Putting Wings or Winglets on a Mavic?

[Gernby]

If anyone has a large / strong fan, it would be cool to see some flight logs of the MP hovering in front of it while slowly rotating...

 

[AlanTheBeast]

Yes that's the only easy way to check it and as expected they designed it slightly nose heavy. I wonder how many grams of extra nose weight it has?

I doubt it's very much. The CofG as I determined is about 7.5 mm from the centre of the fore/aft rotor centre. (within 3 mm I'd hazard).

 

[AlanTheBeast]

I would bet that we could come up with many ways to prove that the MP is nose heavy, but thankfully, it seems we agree it is. However, another factor that I just noticed is that the front props are pitched toward the rear a few degrees, but the rear props are not. I suspect the pitch in the front motors is required to offset the heavy nose.

I'd noticed the offset before and thought "pitch stability" - but after looking at it again I'd attribute it to keeping the rotor tips out of the camera view.

The "heavy nose" is not that heavy IMO. Indeed, in forward flight at higher speeds the force pushing the nose down because the fore part of the fuselage is so wide is probably resulting in more thrust imbalance than due to a slightly forward CofG.

 

[Gernby]

Unless I'm not understanding either one of you, you both failed to mention the biggest reason for the higher flight times when not hovering. It has to do with how clean the air is that the blades move through. When hovering each blade is moving through the rotor wash of the previous blade which is highly inefficient. When any rotary winged aircraft moves forward with sufficient speed, the blades are moving through relatively clean air which greatly improves efficiency. This is only advantageous up to a point where parasitic drag becomes large and efficiency drops back off again.

Back in Vietnam, they would regularly fly helicopters overloaded but to take off the crew chief would run next to the bird until it had some forward speed and then jump on.

I was a helicopter mechanic in the Army and hold a degree in aeronautical engineering although I work as a mechanical engineer.

I didn't fail to mention it. I just disagree that has anything to do with it. A nose-heavy quad will naturally fly forward if all the props are spinning at the same speed with the same pitch. It's clear that DJI had to pitch the front motors backward AND spin them faster in order to hover.

 

[Gernby]

Keep in mind that moving center of gravity forward not only increases the weight that the front props have to support. It also reduces the weight that the rear has to support. That's why weight distribution and corner balancing has such a significant impact on a car's handling (I'm a car guy from way back).

I'm tempted to go get a cheap fishing pole to cut short and attach as a lever arm to the body of the MP, so I can suspend a weight from the end to shift the center of gravity back toward the rear. I suspect that a perfect length / weight tale would extend battery life while hovering.

 

[AlanTheBeast]

Unless I'm not understanding either one of you, you both failed to mention the biggest reason for the higher flight times when not hovering. It has to do with how clean the air is that the blades move through. When hovering each blade is moving through the rotor wash of the previous blade which is highly inefficient. When any rotary winged aircraft moves forward with sufficient speed, the blades are moving through relatively clean air which greatly improves efficiency. This is only advantageous up to a point where parasitic drag becomes large and efficiency drops back off again.

Back in Vietnam, they would regularly fly helicopters overloaded but to take off the crew chief would run next to the bird until it had some forward speed and then jump on.

I was a helicopter mechanic in the Army and hold a degree in aeronautical engineering although I work as a mechanical engineer.

Very true and implicit in the "moving into undisturbed air" where power is concerned. But these are not helicopters. Drones depend to a high extent on raw thrust. The blades are fixed and are "propulsion" airscrews - they are not the near uniform wing shape of a helicopter rotor blade and don't at all change pitch over the rotation to optimize and balance lift as in a helicopter in forward flight.

A S-58 (and former snake) pilot friend of mine mentioned doing pedal turns to reduce torque on the tail rotor, making more power available to the main rotor, to power out of hot and high locations with sling loads in Vietnam.

 

[Wombat55]

Keep in mind that moving center of gravity forward not only increases the weight that the front props have to support. It also reduces the weight that the rear has to support. That's why weight distribution and corner balancing has such a significant impact on a car's handling (I'm a car guy from way back).

I'm tempted to go get a cheap fishing pole to cut short and attach as a lever arm to the body of the MP, so I can suspend a weight from the end to shift the center of gravity back toward the rear. I suspect that a perfect length / weight tale would extend battery life while hovering.

If there's a center-of-weight problem with respect to the center-of-lift, wouldn't that have been easy to correct by the DJI engineers by simply moving the propellers slightly forward or slightly back with respect to the body of the aircraft? Don't know why they would have to resort to tilting the propellers or why people would have to resort to adding additional weights to the drone.

 

[AlanTheBeast]

Keep in mind that moving center of gravity forward not only increases the weight that the front props have to support. It also reduces the weight that the rear has to support. That's why weight distribution and corner balancing has such a significant impact on a car's handling (I'm a car guy from way back).

I'm tempted to go get a cheap fishing pole to cut short and attach as a lever arm to the body of the MP, so I can suspend a weight from the end to shift the center of gravity back toward the rear. I suspect that a perfect length / weight tale would extend battery life while hovering.

As you like but bear in mind that the amount of offset is tiny. Another way of putting what you say above is: Whatever energy more the fore rotors put out the aft rotors put out less so it's likely close to a wash. As I've mentioned elsewhere, the high speed flight impingement on the forward fuselage is probably more "pitch down" that needs compensation than the small weight imbalance.

Note that some intrepids (in good conditions) are hitting 7 km out and back - stock. That doesn't sound like too much energy is being wasted to me...

 

[AlanTheBeast]

I didn't fail to mention it. I just disagree that has anything to do with it. A nose-heavy quad will naturally fly forward if all the props are spinning at the same speed with the same pitch. It's clear that DJI had to pitch the front motors backward AND spin them faster in order to hover.

I'm betting that's just to keep the rotor tips out of the camera view.

 

[AlanTheBeast]

If anyone has a large / strong fan, it would be cool to see some flight logs of the MP hovering in front of it while slowly rotating...

Please elaborate?

 

[AlanTheBeast]

Hmmmm. OK, now my new wild idea is some sort of contraption in which, say, each of the four propellers is not only spinning around at high speeds due to its own electric motor, but each propeller is also slowly rotating around another, separate vertical axis so that each propeller is constantly being supplied with relatively "fresh", non-moving air even though the drone as a whole may be hovering in one fixed location in the air. But maybe that's all starting to get a bit complicated....

In any case, to maintain hover, air is rushing down and that needs to be fought to stay in place.

 

[AlanTheBeast]

Yes that's the only easy way to check it and as expected they designed it slightly nose heavy. I wonder how many grams of extra nose weight it has?

TIC: by the same mass as the gimbal/camera assembly?

 

[grizzard]

Which is fine for helicopters but doesn't translate well to quads as it's a "reaction" machine rather than a "lift" machine. To be sure the props do behave like wings to some degree - just not to the degree they do on helos.

As to the "lifting" body in forward flight, no, the nose is pitched down - if anything it makes a downward thrust.

All rotors are wings. Just less lift for the front rotors than the rear rotors when moving forward. And changing rpms to counteract yaw that the cw and ccw props can't account for. And motor adjustments for wind gusts to keep it on course. You can see it in action when you look up when the Mavic is landing.

Sent from my Nexus 6P using MavicPilots mobile app

 

[grizzard]

Which is fine for helicopters but doesn't translate well to quads as it's a "reaction" machine rather than a "lift" machine. To be sure the props do behave like wings to some degree - just not to the degree they do on helos.

As to the "lifting" body in forward flight, no, the nose is pitched down - if anything it makes a downward thrust.

Well actually, loss of thrust at the front

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[grizzard]

I disagree with all of you guys!

I think it has everything to do with weight distribution! The Mavic is very nose-heavy, which means that the front props have to run at a higher RPM than the rear when hovering. However, when it's pitched forward, the center of gravity moves in relation to the props, allowing the rear props to do more of the work. At the perfect pitch angle (air speed), the 4 props should all be turning about the same RPM, which I assume would be an optimal RPM for this prop design.

It shouldn't be hard to test. Don't the flight logs show RPMs and motor current draw? If so, then hovering in a steady wind should result in the RPM and / or current draw changing based on the relative angle of the MP vs. the wind.

Actually the front rotors have longer arms and therefore takes less energy to lift the front ...

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[AlanTheBeast]

Well actually, loss of thrust at the front

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I put that badly. A downward pitching moment is what I should have said. To counter that, the drone will have to increase thrust at the front.

 

[grizzard]

If you try to balance the MP on your fingers from 2 corners, it will tilt its nose down. That indicates that it is nose heavy. Also, if you video the MP while it is hovering at a high frame rate, the captured pattern indicates that the fronts are spinning at a different RPM than the rear.

If you try to balance an airplane the same way it wouldn't fly. You balance for "in motion".

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[AlanTheBeast]

All rotors are wings. Just less lift for the front rotors than the rear rotors when moving forward. And changing rpms to counteract yaw that the cw and ccw props can't account for. And motor adjustments for wind gusts to keep it on course. You can see it in action when you look up when the Mavic is landing.

Sent from my Nexus 6P using MavicPilots mobile app

No. These are not wings. They are propellers. And there's a huge difference.

In fact the main reason quads are feasible and so stable is because RPM is variable. Helicopters keep the RPM pretty rock steady and control power with angle of attack balanced by torque.

Quads are thrust machines. Helicopters are lift (wing) machines.

Sorry to refute - it's just so.

 

[grizzard]

Unless I'm not understanding either one of you, you both failed to mention the biggest reason for the higher flight times when not hovering. It has to do with how clean the air is that the blades move through. When hovering each blade is moving through the rotor wash of the previous blade which is highly inefficient. When any rotary winged aircraft moves forward with sufficient speed, the blades are moving through relatively clean air which greatly improves efficiency. This is only advantageous up to a point where parasitic drag becomes large and efficiency drops back off again.

Back in Vietnam, they would regularly fly helicopters overloaded but to take off the crew chief would run next to the bird until it had some forward speed and then jump on.

I was a helicopter mechanic in the Army and hold a degree in aeronautical engineering although I work as a mechanical engineer.

Exactly, from another helicopter mechanic in Japan. 1956

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[CactusJackSlade]

The sole way to determine CofG (Shy of the entire parts list and their weights and positions) is by suspension, not balance.

• Get a piece of string with a weight on it.
• Pinch one of the rear rotor heads between thumb and finger
• Dangle the drone so it is hanging freely from your (tightly) pinched finger and thumb.
• Dangle the string from the same point.

Where the string crosses the centre line is the longitudinal CofG.

Verify by doing this with a front rotor - warning: harder to hold - so just hang the antenna over your finger.
Dangle again. String crosses the center line at the same point.

That is the CofG. The MP is only slightly nose heavy.

View attachment 6877 View attachment 6878

I would say the Mavic, relative to the center of thrust, is actually very nose heavy... I found the CG to be 12mm forward of the center of thrust.... I did not realize it was this far. I try to keep my racing quads and all my quads in general no more than 3-5mm difference CT vs CG. Non the less.. the Mavic flies very well.