Are inverted climb outs faster?

[sutty]

I have a question for you guys, only half related to the MB, maybe. Part 2 of the question will be about how this relates to the MB, so I think it is okay to post in here.

Since getting the MB I have become more and more confident at being inverted. A lot of practice has helped too, but even that is as a result of the MB, because since I got my first one, my crash free runs are much longer, and I have so much fun that I try to get out and fly with almost every spare moment.

The result of this confidence is that I spend more and more time inverted. What I think I can now notice, especially on the 250, is that a full collective inverted climb out is faster than a right side up one. I think I read about someone else asking this, but I cannot remember the conclusions.

First thing I did was to double check that I had everything accurate in terms of the setup. Zero was spot on, blades in line and parallel with the boom, when folded back, and the heli dangled from the nose to relieve the strain from the weight of the blades on the links. I did the check at step G, with a servo selected.

Next I double checked the range at K, and had to use a plastic pitch gauge, as my bevel box is just too heavy for the 250, but even though it is just plastic I don't think it is inaccurate. It seems quite easy to see if it is out by probably 1/4 of a degree, but it wasn't. +-12, which is what I had intended.

So I thought, it must be right, and just kept on flying, but again, the more time I spent doing things inverted, the more I became convinced that it doesn't seem to be the same. The 250 was obvious, the 450 less so, and I can barely notice on the 550, in fact it could well be imagination now fuelled by the confusion from the other two. So anyway, after quite some time, I decided I would adjust the 250 to match. :face

Mhhh, this is where I guess I did the wrong thing, as I adjusted the links to the blades. I gave myself one full turn on both, and I swear it did the trick, and I could feel the difference. Thing is, obviously, when I check 0, it is now out. I realise that what I should have done is go back to K, and just given myself maybe 1/2 or 1 degree extra of positive. That way everything would still be right with the set-up, but I didn't. I did it whilst having a hissy fit about it one day, and instead of thinking I ran in and got my ball link pliers.

So anyway, is this a real phenomenon? The answer you guys give me here will determine what I do going forward with the other helis. Funny thing is, since I made this correction I have loved how it flies, but I know if I want to make a change like that I should have done it in K, and should probably go back and re-visit it. This seems especially important after reading a comment by Walter in Bob's thread about the guy who was cheating blue, where Walter said that the control loops wouldn't know where 0 was, since it would be offset, just like mine, but for different reasons.

So here is another question, why does the MB even need to know that. I have obviously caused the same thing here, but I already taught it 6 degrees and blue, and that hasn't changed. Why does the MB care where 0 is? After this is all done and dusted, 6 will still be 6, and it will understand the geometry, as I showed it that before I adjusted the links. How much does it involve itself in collective, such that it even needs to know where 0 is anymore? I realise there would be an issue if I wanted to set up blue again, I would have to do it all right again for that, but that's not what I'm asking.

So, is it real?
Should I adjust it in K, instead of my pigs ear job? (Only asking, as it's flying great, lol.)
Having done it wrong, why does the MB care? (Set-up aside.)

Sorry to go on, but I wanted to explain all my thoughts. Should have just left it with the thread title, lol.

Cheers

Sutty

[TeeJayShelton]

Don't have a mb but inverted climb outs are faster.


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[TeeJayShelton]

If you really want it equal on climb and setup right I would think you should put the links back and just take a degree off your full negative pitch in the curves in your tx.

As prev. Stated I don't own mb


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[sutty]

Thanks for confirming that I wasn't going mad TeeJay. Mind you I probably was with the solution, but I may well leave it as it is on the 250, at least until the next re-build, as it is working now. On the 450 I willl probably adjust it, in the way you said, but in the MB, as it is very easy to do. Hadn't considered the Tx until you said, thanks for the suggestion.

So another question, since it would appear to be real, what is the reason behind it?

Cheers

Sutty

[Bob O]

Sutty,

I haven't paid any attention to the climb-out rates for upright and inverted. I'll make sure to do that next time out.

Regarding the pitch adjustment, if I were in your position, I'd put the mechanics back to where they were, and then make the adjustment in Setup menu point K. Why? It would just be my preference to have zero degrees at mid-stick. Adjusting the pitch curve would accomplish this as well like TeeJay said.

[lyco46]

Pushing the load rather than pulling IMHO it's more efficient

[desertstalker]

Have a look in the main forum, there was a couple of threads on that a month or so ago.

If i recall the effect is usually attributed to the interaction between main rotor tip vortices and the tail rotor (this is the reason some full scale helis have the tail raised above the mains).

[sutty]

Thanks desertstalker, I'll do that, it sounds interesting.

I guess I should make it right Bob. Right's right as they say. Not really like me to accept a half ass solution, even if it is working. First thing tomorrow I'll adjust it.

Cheers

Sutty

[airjawed]

I don't think the difference is due to tip interactions and all that.... especially seeing that it is more noticeable on a small heli.

My guess is that it is related to heli size but in a different way. Large helis have less play in the main shaft and other links... small helis are always full with play. I think you are setting up pitch in the upright position...which is fine but it pushes the main shaft down. When the heli hovers the main shaft moves up a little. Then when you are inverted the shaft then moves back down. Shaft movement translates to a change in pitch. Long story short, the small helis have more movement which ends up having in flight inaccuracy.

I would live with it.

Why does the MB care about 0 pitch? Certainly plays a role if you active tail precomp.

[extrapilot]

You’d first have to define ‘faster’- Z axis acceleration? Z axis max speed with no X/Y component? Conventional climbout nose first? Etc.

I posted on this subject in some thread where it was argued there is some inherent advantage for a pure Z-axis punch. I argue- prove it.

The one general argument involves the impact (pun intended) of the impingement of the induced flow on the airframe. And, to the degree that this is a factor in efficiency, it is a valid argument. The problem is, it is a very small delta for machines like ours. Conversely, there is an argument that accelerating negative Z is less efficient because of the inverted pendulum problem. Many argue this effect does not exist- and reference the rocket fallacy (line of thrust through center of mass is not inherently unstable). Problem is, that premise is faulty- our rotors flex, and line of thrust changes constantly relative to center of mass.

It is much more probable that you have asymmetry in your rotor. It does not have to be a simple pitch error. It can be something like asymmetric coning, where you have asymmetric wear in your grips from an upright bias. It might be related to the inverted pendulum problem- where the additional cyclic bias required may present itself as a net increase in collective. It could be as simple as visual or audible perception.

[labmaster]

In fact it is quite complex to explain exactly why negativ pitch is most times more effective than possitive.
A simply explanation is, that inverted, the air on the push side (inverted downside) don't have to bypass the heliframe.

Usually we simply get around this by using slightly assymetric pitch range, e.g. 11° pos. and only 10° neg.

You can simply adust this in Menu K or you set it symetric and then decrease the neg. range in your radio. When doing in the radio, make sure you do not shift the 0° point.

MB needs to know the 0° pitch point for the control loop calculations on cyclics.

Walter

[sutty]

Thanks airjawed and extrapilot, Extrapilot, no intentional X/Y component, as best I can manage, and from level, with full collective. Not impossible that it is perception thing, but fairly unlikely, as on the 250 it seemed very obvious. I accept the arguments about small helis from airjawed, and your suggestions with regard to assymetric coning, and grip wear bias, as opposed to a simple pitch error, since I suspect these things would apply significantly more to the smaller heli.

With regard to your suggestion, prove it, if I could prove it, I would, but I can't. Even if I were to video two perfect climbouts, one +ve and one -ve, I still could not prove that my set-up was 'perfect', or that I didn't have blades that coned more easily one way than the other, etc, etc.

Thanks Walter, I had thought about the airframe, but I had thought it might well be equally disruptive to the airflow entering into the disc from above when inverted, so I had dismissed this as probably a symetrical effect, but if this is a contributor, then fair enough. I'm sure that a full explanation is likely very complex, so I will accept it as, 'it is what it is', and anyway I have now fixed 0, and created an offset at K, which seems to give the same results, but it is early days, just a couple of packs.

Maybe a moot point now, as it is fixed, but I hadn't noticed any odd results from having mechanically moved the mid point after completing the main set-up. What issues might this have cause in the cyclic control loops? Not too important, as I accept the error of my ways, and as I say it is corrected, but I was just curious.

Thanks all.


Cheers


Sutty

[TeeJayShelton]

With regard to your suggestion, prove it, if I could prove it, I would, but I can't. Even if I were to video two perfect climbouts, one +ve and one -ve, I still could not prove that my set-up was 'perfect', or that I didn't have blades that coned more easily one way than the other, etc, etc.


Sutty


In heli Races the pro's always climbout inverted....
Sutty, I dont think there is a need to try to prove "beyond doubt". There is ample evidence ie. Trying it on your heli, The way heli drag race are conduted, Testimony of many others(there are other threads on this)
I have no idea why it happens, but unless your setup(pitch & or throttle) is not symetrical, your heli will climb out faster inverted.

[jperkosk]

Pushing the load rather than pulling IMHO it's more efficient

Yep, that's very close to what's happening here, mainly for the reason of the airflow.


The one general argument involves the impact (pun intended) of the impingement of the induced flow on the airframe. And, to the degree that this is a factor in efficiency, it is a valid argument. The problem is, it is a very small delta for machines like ours. Conversely, there is an argument that accelerating negative Z is less efficient because of the inverted pendulum problem. Many argue this effect does not exist- and reference the rocket fallacy (line of thrust through center of mass is not inherently unstable). Problem is, that premise is faulty- our rotors flex, and line of thrust changes constantly relative to center of mass.

It is much more probable that you have asymmetry in your rotor. It does not have to be a simple pitch error. It can be something like asymmetric coning, where you have asymmetric wear in your grips from an upright bias. It might be related to the inverted pendulum problem- where the additional cyclic bias required may present itself as a net increase in collective. It could be as simple as visual or audible perception.

Too many equations for my aging brain here, and it feels like these effects are marginal at best anyway...


A simply explanation is, that inverted, the air on the push side (inverted downside) don't have to bypass the heliframe.

Usually we simply get around this by using slightly assymetric pitch range, e.g. 11° pos. and only 10° neg.


+1 I'm a strong believer in simple explanations. Plus, I did some experiments on industrial fans years ago and I found a few percent of a difference in the airflow when inverting the fan blade; the body of the motor/mounting frame in the slipstream was causing enough turbulence to make an airflow difference measurable, I imagine that heli frame will affect the airflow even more...

[airjawed]

Yep, that's very close to what's happening here, mainly for the reason of the airflow.



Too many equations for my aging brain here, and it feels like these effects are marginal at best anyway...



+1 I'm a strong believer in simple explanations. Plus, I did some experiments on industrial fans years ago and I found a few percent of a difference in the airflow when inverting the fan blade; the body of the motor/mounting frame in the slipstream was causing enough turbulence to make an airflow difference measurable, I imagine that heli frame will affect the airflow even more...
How much of a difference did you see in terms of percentages? If the percentage is high then I'll buy it.... because in order to see a heli move noticeably faster its got to be greater than the order of a few percent.

Racing...I'm thinking inverted is better because you can get the blades closer to the ground which is much better to push off of than air.

Anyway...lots things to consider but in the end the obvious is usually the answer.

[extrapilot]

Sutty-

Of the potential subsystems, look at the one (canopy drag) people intuitively seem to hold as primarily responsible for the reported inverted advantage-

If you do the math, and assuming mine is correct, you will find that the flow velocity in question is low; average flow velocity change through a stock 450 rotor system at max power is approx 17 fps, or about 11.5mph. Half the acceleration happens on the inflow side, half on the outflow side. So, inverted, same scenario, the canopy sees 8.5fps.

Lookup the coefficient of drag for a 450 canopy in the Z axis, or measure it yourself. It produces about 2x the drag for a given air velocity coming at it from its base vs from its top (Cd is approx 2x higher for negative Z flow).

Then, consider that the speed of the outflow is not consistent across the rotor radius- it is faster near the tips, slower near the root. The canopy lives in a slower area of flow. But assume it doesn’t- and that it sees this 17fps delta upright, and 8.5fps inverted.

In that scenario, the inverted canopy at start of pitch pump generates about 35% less drag than the upright canopy. But, its advantage rapidly disappears as the machine accelerates. When the machine hits 10mph vertically, the canopy drag advantage is lost. Above that speed, the drag for the inverted canopy becomes a disadvantage.

If you calculate it for the reduced outflow speed the canopy sees due to its location nearer the hub, the breakeven happens even sooner, at approx 4mph.

The point of this is- the magnitude of form drag delta in question- including the boom, the gear, tail fin, etc, is generally quite small (< 5%) at the speeds we are talking about (Z-axis climbouts). And, in some cases, advantage becomes disadvantage as the system state changes. If the margin is substantially higher than 5%, probably makes sense to look at other subystems (tail rotor seems high on that list)

Cheers mate-
R

[airjawed]

Can't argue the physics!

If you've got time and curiousity set up your pitch with the heli hanging from the skids. I am betting on the heli becoming faster upright than inverted. My money is on small heli having more slop than larger helis relative to size.

[desertstalker]

Can't argue the physics!

If you've got time and curiousity set up your pitch with the heli hanging from the skids. I am betting on the heli becoming faster upright than inverted. My money is on small heli having more slop than larger helis relative to size.

Doesn't explain the drag racing inverted, since they will be right on the edge pitchwise (any more and they lose thrust as RPM drops) a little extra pitch would have the reverse effect.

[extrapilot]

DS

Im not sure I understand what drag racing is in your context. Is this pure Z motion? If this is an X/Y thing, that is a whole other issue, very specific to a setup.

That said, in either case, Im curious- do you believe these machines are operating at anything like peak efficiency? As they accelerate, the inflow accelerates, which changes the AOA- which, incidentally, is not consistent across the blade regardless. Blades experience centrifugal twisting moment, which further alters the AOA as headspeed changes.

Even if one knew the peak power delivery point for his motor/ESC combo, that may not translate well to the rotor system (i.e. maybe he is swinging too much rotor for the motor, or vice versa). It is a huge multivariable, and it requires some sophisticated logging to evaluate etc.

Regards
R

[wlfk]

If you look at the tail rotor, the majority of them push the air away from the tailfin rather than towards it, as it's more efficient this way round - air sucked into the tail rotor comes from a larger area than the jet of air blown away from it.

In the same way, air blowing away from the main rotor has a higher velocity than air sucked towards it, so although airflow over the shape of the canopy may be more efficient in one direction rather than another, we would also expect the helicopter to fly more efficiently if the airflow from the rotor does not have to pass over the canopy.

[airjawed]

All excellent points here from everyone. I don't think anyone is disputing all the effects and variables at play on a heli. In fact, this is definitely a big part of what makes this hobby so much fun for me.

Anyone have a data logger for power consumption and altitude meter on the heli? You could get some info on overall power used vs power done. Compare upright vs inverted climbouts and that should be a very good comparison of aerodynamic efficiency.

[extrapilot]

If you look at the tail rotor, the majority of them push the air away from the tailfin rather than towards it, as it's more efficient this way round - air sucked into the tail rotor comes from a larger area than the jet of air blown away from it.

In the same way, air blowing away from the main rotor has a higher velocity than air sucked towards it, so although airflow over the shape of the canopy may be more efficient in one direction rather than another, we would also expect the helicopter to fly more efficiently if the airflow from the rotor does not have to pass over the canopy.


Wlfk

There are 10s of variables involved in the question, most of which interact. But I picked the canopy question because it is easier for most people to understand, and they seem to focus on it as a cause.

The data I presented took some effort to calculate. If you have evidence to suggest the finding is incorrect, Id like to see it.

The reality is, air flowing over a stock canopy from the bottom (i.e. in an inverted hover) creates more drag than it does when it flows over the top of the canopy (upright hover). In a hover, the airspeed over the canopy in upright hover is approx 2x higher than inverted, and drag is a function of airspeed squared. So, if you do the math, inverted hover for a stock canopy is more efficient; it has a higher drag coefficient, but is seeing slower air. But as the machine accelerates inverted, the drag advantage drops rapidly, because the difference is airspeed is no longer 2/1. By the time you reach 8.1mph, the airspeed ratio is 1.45/1, which is where the drag becomes a break-even. Above that speed, the drag becomes a net negative inverted.

Just an sort of an interesting visual, have a look at this vid:
http://www.youtube.com/watch?v=UoZQurz3DMw

Aside from it being a cool video with a great soundtrack and a very smooth pilot, there are sections where the canopy has collected rain. You will see in those sections the surface flow pattern over the canopy. Look at 3:30 for example, and on departure, you will see how dramatically the water droplets change from a vertical path to horizontal as he accelerates. That shows you the velocity components in play- where even at a forward airspeed of 10-15 mph, the droplets are nearly horizontal. There is just not that much wash on the canopy, and it is far less for our models, which operate MUCH lower disc loading.

[extrapilot]

All excellent points here from everyone. I don't think anyone is disputing all the effects and variables at play on a heli. In fact, this is definitely a big part of what makes this hobby so much fun for me.

Anyone have a data logger for power consumption and altitude meter on the heli? You could get some info on overall power used vs power done. Compare upright vs inverted climbouts and that should be a very good comparison of aerodynamic efficiency.


Well, the original question is 'which is faster', not which is more efficient. Not necessarily the same:( I do have the hardware for testing all the rates/power levels, but without a tunnel, it would not be possible to eliminate stuff like head asymmetry.

This really needs a wind tunnel with proper high speed cams and all that...

[airjawed]

True the original was 'which is faster' but that was under the presumption that the head was setup symmetrically and slop was not a factor. I'm 100% sure that there are all the other variables at play such as described by yourself and others.

However, when it comes to 250 size helis I feel it is a slop/asymmetrical pitch issue. Then, when it comes to pros it is the effect of ground push-off and the good old 'cool factor'... IMHO

On a small heli when pitch is setup using a gauge the main shaft/head is pushed downwards which biases the slop to falsely show additional positive pitch. Additionally, the pitch gauge is typically not balanced on small blades well. This buts weight typically on the trailing edge of the blades.... biasing the pitch additionally to the positive side. End result is the in reality if for example you think you have a -10,0,+10 setup you in actuality have -11,-1,+9..... this makes for a faster inverted climbout.

[extrapilot]

Ground effect exists practically only for a rotor diameter’s distance from the ground, and even then, only adds about 15% efficiency. The rotor diameter/ground effect rule is independent of upright/inverted; it is simply disc to ground.