Pitch at take-off and landing

[npomeroy]

It's been mentioned from time to time that destructive wobbles on spool-up and spool-down are especially associated with the transition from negative to positive pitch. I can see why a few degrees of negative pitch is needed during flight, but is it really any benefit during spool-up and down? I'm thinking about setting my take-off flight mode PC to start at zero, and also the PC for throttle hold. Once in the air I can switch to the next mode which would include some negative pitch. Of course the PCs would need to be matched above, say, 20% stick so that I didn't get an embarrassing sink when switching flight modes.

Any thoughts?

EDIT - I see this is discussed here https://www.helifreak.com/showthread.php?t=705407
Hope this is not duplicating the discussion too much.

[Andy01]

Nelson

I was going to refer you to the other thread I think it possibly has most of the info on offer.

As mentioned there I usually spool up with a couple of degrees of positive pitch, and if I want to switch modes, do so just before take off. The only difference between my modes is the headspeed setting in the Castle governor. Often I just fly in Normal, although I use the flight mode switch to start the timer on my 11X just before lifting off.

Colin

[Keyrigger]

Hi Colin and Nelson:

I have had a few times (3, if I am not mistaken) where I flew in Normal mode and while doing an angled approach, cut power (remember, no negative pitch in Normal) to the motor and had some disastrous results ensue. The angled approach demands some negative pitch to work properly, especially in windy conditions, so when the helicopter does not want to come down, the normal reaction is to apply more negative collective but as there is none in Normal Mode, when the stick is dropped, so is the motor.

So with that explained, I now am setting my Castles so that I have a much slower head speed in Normal Mode, striving for 1000rpm at the most. This provides an obvious state that the helicopter is not ready for takeoff and will prevent such an episode from happening in the future, I hope. I now use only two of the mode switch positions for flying and the last one is seldom used as I have finally worked out what head speed the models are good at so that becomes Idle 1.

Normal mode is still set with no negative pitch and many takeoffs and landings on asphalt and concrete have shown that the system works just fine. I never got the opportunity to try the Airwolf on a hard surface so I have only had excellent success with skid based helicopters. I think setting the blades out is more important to a smooth spool-up than anything else, provided the blades are properly balanced. Hope this helps out.

Don

[npomeroy]

Ok, thanks for the comments Colin and Don.
I can see a range of solutions there.
What about setting (in degrees on a 7-point PC)
Normal = 0 0 +2 +4 +6 +8 +10
Idle 1 = -4 -2 +2 +4 +6 +8 +10
Both with governed flat rpm's. So you could take off in N and switch to 1 at any stage. And you could change back to N before touch down. I'd set Throttle Hold the same as N. Of course the PC need not have exactly those numbers or linearity but the point being to match the curves above about 1/4 stick. Also, with governed rpm even on N there should not be Don's case of the the drop out problem at lower stick.

[goodhunting]

Depends on the ESC but CC can be set to low change of RPMs... I get in a hover in Normal mode.... Then I switch to IU1 just as I transition to forward flight... The RPM increase works as a booster anyway and the slow change makes it unnoticeable. Gh

[Andy01]

I use the same 2 point PC in all 4 modes (incl. TH). It simply goes from around -2 or 3° at low stick, and up to around +10 or 11° at high stick. Usually hits around 4 or 5° around midstick. I try to stick with the KISS principle.

Three of my four scale helis weigh more than 19lbs, so getting them back down is usually not a problem - gravity works a treat - sometimes too well

I spool up to 1 graduation mark below half stick, which generally gives me about +2° of pitch, wait for it to spool right up, switch to ST1 to start timer, and then usually switch back to Normal, but there would be no reason to switch out of ST1 if you wanted to fly there. I always land under power (except when there is no power for some unforeseen reason), and drop to low stick once fully grounded - in Normal mode that obviously cuts the motor out and for insurance I hit TH.

The only time I have ever had a ground resonance problem was last weekend when I mistakenly bolted on the 4th HeliTec 820 x 64mm 235 gram asymmetrical blade on my Baumann EC-145 backwards (I have to take 1 blade off to fit it in the car). The spool-up got about halfway and looked like the mechanics or tail boom was not bolted in. I wouldn't recommend it

Throttle is controlled by the Castle Set RPM governor, with 3 speeds for the 3 flight modes.

My biggest "risk" is that I manage to touch zero stick in Normal mode and the Castle ESC goes into its soft start mode, which would not end well. Hasn't happened since I got my first Ice 50 in 2008 (I think - lots of sleeps ago).

Nelson

Do whatever works for you. Curves are a fairly personal thing, especially in scale, and it is easy to experiment.

Colin

[stevolution]

Negative pitch on spool up and spool down are very helpful on those windy days to prevent tip overs.

[Keyrigger]

stevolution:

The main reason that negative pitch is avoided on a scale helicopter, especially with multi-blade heads, is the tendency for the blades to flutter at the point they transition from negative to positive. Spooling down and dropping the pitch at higher rpms can also include the same flutter, and with that, the dancing from one skid to the other (or wheel). What might work on a light helicopter can be a disaster on a heavy scale bird and there are a few videos on YouTube that show exactly what we are attempting to avoid. If it works for you, great, but do be careful of it.

Nelson:

I use a very tight line for my CC ESC. I start at 0 and with 10% movement of the throttle stick, I am at the flat line setting for the speed controller and the rpm level I have set. I use only three points on the curve in Normal and two points in Mode 1 and 2. I have attempted to set the throttle "Off" point low enough to minimize the potential for an ESC shutdown but I have still managed to do it. Hope this helps.

Don

[stevolution]

[QUOTE=Keyrigger;6661138]stevolution:

The main reason that negative pitch is avoided on a scale helicopter, especially with multi-blade heads, is the tendency for the blades to flutter at the point they transition from negative to positive. Spooling down and dropping the pitch at higher rpms can also include the same flutter, and with that, the dancing from one skid to the other (or wheel). What might work on a light helicopter can be a disaster on a heavy scale bird and there are a few videos on YouTube that show exactly what we are attempting to avoid. If it works for you, great, but do be careful of it.

Ah i missed that this was on a scale heli.. But even so i would have known that. Learn something new every day