Putting Wings or Winglets on a Mavic?

[cobra46]

Something I find interesting about the Mavic is that the rotor/prop tip speed at 600 rpm is only 15 mph. This means the retreating blades are producing negative lift at full speed.

For reference a Huey UH-1 has a tip speed of about 509 mph so the retreating blade is still producing lift. As someone mentioned this is why the angle of attack on the retreating blade is increased during forward motion.

Tip speed is a design consideration for most aircraft to keep it subsonic. This is one reason why some helicopters have swept tip blades.

 

[JOHNNYBRAVO]

Well that was a fun read. So from all the banter, I would suggest DJI experiment with this. If indeed it works, as from the logic explained in this post by some people "in the know" I suggest that DJI sell some blades with "winglets". I would buy them and I don't care having to attach and detach them each time I flew if it increased my flight time or distance etc.

Informative post except for the whole "speed up so I don't have to slow down" aspect. Keep it friendly peeps.

 

[CactusJackSlade]

Hummm, maybe #2 above, changing the angle a couple degrees would be insignificant as far as camera view goes.

The blades point up a little wrt to the level camera view. Less likely to be seen by the camera.

Another reason for the angle may be for folding and having the blades lie flat when folded.

Or longitudinal pitch stability. Or to offset the pitch moment due to the slightly forward CofG as Gemby proposes. Or because it looks cool.

(Why don't I think it's for pitch moment correction? because it would have been far simpler to correct the CofG by distributing the parts inside correctly).

Hummm, maybe your #2 above having the blades stow properly when folded, changing the angle a couple degrees would be pretty insignificant as far as camera view goes.

 

[BadBob]

The stock props have to work well enough across the flight range. Unlike helicopter blades there is no pitch adjustment over the course of one turn in forward flight. Unlike helicopter blades the RPM is very variable.

They (DJI or 3rd party) could add tip fences or other clever anti-vortex changes - but in the end, this is a flying camera, not a fuel gulping revenue machine. (Note the fence can go upward to pretty much the same effect like winglets on an airplane to avoid the down tip hitting the drone...).

I even wonder if a ducted fan could be more efficient - while providing more protection to the rotors and people...

I agree


Sent from my iPhone using MavicPilots

 

[Logger]

Something I find interesting about the Mavic is that the rotor/prop tip speed at 600 rpm is only 15 mph. This means the retreating blades are producing negative lift at full speed.

That doesn't sound right to me @cobra46.
I believe you have a decimal error, perhaps due to the way the RPM units display on the Radar thingummy. When it display 600 it is really ~ 6,000rpm . That will give you are more plausible tip speed I guess.

 

[AlanTheBeast]

I plan to repeat that video tomorrow evening (my daughter is here tonight), without the couch behind it, and with a better camera. I also plan to get a better video of the props from above, with more light and a 360 degree view.

Great. We might not solve this, but we're learning interesting things!

 

[AlanTheBeast]

If I have time, I also plan to drop paint balloons into each prop, so we can observe the resulting spatter patterns. I theorize that the front rotors will produce smaller paint spots on my walls and furnishings than the rear rotors. Of course, I need to use 4 different high contrast paint colors to differentiate the spatter patterns...

At least 8 colours as repeating experiments to validate them is absolutely necessary. I'd also appreciate audio of your wife observing it all.

 

[AlanTheBeast]

Aside from the science talk, I totally agree with AlanTheBeast's comment about packaging. If the "interesting" design choices about the Mavic were based on physics and flight characteristics alone, then I would expect the Phantom 4 to share those design choices. I am sure that the Mavic missed out on many ideal design parameters, so it could achieve its obvious objective (compactness).

The P4/P is just the continuation of that line of drones so they weren't about to make any major packaging changes. Add sensors? Sure. Better camera? Sure. The interesting thing now is to see if DJI put an end to the Phantom line - or at least the "shell" design. Maybe one last iteration before drones becoming more "Mavic" like and less Phantom like. And here I am wanting a P4P. Maybe I should wait until the fall...

One thing that constantly comes out from new Mavic owners: "sure the camera is not as good as my Phantom - but the Phantom takes longer to setup and is bigger/heavier/cumbersome to lug around". This paints Phantom's demise and a better camera'd M2. And yes it would cost more. That's okay, the MP (1) seems to be designed to woo new entrants to the "real capability" sub kg drone market and to launch a series of drones that are going to wow us for the next few years.

(It occurs to me that if DJI would simply come up with folding props for the Phantom 4 and use the MP's controller, that would go a long way to making P4 customers happier to use their machine more often.)

DJI may be entering an era of very interesting, clever and advanced mechanical design for drones (Inspire, Mavic) to improve capability, portability, customer experience and overall performance. Note that "overall performance" is a marketing term for engineering compromises - there will never be a commercial mass market drone that will satisfy those who want energy efficiency in a chosen flight regime.

If they would step up to the quality/customer service plate with the same verve and vigor as the Mavic and Inspire engineering team, they'd really become the standard bearer of the small drone market.

 

[AlanTheBeast]

Something I find interesting about the Mavic is that the rotor/prop tip speed at 600 rpm is only 15 mph. This means the retreating blades are producing negative lift at full speed.

For reference a Huey UH-1 has a tip speed of about 509 mph so the retreating blade is still producing lift. As someone mentioned this is why the angle of attack on the retreating blade is increased during forward motion.

Tip speed is a design consideration for most aircraft to keep it subsonic. This is one reason why some helicopters have swept tip blades.

The display on the RC shows RPM/10. 600 is 6000.

 

[cobra46]

That doesn't sound right to me @cobra46.
I believe you have a decimal error, perhaps due to the way the RPM units display on the Radar thingummy. When it display 600 it is really ~ 6,000rpm . That will give you are more plausible tip speed I guess.

You very well could be right. I was a bit surprised by the small rpm values displayed not to mention that it looks much faster than 5-600. We already know DJI is not very diligent with units....fps versus mph issue.

6000 rpm is 148 mph

I have an rpm meter at work. Maybe I'll throw it on my Mavic and see what it reads versus the RC.

 

[Helo]

Something I find interesting about the Mavic is that the rotor/prop tip speed at 600 rpm is only 15 mph. This means the retreating blades are producing negative lift at full speed.

For reference a Huey UH-1 has a tip speed of about 509 mph so the retreating blade is still producing lift. As someone mentioned this is why the angle of attack on the retreating blade is increased during forward motion.

Tip speed is a design consideration for most aircraft to keep it subsonic. This is one reason why some helicopters have swept tip blades.

You'r right; the blade angle on the retreating blade needs to increase to maintain equal lift on the rotor system the problem as you mentioned, keeping it subsonic is a big consideration. The other is that as aircraft speed increases the amount of pitch increases (on the retreating blade) and you will eventually get blade stall just like a fixed wing aircraft. You can usually feel it coming on and correct it by reducing pitch. If you don't the aircraft would do a roll to the retreating blade side and very very bad things would happen.

 

[Wombat55]

You very well could be right. I was a bit surprised by the small rpm values displayed not to mention that it looks much faster than 5-600. We already know DJI is not very diligent with units....fps versus mph issue.

6000 rpm is 148 mph

I have an rpm meter at work. Maybe I'll throw it on my Mavic and see what it reads versus the RC.

My fingers liked the idea of 15 mph a lot better 148 mph. Thanks for the info. Won't find me trying to hand-launch or hand-catch a Mavic, for sure.

 

[Gernby]

Well ... flock a bunch of moda farggin' ice-holes! I crashed my MP into my fish tank while recording a "better" video of the MP hovering in my living room. The damage to my aquarium reduced my interest in the paint balloon test, so I'll probably do that one tomorrow.

Here is the raw footage, since I wasn't up for an edit...

 

[cobra46]

I was admiring your reef tank on your previous video. What damage occurred to the tank? Sounds like props broke on the MP. Anything else?

Any idea how this happened? It looked like it was pretty close but stable before the crash. Did it just drift?

 

[Gernby]

I was admiring your reef tank on your previous video. What damage occurred to the tank? Sounds like props broke on the MP. Anything else?

Any idea how this happened? It looked like it was pretty close but stable before the crash. Did it just drift?

Thanks. It seemed clear that I reduced the level of light in the room just enough to render vision positioning completely ineffective. As soon as I turned off the lights on the ceiling fan, the MP immediately flew toward the aquarium.

 

[Gernby]

The MP doesn't seem damaged, but the aquarium cabinet has some impressive scratches

 

[AlanTheBeast]

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Well ... flock a bunch of moda farggin' ice-holes! I crashed my MP into my fish tank while recording a "better" video of the MP hovering in my living room. The damage to my aquarium reduced my interest in the paint balloon test, so I'll probably do that one tomorrow.

Here is the raw footage, since I wasn't up for an edit...

I found a frame in there where the laser line matched the wall and drone meaning the camera was pointing in the same plane as the laser plane.

Annotated that with lines for the two rotor planes. (pale green).

Took that to Photoshop to normalize the image (make the laser line 0° in PS), and then measured the rotor plane lines. (The pale blue line is just a copy of the forward pale green line).

There's a 2.4° difference between them with the rear rotor having the greater pitch. So the front rotor may be doing more vectorial work against the forward CG.

The only thing we don't know is the difference in RPM fore/aft

 

[AlanTheBeast]

Well ... flock a bunch of moda farggin' ice-holes! I crashed my MP into my fish tank while recording a "better" video of the MP hovering in my living room. The damage to my aquarium reduced my interest in the paint balloon test, so I'll probably do that one tomorrow.

Here is the raw footage, since I wasn't up for an edit...

Sorry about the fish tank test. I feel guilty having urged you on with this...

 

[Gernby]

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.

 

[Gernby]

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I found a frame in there where the laser line matched the wall and drone meaning the camera was pointing in the same plane as the laser plane.

Annotated that with lines for the two rotor planes. (pale green).

Took that to Photoshop to normalize the image (make the laser line 0° in PS), and then measured the rotor plane lines. (The pale blue line is just a copy of the forward pale green line).

There's a 2.4° difference between them with the rear rotor having the greater pitch. So the front rotor may be doing more vectorial work against the forward CG.

The only thing we don't know is the difference in RPM fore/aft

I finally had a minute to analyze your image, and am pretty surprised by the numbers I calculated from it. As I mentioned before, it's been a LONG time since I did any trigonometry, so please help me verify this...

I think that the magnitude of the horizontal vectors (Fh) for each prop would be related to the vertical vectors (Fv) like this:
Front: TAN(3.9) = Fh(front) / Fv(front)
Rear: TAN(6.3) = Fh(rear) / Fv(rear)

Since we know that Fh(front) must equal Fh(rear) while hovering (opposite direction, but equal magnitude), we can substitute:

Fv(rear) * TAN(6.3) = Fv(front) * TAN(3.9)

Therefore:

Fv(rear) = Fv(front) * TAN(3.9) / TAN(6.3)

Which is:

Fv(rear) = 0.62 * Fv(front)

If this is correct, then the vertical component of thrust from the rear props is just 62% of the magnitude of the vertical component of thrust from the front props. Since the angles are relatively small for front and rear, the magnitude of the total thrust from the rear is also about 62% of the magnitude of the total thrust from the front. If we assume that the props produce thrust with a linear relationship to RPM, then it seems that the front will be spinning about 62% faster than the rear. However, I doubt the props will have the same efficiency across such a wide RPM range, so it may be an even greater difference.