Low head speed nose ballooning on very fast passes

[AndrewJHawes]

Hi all,
This may be seen as an exercise in futility, but I am really enjoying flying my Compass 7HVU on 6S pack, with a head speed of about 1080 rpm, but am really getting some ballooning up on fast speed runs. I know that low head speed and 'speed' runs may sound mutually exclusive, but just humor me please.
I have a very stable, well behaved machine, except when I push the speed in a dive followed by level run. She noses up chronically before giving me back elevator control!
I have reduced the decay rate from 5 to 3 now, and this has improved things a little. Do I need to increase the cyclic a bit? to improve things further, or am I just doomed not to have the fastest, slowest head speed machine around? There is no issue at the higher head speeds. If the machine had not behaved so impeccably at 1000 odd head speed otherwise, I would not press the point and bother trying to fix this.
I will post all the settings if this is necessary.
Thanks for your opinions,
Andy

[ArchmageAU]

Hope you get someone to help with this. Yours is the quietest 700 I have ever barely heard (and it flies for 7+ mins on a single 6S pack).

Was surprised to witness the elevator action on this during runs (The faster you are going, the more aggressively it pulled up - and you have little room for error at those low head speeds and high collective angle).

[AndrewJHawes]

Thanks ArchmageAU,
I am sure that this is a side-line issue due to my extreme low head speeds of 1000 rpm or less - mostly an academic exercise because, and mostly because this machine flies otherwise so nicely in all respects at this head speed. The whole point of such head speeds does not at the same time easily entertain notions of fast forward flight and speed runs.
I watched the piro flips with the Taranis - starting to look good. I did get the last flight today though in the wind.
I ironed out some of the ballooning with the reduction of decay rate, but the problem is still present, and when it occurs it is pretty extreme. Not difficult to deal with, just need to wait until it has done its thing and lets you control the elevator again!
It would be easy to say just raise the head speed and forget the problem. I am a little more pig-headed and stubborn though. By the way I get about 12 minutes with a 6S 5000mAh pack at 1090 rpm.
Regards,
Andy

[Error401]

Sounds like you're trying to dial out retreating blade stall.

I fly an MA Whiplash at around 1000 HS, and everything I've read about low head speed flying says to manage collective, and to be careful about FFF (due to RBS). I'm running an iKon in my whip, and I didn't do a lot of tuning past cyclic and tail gain, and it does have a tenancy to nose up when I get too fast, but it's more of a hint than a problem.

I hope you find the settings to get it to where you're ok with it, but I'd be mindful of trying to dial out something and end up making it a potentially bad problem when the BD can't compensate for aerodynamic forces.

[Sonixs]

Where have you got your head gain and p set?


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[AndrewJHawes]

Hi Sonixs, my head gain is set at 55%, and P gain at 14.
I did try to mess with the Aileron and Elevater % which were 80% to a little higher, and a couple of other tweeks, but just made general flying worse. Attitude hold I increased from 10 to 12 and reduced Decay rate from 5 to 3 then to 2.
I am inclined to bring everything back to my original settings and avoid the fast speed runs. It was flying really sweetly at 1090 rpm, except for very fast passes at this.
I certainly think I was getting into retreating blade stall territory.
Andy

[ArchmageAU]

Originally Posted by AndrewJHawes

Hi Sonixs, my head gain is set at 55%, and P gain at 14.
I did try to mess with the Aileron and Elevater % which were 80% to a little higher, and a couple of other tweeks, but just made general flying worse. Attitude hold I increased from 10 to 12 and reduced Decay rate from 5 to 3 then to 2.
I am inclined to bring everything back to my original settings and avoid the fast speed runs. It was flying really sweetly at 1090 rpm, except for very fast passes at this.
I certainly think I was getting into retreating blade stall territory.
Andy

Was thinking about this last night (as to why you have no control over the ballooning) and am convinced it is RBS (retreating blade stall).

Due the way a helicopter controls rotor tilt, RBS occurs on the left hand side (CW spinning blades) stalls and loses the ability to generate lift. If you want to tilt the rotor forward, you want greatest lift on the left hand side (the side that has stalled). Adding more AoA (angle of attack) on that blade will not give you lift as it has already stalled, so the helicopter will nose up. There is nothing the FBL unit can do to help you (as the retreating blade is in stall state and you need the retreating blade lift to nose down).

You can bank left or right during RBS, but you cannot nose down till the heli has slowed enough for retreating blade to regain lift.

Welcome to the fun of low head speed.

Come to think of it, this dysymmetry of lift could be a contributing factor to dolphin behavior in fast forward flight which the FBL unit gains try to cope with. You do not get the extreme behavior seen in RBS, but the large variations in lift can be expeced on small differences in attitude and blade AoA.

There is a lot going on with physics with these things.

[AndrewJHawes]

Thanks Archmage,
What you say has some merit. I studied aeronautical engineering at Imperial when I was a youngster in a former life, and have made several calculations in this area, but the Reynolds number simulations are quite complex in reality. Also simplistic tip speed calculations can be very misleading, so beware.
Andy

[Sonixs]

As a comparison my head gain is at 70 and my P at 35.

Retreating blade stall could well be your issue. I've flown my TDR at 1300 without issue although I'm not sure whether 210 rpm more improves the performance.

My gut is telling me that the lower the HS the higher the head gain and P. Have you considered loading Popokatepetl's 700 defaults and giving it a go?


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[AndrewJHawes]

I had forgotten about Popokatepetl's settings, and where to find them.
It would be nice to have the option of two set-ups, one on 12S with a higher head speed, and one for 6S, although I already use my rotary knob for agility for different flying styles.
I have forgotten, can I set two variable parameters?
Andy

[ArchmageAU]

Originally Posted by AndrewJHawes

I had forgotten about Popokatepetl's settings, and where to find them.
It would be nice to have the option of two set-ups, one on 12S with a higher head speed, and one for 6S, although I already use my rotary knob for agility for different flying styles.
I have forgotten, can I set two variable parameters?
Andy

From: Correct tuning procedure

Initial settings

Use this predefined files for different sizes of helicopter.

Class 450
Class 500
Class 550-600
Class 700-800


Only one RTT parameter at a time unfortunately.