Putting Wings or Winglets on a Mavic?
- Thread starter Wombat55
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AlanTheBeast
Well-Known Member
In the end, that's pretty well on the money. I approached it differently by solving for the force vector on the front rotor that would result in equal opposing horizontal force vectors.
Result? Front rotor has to be working at an incredible 61.34% more rotor force than the rear in hover. I'm gobsmacked.
So what the hell were the DJI engineers thinking? And I suppose it was, "hey! We're not making this thing for hover! - we're making it to go somewhere else and hover there."
So now the challenge is to find out how the thrust is distributed at other speeds.
AlanTheBeast
Well-Known Member
Thanks for marking up the image. I haven't put much thought into the math yet, but it seems that we have all the information needed to calculate the RPM difference, assuming some ideals.
I didn't lose any sleep over the crash, and will probably have a few more crashes like it soon. This was actually my 2nd crash inside my house, since my interest in the MP is mostly related to the development of autonomous flight without GPS.
I also have a tendency to push the limits of things, even if the risk vs. reward doesn't see favorable.
Not sure I get the risk as long as it's a front wheel drive car? Is there a risk the boat will "lift" the hitch of the car? Doesn't it just "float off" of the trailer?
I'm not a boat kinda guy...
Glad you guys got on board! 
I actually think DJI made some a ton of really great design choices, even though the final result isn't "ideal". If DJI had wanted to achieve an ideal / efficient hover, they could have simply reduced the opposing pitch of the props, and allowed the front rotors to swing a few degrees farther forward. However, as AlanTheBeast pointed out, that would cause the props to show up in the camera's view more often.
I actually think DJI made some a ton of really great design choices, even though the final result isn't "ideal". If DJI had wanted to achieve an ideal / efficient hover, they could have simply reduced the opposing pitch of the props, and allowed the front rotors to swing a few degrees farther forward. However, as AlanTheBeast pointed out, that would cause the props to show up in the camera's view more often.
The risk factors of towing my boat with my Honda S2000 were:
* The max rated towing capacity of the S2000 is 0 lbs
* The S2000 weighed about 2800 lbs, but the boat and trailer weighed about 3500 lbs
* The S2000 is a "sports car" with a relatively stiff suspension and very limited suspension travel
* The S2000 is rear wheel drive with low torque and a weak clutch
AlanTheBeast
Well-Known Member
Ah, yes, high tolerance for risk indeed!!! Going downhill with that rig could get very, very interesting.
As a helicopter pilot for 50+ years, the reason a helicopter needs less power in forward flight is called "translational lift" .. as the fuselage moves forward the forward rotating blades see additional lift from the additional forward speed of the helicopter .. simple. Now look up "retreating blade stall" and see what happens if you go to fast!! I doubt you could design a wing that would work in a quad-rotor ...
Helicopter Aviation
Helicopter Aviation
Found an interesting article on quadcopter design by someone who is actually an engineer who has experience with quadcopter design. Some interesting facts revealed by the article:
(1) Quadcopters are inherently less stable and less efficient than equivalently sized helicopters.
(2) The one big advantage of quadcopters over helicopters is mechanical simplicity.
(3) Constant electronic stabilization is the key to making quadcopters work. (Demo shown of what happens when electronic stabilization is turned off.)
(4) Constant electronic stabilization uses a significant amount of battery power. (Again, quadcopters are not very energy efficient).
(5) Regenerative braking can re-capture some of the energy used in constantly speeding up and slowing down the propellers due to electronic stabilization, but apparently a lot of drone makers don't do that. (Perhaps something to look forward to in the 'Mavic 2'?).
To demonstrate the paramount importance of electronic stabilization over mechanical design, the author shows a simple quadcopter made by attaching four propeller-motors together in an assembly with wooden sticks and duct tape. It flies fine! Always thought that DJI always gave a great deal of thought to all aspects of the Mavic's design including the slight tilt of the forward and rear propellers of the Mavic. But perhaps this isn't so? From the sticks-and-duct-tape demonstration of this article, it appears that electronic stabilization can largely correct for any small deficiencies in the layout of the mechanical design of a quadcopter.
What Makes The Quadcopter Design So Great For Small Drones?
(1) Quadcopters are inherently less stable and less efficient than equivalently sized helicopters.
(2) The one big advantage of quadcopters over helicopters is mechanical simplicity.
(3) Constant electronic stabilization is the key to making quadcopters work. (Demo shown of what happens when electronic stabilization is turned off.)
(4) Constant electronic stabilization uses a significant amount of battery power. (Again, quadcopters are not very energy efficient).
(5) Regenerative braking can re-capture some of the energy used in constantly speeding up and slowing down the propellers due to electronic stabilization, but apparently a lot of drone makers don't do that. (Perhaps something to look forward to in the 'Mavic 2'?).
To demonstrate the paramount importance of electronic stabilization over mechanical design, the author shows a simple quadcopter made by attaching four propeller-motors together in an assembly with wooden sticks and duct tape. It flies fine! Always thought that DJI always gave a great deal of thought to all aspects of the Mavic's design including the slight tilt of the forward and rear propellers of the Mavic. But perhaps this isn't so? From the sticks-and-duct-tape demonstration of this article, it appears that electronic stabilization can largely correct for any small deficiencies in the layout of the mechanical design of a quadcopter.
What Makes The Quadcopter Design So Great For Small Drones?
AlanTheBeast
Well-Known Member
Glad you guys got on board!
I actually think DJI made some a ton of really great design choices, even though the final result isn't "ideal". If DJI had wanted to achieve an ideal / efficient hover, they could have simply reduced the opposing pitch of the props, and allowed the front rotors to swing a few degrees farther forward. However, as AlanTheBeast pointed out, that would cause the props to show up in the camera's view more often.
See esp. the para below on retreating blade stall and why it is not at all of the same importance as for a helicopter.
We've stomped all over this. A quad is not a helicopter (mostly). Helicopters have one (or 2) variable pitch, constant speed rotors that are more aptly described as wings. A quad pushes air down fast for reaction lift throughout most of its envelope. And yes, these fan blades are airfoils, but that's just to make air move efficiently.
Which is why quads (and helicopters) need more power in out-of-ground-effect hover. It is sitting in its self made down rushing column of air. Put your hand under a hovering MP to get a "feel" for that.
Move it forward and it's fresh still air.
Yes, this is very analogous to the same advantages for a helicopter moving into still, unperturbed, no down vector air - but not quite the same.
Drones are not as dangerously affected by retreating blade stall. Since that rotor is paired with a rotor beside it [1], there is no roll induced as there will be in a helicopter. The "fan" is merely less effective and so is its opposite lateral fan. The roll cancels out. The drone may not be able to maintain altitude at that speed, but it won't roll into an uncontrollable condition. DJI limit the speed of their drones and such is probably one of the many reasons why. Or maybe to not fry the motor. So no lack of control - no loss of altitude[2].
Importantly, don't forget that stall is not a result of lack of speed, but is due to an angle of attack that causes the air over top the airfoil to stop adhering to the top of the airfoil - so lift is lost. Since drone blades are fixed they cannot stall in quite the same manner - merely cease pushing air downward as effectively. See above for why this is not so much of an issue as it is with helos.
[1] - could also say paired with the rotor behind (or ahead) of it - but in the case of the MP the incidence angle of the rotors are different fore and aft so one "pair" of rotors may have retreating blade stall ineffectiveness, the other pair (fore or aft) would not at the initial onset for the other pair.
[2] I have seen one (1) report of a MP user getting a rotor high speed limit warning, and this may be a case where rotor thrust imbalance fore and aft becomes an altitude loss issue. Not enough info to date.
I recorded a quick video of the MP with a very non-ideal tail, just to see how it impacted the overall pitch. This is a stainless steel rod, which is way longer than it would need to be. I don't know how much it weighs, but it ain't a feather. If AlanTheBeast has some time to analyze the pitch similar to the other video, that would be appreciated.
AlanTheBeast
Well-Known Member
Certainly.I recorded a quick video of the MP with a very non-ideal tail, just to see how it impacted the overall pitch. This is a stainless steel rod, which is way longer than it would need to be. I don't know how much it weighs, but it ain't a feather. If AlanTheBeast has some time to analyze the pitch similar to the other video, that would be appreciated.
Angles are wrt the laser line.
We can have a wing on top of the MP in the form of a "limousine antenna" but with straighter thicker wings and increased AOA. This would prevent prop downwash from hitting the wings.
AlanTheBeast
Well-Known Member
Would need to be a pretty deep camber wing at the relatively low speeds of the MP to get any lift - at that point the weight is probably not worth it. But a top wing is a smart idea indeed.
I'm still most interested in how much performance could be increased by converting the arms to vertical airfoils.
AlanTheBeast
Well-Known Member
Which performance? Hover? Slow flight? Fast flight?
Visualize the airflows for each of these and you may prefer tubular arms...
I spent some time analyzing the flight logs from my last hover test (from the MP internal flash), and was really pleased by the data that it contains. It does contain a high resolution RPM, PWM, Volts, and PPM for each rotor (not sure what PPM is), as well as many other usefull things. However, the DJI viewer is pretty weak, and I haven't figured out how to convert the data to a CSV.
That said, it's clear that my hover testing may have been somewhat corrupted by a couple blemished rotors. I have 1 rotor in particular that shows a lower RPM despite having a higher PWM, which seems to indicate higher drag. If so, I suspect that the MP might have needed to adjust the other rotor speeds in various ways to stay in place without rotation. It seems possible that might even contribute to the surprising angles we discovered.
I've ordered another pair of fresh rotors, so that I can repeat all of the tests.
That said, it's clear that my hover testing may have been somewhat corrupted by a couple blemished rotors. I have 1 rotor in particular that shows a lower RPM despite having a higher PWM, which seems to indicate higher drag. If so, I suspect that the MP might have needed to adjust the other rotor speeds in various ways to stay in place without rotation. It seems possible that might even contribute to the surprising angles we discovered.
I've ordered another pair of fresh rotors, so that I can repeat all of the tests.
I cant help but thinking about the overall design of the MP. I mean, all other dji drones are like square or circle. but the MP is more like rectangle. sort of like it's halfway optimized for a forward flight hybrid craft thing. I am still trying to envision how the legs can be made into airfoil themselves and still preserve the folding ability. but yes, given the design shape of the MP just can't help thinking that it's leaning toward a hybrid craft in that way.
AlanTheBeast
Well-Known Member
If you look at the MP in forward flight, esp. at high speed, it is quite nose down. So the air is hitting that large top surface, in particular the front. I would bet that this is a much higher energy loss than air impingement on the arms as, esp. the front rotors would have to work harder to overcome the forward down moment from the body impingement plus the moment from the slightly nose heaviness of the unit.
Despite all that, this little 43.6 Wh battery machine is doing 14 km round trips at 50 km/hr ... this is a fine machine.
I'm not worried much about it.
AlanTheBeast
Well-Known Member
Look forward to it.
Also, next video, please add the laser going through the rotor heads and photograph coincident with the wall. It occurs to me that the way we've done it to date might exaggerate the angles a little, more so for the front rotor.
According to the article linked here Putting Wings or Winglets on a Mavic? airfoil arms reduced drag by 60%, so I guess the answer would be 'all of the above.'
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