Recommended thrust to weight ratio for a hybrid VTOL UAV

ArduPlane VTOL Plane
1

Hi there,
I’m somewhat new drone building etc, however I’ve joined an engineering project at university and have been tasked with selecting the motors/props.

The layout is similar to the one pictured below, however the pusher props are also located along the wing. (not my design, imo the one pictured below is much better)

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The maximum total weight is 15kg, wingspan of 2 to 3m. Based on the standard quadcopter thrust/weight ratio of 2:1, we would require 4 motors producing up to 30kg total, of which I’ve found a couple on t-motors under the V-TYPE section.

However I’m not sure if this ratio is correct - there’s two avenues of thought that occur to me:

  1. the 2:1 ratio is so that a quadcopter is able to comfortably move in an agile manner, however as we are only using the VTOL props for a brief period of time to reach a desired height, manoeuvrability is not so much of a concern, as such we can get away with a lower ratio. This seems to be supported by this paper (bottom of page 2/12), which suggests 1.2-1.5 however I’m not sure if that only applies to tail sitter UAVs.

  2. due to the large wingspan of the VTOL plane, the wings create a lot of drag, which means the motors need to produce even more thrust than they would on a quad, as such a greater ratio is required.

What thrust to weight ratio would you guys recommend? I understand that is hard to say for sure given the lack of specifications however a ballpark figure or even just pointing me to somewhere I can find out more would be a great help.

In addition the structures team have told me that the VTOL propellers should be less than 13 inches in diameter. The configuration I was previously looking to use was MN505-S KV320 with MF2211 props producing ~6.7kg each. I mainly chose these because they are rated for 6S as opposed to the 12S that a lot of the V-TYPE motors are rated, and it seems to be quite hard to find 12S batteries. Unfortunately they use 22 inch props, so they’ve asked me to shop around for a different configuration. Looking through the T-Motors store, I have a hard time finding any motors that are spec’d for 13 inch props that provide even 4kg of thrust. Is this an impossible task?

Any and all help would be greatly appreciated!

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Hi, I’m going through the same design process for a drone that is slightly larger (MTOW 25 kg, wingspan 4 m). I’m doing this for the second time, so I’m not an expert, but maybe my conclusions will be of help to you.

The 2:1 ratio is so that a quadcopter is able to comfortably move in an agile manner, however as we are only using the VTOL props for a brief period of time to reach a desired height, manoeuvrability is not so much of a concern, as such we can get away with a lower ratio. This seems to be supported by this paper (bottom of page 2/12), which suggests 1.2-1.5 however I’m not sure if that only applies to tail sitter UAVs.
What thrust to weight ratio would you guys recommend?

I found the same paper and based my work on it. If you look at the source given in the paper, it says that a minimum of 1.05 is required, but most VTOL aircraft (not just tail sitters) aim for 1.2 to 1.5, so that is also the range I use and have used before with good results.

Due to the large wingspan of the VTOL plane, the wings create a lot of drag, which means the motors need to produce even more thrust than they would on a quad, as such a greater ratio is required.

I’m not worried about that, as the VTOL system is only used to achieve very low vertical speeds, especially with the high MTOW in both of our cases.

I mainly chose these because they are rated for 6S as opposed to the 12S that a lot of the V-TYPE motors are rated, and it seems to be quite hard to find 12S batteries.

It is difficult enough to find a satisfactory drive system, do not limit yourself to 6S. Since the VTOL propellers are limited in size, the motors compensate for this with higher power, for which you need higher voltages. Its much simpler to find 12S batteries, than motors for 6S

I have a hard time finding any motors that are spec’d for 13 inch props that provide even 4kg of thrust. Is this an impossible task?

I used eCalc to find my setup. Enter your requirements and then try different configurations until you find one that works. This is much easier than digging through manufacturer specifications and websites.

I hope some of this was helpful.

1 Like
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You just put 2- 6S batteries in series to get 12S. There are a few that make 12S though.