Mavic vs P4P stability

[AyeYo]

A couple months ago in the MP group we discovered a funny thing about the MP. Because it's designed to fold, the engineers made some compromises on rotor plane angles. Such that the difference between fore and aft rotor planes is a whopping 10.2°. This results, in a stable, no wind hover, in the front rotors working 61% more than the rear (without accounting for the slight nose-heaviness of the MP). So the MP may be headroom limited due to that. (How those angles play in forward flight is harder to say...).

This has nothing to do with folding angle and everything to do with forward flight efficiency when the power once again balances out and the Mavic is able to achieve greater forward speed with less forward pitch (and thus greater air resistance).

 

[AlanTheBeast]

Both Mavic and P4P are very stable in wind. The difference is the Mavic can't fight much more than a 15mph wind, while I've flown the P4P in 25mph wind without running out of power.

I've had the Mavic up in stronger winds than 15 mph. But I needed S-mode to get it back. It will do 40 mph after all.

The "book" (manual) max wind speed (Mavic Pro) is 10 m/s. (36 km/hr, 22 mph).

Monday this week I was flying the P4P in 20 kt winds with gusts to 30. Champed it of course.

The "book" max wind speed for the P4P is .... (see above for the MP - same thing).

 

[AyeYo]

I've had the Mavic up in stronger winds than 15 mph. But I needed S-mode to get it back. It will do 40 mph after all.

The "book" (manual) max wind speed (Mavic Pro) is 10 m/s. (36 km/hr, 22 mph).

Monday this week I was flying the P4P in 20 kt winds with gusts to 30. Champed it of course.

The "book" max wind speed for the P4P is .... (see above for the MP - same thing).

The other difference between the two is that the Mavic's gimbal is easily affected by wind gusts and gets slammed around into the frame causing video issues, especially when heading into the wind. The P4P's gimbal seems capable of dealing with any wind the aircraft can.

 

[AlanTheBeast]

This has nothing to do with folding angle and everything to do with forward flight efficiency when the power once again balances out and the Mavic is able to achieve greater forward speed with less forward pitch (and thus greater air resistance).

I really can't break the code on that. Here's how I see it:

It has everything to do with the angles of the prop planes[1]. See the image below. (MP in a stable hover - laser is level and vertical).

If the MP were in forward flight with the nose down 10°, then the forward disk is at -6.1° and the rear disk is at -16.3° (v horizontal). Load on the upper body is almost equal front to back.

So the rear motors have to work much harder than the front to achieve required (balanced) vertical component (if they are not balanced, the pitch rate won't be 0).

If one assumes that there is higher load on the front (wide nose area), then the rear motor doesn't have to work as hard as described above ... but the difference won't be that great.

This could explain the "over current" warnings that happen in strong winds...

[1] the prop plane angles are (IMO) a consequence of the packaging required to achieve the MP's folded configuration.

 

[AlanTheBeast]

The other difference between the two is that the Mavic's gimbal is easily affected by wind gusts and gets slammed around into the frame causing video issues, especially when heading into the wind. The P4P's gimbal seems capable of dealing with any wind the aircraft can.

That's possible. I was flying in some good gusts the other day and didn't notice any gimbal bashing at all.

Heading into the wind? Always doing that. Do you mean into gusts?

 

[AyeYo]

If the MP were in forward flight with the nose down 10°, then the forward disk is at -6.1° and the rear disk is at -16.3° (v horizontal). Load on the upper body is almost equal front to back.

With rear motors already pitched forward right from a hover, you can achieve more forward speed with less forward body pitch (which is also less forward surface area) or higher forward speed at the same body pitch and more forward speed with less work from the rear rotors to keep the craft pitched up into the wind. The front rotors only need to pitch to level or just past and the rear rotors are already pitched enough to create plenty of forward thrust. It also gives you more gimbal range, as the gimbal is more level in forward flight.

 

[PhantomEvan]

That's possible. I was flying in some good gusts the other day and didn't notice any gimbal bashing at all.

Heading into the wind? Always doing that. Do you mean into gusts?

Let me know how the test goes.

 

[AlanTheBeast]

With rear motors already pitched forward right from a hover, you can achieve more forward speed with less forward body pitch (which is also less forward surface area) or higher forward speed at the same body pitch and more forward speed with less work from the rear rotors to keep the craft pitched up into the wind. The front rotors only need to pitch to level or just past and the rear rotors are already pitched enough to create plenty of forward thrust. It also gives you more gimbal range, as the gimbal is more level in forward flight.

I think we're saying the same thing differently?

It's not about the forward thrust amount. It's about steady state forward flight with the pitch rate at 0. For the pitch rate to be at 0 the vertical thrust component has to be equal front to back (offset slightly depending on the longitudinal load).

So due to their angle the rear props have to work harder than the front. Yes, this results in a greater forward thrust contribution from the rear ... but that's incidental. What's important is the pitch moment is 0.

 

[AyeYo]

So due to their angle the rear props have to work harder than the front. Yes, this results in a greater forward thrust contribution from the rear ... but that's incidental.

Working harder than the front, yes, but neither front nor back working as hard to maintain forward flight as if all four props were perfectly level. This is why tilt rotor FPV racers are taking over, because rather than requiring the body of the craft to be pitched into the wind at 20-45 degrees and creating a huge frontal cross section (and huge drag), you can instead tilt the rotors forward to achieved the necessary AOA while leaving the body horizontal or at least less pitched for less drag. The Mavic rear rotors allow the body to stay horizontal or more horizontal in forward flight, which increases cruising efficiency.

 

[sar104]

Working harder than the front, yes, but neither front nor back working as hard to maintain forward flight as if all four props were perfectly level. This is why tilt rotor FPV racers are taking over, because rather than requiring the body of the craft to be pitched into the wind at 20-45 degrees and creating a huge frontal cross section (and huge drag), you can instead tilt the rotors forward to achieved the necessary AOA while leaving the body horizontal or at least less pitched for less drag. The Mavic rear rotors allow the body to stay horizontal or more horizontal in forward flight, which increases cruising efficiency.

Presumably the penalty for that is that backward flight is slower and less efficient, which is probably fine as a tradeoff.

 

[AyeYo]

Presumably the penalty for that is that backward flight is slower and less efficient, which is probably fine as a tradeoff.

And, because the Mavic's rotors can't tilt back to level, as Alan pointed out it also results in less efficient hover.

 

[sar104]

And, because the Mavic's rotors can't tilt back to level, as Alan pointed out it also results in less efficient hover.

Yes - I wasn't sure that I understood that though. The Mavic rotor angles are divergent downwards (front to back but not side to side) and so there is no aircraft attitude at which all four rotors are horizontal - is that the point? That is less efficient, but some divergence is also beneficial in avoiding VRS conditions during zero-lateral-airspeed descent. This may be more than necessary for that, however.

So what's your final conclusion on the reason for the Mavic rotor angles - faster forward speed?

 

[AyeYo]

Yes - I wasn't sure that I understood that though. The Mavic rotor angles are divergent downwards (front to back but not side to side) and so there is no aircraft attitude at which all four rotors are horizontal - is that the point?

It's a stability thing. Look at any camera platform multirotor from any manufacturer, they all have rotors that have a slight inward (towards the COG of the platform) pitch to allow for inherent stability - not all that dissimilar from dihedral on a plane wing. I think the lack of side-to-side pitch on the Mavic is the actual packaging compromise.

That is less efficient, but some divergence is also beneficial in avoiding VRS conditions during zero-lateral-airspeed descent. This may be more than necessary for that, however.

So what's your final conclusion on the reason for the Mavic rotor angles - faster forward speed?

I think it's an efficiency thing. More battery life at the same speed or more speed at the same battery life, really only depends on how you're looking at it. With stuff like sport mode and the new fixed wing mode, DJI is well aware that people will be flying the Mavic, not hovering it.

 

[AlanTheBeast]

Working harder than the front, yes, but neither front nor back working as hard to maintain forward flight as if all four props were perfectly level. This is why tilt rotor FPV racers are taking over, because rather than requiring the body of the craft to be pitched into the wind at 20-45 degrees and creating a huge frontal cross section (and huge drag), you can instead tilt the rotors forward to achieved the necessary AOA while leaving the body horizontal or at least less pitched for less drag. The Mavic rear rotors allow the body to stay horizontal or more horizontal in forward flight, which increases cruising efficiency.

In the end I don't think this arrangement has very much to do with efficiency for the MP - at least not in flight. It was designed to fold, compactly, with the propellers attached - and that's what drives the current prop angles. That it has some benefit in forward flight for efficiency may have indeed driven the final configuration (which props folded which way).

 

[Howick]

I just want to put in my two cents worth and experience.

I got caught out a while ago with a suuden wind change and found myself flying in winds much higher than I wanted.

I found the P4P was being pushed around and would end up possibly 15 meters off course before fighting it's way back and then being pushed off course again. I was trying to fly home in a Northerly direction at the time and in P-mode. All in all, I think that the P4P did a remarkable job in such conditions and I felt "almost in control most of the time"

I haven't flown a Mavic but I'm not sure if it could handle that! I have heard a couple of people with both birds say that the P4P pro is noticeably more stable in wind.

As stated: Just my two cents worth

Bruce

 

[AlanTheBeast]

I have my MP back so I'll be able to test if the conditions present themselves. The MP does have a 10 km/h disadvantage for fighting a headwind.

 

[wsalopek]

Both Mavic and P4P are very stable in wind. The difference is the Mavic can't fight much more than a 15mph wind, while I've flown the P4P in 25mph wind without running out of power.

This makes no sense...penetrating wind is almost all about simply how fast an aircraft can go...if both can go X miles per hour (and the MP and P4P are very close), then they will have the same success (or failure) flying into a given headwind.

That said, I'm open to other interpretations.

--

Bill

 

[sar104]

This makes no sense...penetrating wind is almost all about simply how fast an aircraft can go...if both can go X miles per hour (and the MP and P4P are very close), then they will have the same success (or failure) flying into a given headwind.

That said, I'm open to other interpretations.

--

Bill

That was my conclusion too. Frame of reference is unimportant.

 

[Russ43Phantom]

My first question is "Why fly in heavy wind". As a photographer, light wind provides a more stable platform. Why not come back another day. I am working with a sailor who wants to capture his sailboat with the "rail in the water" on a windy day. He can achieve this level of sailing with 15-18 mph wind. Why do it on a day with winds at 25 mph?

 

[sar104]

My first question is "Why fly in heavy wind". As a photographer, light wind provides a more stable platform. Why not come back another day. I am working with a sailor who wants to capture his sailboat with the "rail in the water" on a windy day. He can achieve this level of sailing with 15-18 mph wind. Why do it on a day with winds at 25 mph?

That's a good question, but a different question than the one being discussed.