD_FBW_MIN: 17*1.2 = ~21 m/s
3. Range of cruise speeds to test at: 24 to 30 m/s

Thanks

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Yeah that looks like a good starting point.