Multi-rotor BMFA B cert

[Biggles' Elder Brother - Moderator]

A little while ago I was browsing through the Achievement Scheme pages - as you do - when I came across the multi-rotor B-test specification. Looking through the elements everything looked fairly straight forward - well at least it was clear! Then I came upon this:

"(j) Perform One Loop

The model should be flown out to a point between 30-50 metres past the pilot, then flown back past the pilot on a standard height and line, at the point the model reaches in front of the pilot a loop of approximately 15-25 metres diameter should be performed. A perfect loop is not required but the exit height and line should be very close to the original."

I'd be very interested to hear from anyone who can do that with standard multi-rotor - because I can't! I can flip it - but I can't fly a loop - or even anything approximating it.

I've searched Youtube for any video showing someone flying a loop with a multi-rotor. Many claim it - but when you look,....its just a flip, not a loop. Its certainly not "15-25 metres in diameter exiting at the same height" or anything like it!

I recently asked someone I know who has been flying multi-rotors since the very early days - prior the current interest. This guy was one of the pioneers, today he's one of the top multi-rotor pilots around with a well-known company in the field. He couldn't see how its possible to do this either! So its not just me.

Well, is there anyone out there who can do this? Someone who has a multi-rotor B-cert? An examiner? Anyone from the achievement scheme committee? Anyone at all? If so, please explain, because I for one am very confused!

BEB

[stu knowles]

You could say much the same about the bunt in the fixed wing schedule......because its there is probably the best explanation. Both may require a specific model capable of such a manouvre

[FlyinBrian]

Posted by stu knowles on 26/06/2015 20:13:41:

You could say much the same about the bunt in the fixed wing schedule......because its there is probably the best explanation. Both may require a specific model capable of such a manouvre

ANY half decent aerobatic model will perform a bunt (negative loop performed from the top down)

I must say I have never seen a multi rotor perform a loop.

[Biggles' Elder Brother - Moderator]

Stu - as my old instructor told me - "If a model will loop then it will bunt - its just a matter of confidence and belief"

But this is quite different - because I can't see how this is even possible! I would be very happy to be shown otherwise!

BEB

[WolstonFlyer]

Better ask the BMFA to explain what they mean, a big loop certainly sounds impossible unless you have one of the fancy 3D capable multi-rotors that can change the blade pitch to hover inverted etc.

[john stones 1 - Moderator]

I don't know enough about them to say BEB, i'll have a word with our club multi flyer and see if he knows owt.

John

[John Privett]

I wonder if this has changed recently? I recall when the multi-rotor A & B tests first appeared they seemed just to be a direct copy of the Heli A & B tests with the word "Helicopter" replaced with "Multi-rotor".

I must admit I didn't study the details that closely, but the loop seems (now/always?) to be in place of the double stall turn in the Heli B.

[Biggles' Elder Brother - Moderator]

Yes John, I'd noticed that as well - ie the fact that the MR tests seemed to be just the heli tests with the names changed. Initially I wondered if that was the problem - after all a heli with collective pitch could indeed fly a loop. So I thought that maybe it was just oversight and the loop had been accidentally carried over from the heli test.

But no. There is no loop in the heli B test (somewhat surprising I think, but there we are!). This MR B test is basically the heli B test but, as you say, with the double stall turn swapped for this loop; a manoeuvre, as far as I can see, a MR can't do!

BEB

[stu knowles]

Stu - as my old instructor told me - "If a model will loop then it will bunt - its just a matter of confidence and belief"

I think that we might have to agree to differ on that one but either way, I could happily fly all of my scale models without troubling them with a Bunt. anyway back to the subject.....

[Cuban8]

Fast forward to 1:20. Looks like loop to me. Lots of forward speed and eased over the top. A bit easier to see in full screen.

 

Edited By Cuban8 on 26/06/2015 22:40:43

[Chris Jones 7]

Yup entirely possible.

The key is realising that the quad does not start the loop upright as a fixed wing would, it starts at 45-60 degrees nose down. So at the top of the loop the model will not be inverted but most likely just passing the verticals orientation. At this point power is held until the model rotates past the vertical point and starts to come down. Lateral momentum allows the quad to fall through the backside of the loop while a smidge of power lets the model continue to rotate around it's axis. Exit the loop at 45-60 degrees nose down again.

[Biggles' Elder Brother - Moderator]

@Cuban: Well, mmmm. I agree the video is closer than many I've seen, but I'm still not entirely convinced that satisfies the description. First of all he didn't enter from level flight - he was flying a line upward at about 45 degrees. Secondly because the line is partially away from us I suspect it looks more circular than it is. In summery I'd call that a "big flip" rather than a loop.

@Chris - yes Chris I understand that the quad has to be running fast and nose down - but what you describe again is just a flip with some forward maomentum - it won't be a loop - surely not circular?

If these "approximations" are what they want then why no say that?

BEB

[john stones 1 - Moderator]

I've no idea which way up it is, but it do's a bit

leap 450 3d

John

[Biggles' Elder Brother - Moderator]

Nice John. But the Leap 450 is far from being a standard quad - it has motors that can be reversed in flight! That why it can fly inverted. Standard quads can't do that!

BEB

[Biggles' Elder Brother - Moderator]

Chris - I've been thinking about what you say. So you're saying that at the top of the loop - in the 12 o'clock position - the quad is not inverted, the inversion point comes sometime later in the downward portion? At that point you just have enough power on to continue rotation? Yes I can see that could be possible. OK, I'll try it!

BEB

[Biggles' Elder Brother - Moderator]

Well I've given that a go! And Ok, its better once you let go of the objective of having the quad inverted at the top of the loop - but its still far from what I would describe as "a loop"! It will get better with practice I'm sure - but its never going to be "right"! I still think that the term "loop" is inappropriate for this manoeuvre! But I'm not sure what you would call it! I'm also less than sure it would satisfy an examiner used to testing planes and helis that that is the best an MR can do! I'll have to work on him! Anyway - thanks for the pointer Chris

BEB

[Chris Jones 7]

Excellent glad to be of help.

to make the loop more circular on the back side you need to experiment a bit with the throttle. My Armattan 355 needs a small burst of extra throttle at the 12 o'clock position to increase momentum before reducing it almost completely for the 12-3 quater. Between 3-6 I need the quad to be facing back the way it entered the loop so I can apply around 1/4 throttle to bring the momentum back the other way before accelerating away at the bottom of the loop. Timing is everything!

[Bob Cotsford]

First, I've never flown an MR, so all that follows is either based on the laws of physics or complete drivel

A loop is a positive G manoeuver so is the key to performing one maintaining sufficient speed to generate enough centripetal force to stop the contraption dropping out of the sky i.e, performing a flip? In the case of a fixed wing surely the wing is generating lift all the way round, just as your rotors are?

 

Just had a thought - the MR flip sounds remarkably like the egg shaped 'loops' you get from underpowered trainers or WW1 models - again making me think you need more power and/or speed.

Edited By Bob Cotsford on 28/06/2015 09:56:45

[Dave Bran]

There, and I thought the "B" test standard was set to allow the person being tested to demonstrate routine and regularly performed moves in an accurately and safely controlled manner.

When you have pretty experienced people stating its not normal, and its taken as much effort to even find one (or a rough representation of one not actually to demanded standard) it isn't!!!

Can I truly "loop" any of my five Multi's as the schedule demands? No. (extended flips, yes, true loop, no way at the power levels available from mine).

Have I ever directly seen one performed? No

Ergo, as described, its IMO inappropriate. Seems the same situation with the BMFA when 3 axis control systems were fast becoming the "norm" with heli's, yet they were left wrong footed as to how to integrate the technology to a pilot skills test (and I can sympathise to a degree)

[DH 82A]

Just more Bumfa nonsense.

[Biggles' Elder Brother - Moderator]

Posted by Bob Cotsford on 28/06/2015 09:49:12:

First, I've never flown an MR, so all that follows is either based on the laws of physics or complete drivel

A loop is a positive G manoeuver so is the key to performing one maintaining sufficient speed to generate enough centripetal force to stop the contraption dropping out of the sky i.e, performing a flip? In the case of a fixed wing surely the wing is generating lift all the way round, just as your rotors are?

Just had a thought - the MR flip sounds remarkably like the egg shaped 'loops' you get from underpowered trainers or WW1 models - again making me think you need more power and/or speed.

Edited By Bob Cotsford on 28/06/2015 09:56:45

The point about maintaining positive G is well made Bob - but I think its rather more complex than that because comparisons with fixed wing don't really hold up well - there are several reasons for this:

1. The "lift" a multi-rotor generates is entirely one-directional. With a wing this isn't the case - wings can generate significant "upward" lift even when close to inverted - otherwise inverted flight would be impossible. So although a wing performing a loop does generate "lift" directed to the centre this is not uniformly so because although the AoA is usually +ve all the way round it is not uniformly so. But with a MR it is - the "lift" is always inveriably towards the centre.

2. In an MR the "lift" and the maneuvering ability are linked 100%. There are times in a loop when the MR's uni-directional lift is an embarrassment - you would like to lose it - but if you do so you have no control at all! This is not the case with a fixed wing aircraft where the control (via the elevator/ailerons and rudder) and the lift (from the wing) are far more independent.

3. Similarly in an MR your "lift" and your ability to produce translational flight - ie move forwards and backwards - are also 100% connected. There are times when you want power in order to widen the loop - but the orientation of the quad at that instant means that more power will simply make you descend even faster!

I totally agree - this is a high energy move. And you need to carry a lot of forward momentum into the loop which you will convert to height hopefully with some lateral "roundness" factored in. But I don't think the MRs's difficulty with the loop stems from lack of power so I don't feel the trainer/WW1 analogy really holds. The hexacopter I was flying yesterday has enough power to shoot vertical at very high rates of acceleration and ultimately high speed - so no lack of power there. The problem lies in the almost painfully subtle way the power has to be applied with impeccable timing in order to produce something which is, in the final analysis, still a pretty poor impersonation of a loop!

I agree with Dave Bran this is not a maneuver well suited to this type of aircraft and I think BMFA should seriously reconsider swapping it for something more in keeping - a "funnel" perhaps or a four direction flip? If they want to make it really difficult, make the flips diagonal!

BEB

[john stones 1 - Moderator]

Being a fixed wing examiner I would need an Heli examiner as lead to take someone for a B test. It states you can take the test with a fixed or collective pitch multi, is it this statement you take issue with, it then go's on to state you cannot use the models lack of suitability to miss or change a manoeuvre ? I have flown helis myself but not to B standard so understand some of what you're saying but don't know enough about multis to comment without listening first.

John

[Biggles' Elder Brother - Moderator]

A loop is no problem with a heli that has collective John - because you can alter the direction of the thrust by altering the pitch of the blades. But with 99.9% of MR's that isn't the case - the pitch is fixed at a positive angle. And there's the rub.

Its all very well BMFA saying you can't use the model's limitations as an excuse - but this particular "limitation" takes in the vast majority of the type!! MR's with collective are very much the exception not the rule.

I think my real issue is that the chance of getting an examiner that understands the problem is, at present, very slim! A heli examiner might do - if he has done aerobatics with a fixed pitch heli - but that's pretty rare - most heli pilots that do aerobatics will use collective - so this issue simply doesn't arise for them. I can just see an examiner failing people because the loop wasn't at the standard he'd expect to see from a helicopter - despite the fact that the schedule says "the loop doesn't have to be perfect" - but, as I say, if they judge by conventional helicopter standards then that puts the bar, on this particular element, very very high indeed for MR pilots.

And to return to an earlier debate I think the chance of finding an examiner who could demonstrate this element is almost zero!

BEB