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500 size Apache Gunship Build

Konrad

Posting Elsewhere
#21
Neg 1° or 2° pitch. Wasn't aware that the range was so limited. I would have thought that for auto rotations one would need more neg pitch.

I assume that the trailing blade's performance is still the limiting factor for fast forward flight, both in models and full size?

Please let us know how CNC Helicopter treats the issues you found. As you might have gathered, how a firm treats the customer with regard to real technical issues is how I measure a firms level of customer support.

All the best,
Konrad
 

F106DeltaDart

Well-known member
#22
Neg 1° or 2° pitch. Wasn't aware that the range was so limited. I would have thought that for auto rotations one would need more neg pitch.

I assume that the trailing blade's performance is still the limiting factor for fast forward flight, both in models and full size?
Generally, you want little to no negative pitch in a real helicopter. In fact, most helicopters that I know were limited to flat pitch at full down collective. Since you idle with collective full down, that imparts a negative loading on the rotor. That can create ground resonance easily if not properly damped by the gear. It also doesn't take any negative pitch for a full sized helicopter to autorotate. RC Pilots use it because RC's have a very low rotor inertia, where it would be extremely difficult to achieve the right balance to maintain RPM. Applying negative pitch in an auto would most likely overspeed the rotor in a full scale. For a more comprehensive guide to autorotative aero, this is fairly well written: http://www.copters.com/aero/autorotation.html.

And yes, retreating blade stall is still a limiting factor in speed, as is vibration. That is why most modern high speed helicopter concepts are either tiltrotors (Bell V-280) or compound with active vibration suppression (Sikorsky X2).

I'll post when I hear back from CNCHelicopter.
 
#24
RC Pilots use it because RC's have a very low rotor inertia, where it would be extremely difficult to achieve the right balance to maintain RPM
It depends on the size of the helicopter. With my 500 I use maybe 2-3 degrees of negative pitch to autorotate. With my 600 with much disc loading I feather the rotor and just use cyclic to drive it.

Most RC helicopters, due to quite low disc loading, do require that you have some negative pitch available for windy conditions where ETL will cause the helicopter to flare and it won't come down on zero pitch. Light utility helicopters in full size are in the range of 2.5lb/ft^2 disc loading. RC heli's are rarely above 1 lb/ft^2, and 0.5 - 0.6 lb/ft^2 is more common.
 

F106DeltaDart

Well-known member
#25
It depends on the size of the helicopter. With my 500 I use maybe 2-3 degrees of negative pitch to autorotate. With my 600 with much disc loading I feather the rotor and just use cyclic to drive it.
Interesting, thanks for the info! I've only flown up to 500 size RC helis, so mine have always needed a bit of negative pitch. I haven't dared try it with any of my scale birds yet due to their extra weight though. The full size birds have the inertia of much heavier rotor systems on their side though. They tend to maintain RPM very well and can be comparatively forgiving when compared to the smaller birds, especially 450s and smaller.
 
#26
A scale model would be very difficult to auto because of the weight and the altitude they are normally flown at. My 500 weighs 7.2 lbs because of a 10A 4S battery in it. From 100 feet I can fly it in a full circle with the rotor at about -2 and using a lot of forward cyclic to drive the rotor, and land it just it does with power. The forward cyclic drives the rotor far better than just letting it drop. I normally run 2,100 rpm headspeed with it. I can get the rotor to 2,500 in autorotation driving it with forward cyclic (I have a headspeed tach on my radio with telemetry).

From 50 feet it is a quite harsh landing at 7.2 lbs takeoff weight. From 25 feet it is impossible to auto it from hover. In forward flight it can be done from 25 feet, but the flare is more of a plop and quite hard landing than a real flare.
 

F106DeltaDart

Well-known member
#27
Thanks for info, that's kind of what I figured. I'm guessing the the thin-chord blades wouldn't be of particular help either. The headspeed tach is a pretty cool feature to have!
 
#28
Thanks for info, that's kind of what I figured. I'm guessing the the thin-chord blades wouldn't be of particular help either. The headspeed tach is a pretty cool feature to have!
Yeah, practicing autorotations is one thing. Actually having a power failure in flight is another. When you practice there is no delay due to reaction time. When power fails in flight you lose 9 feet of altitude in the first second due to the rotor slowing and stalling. 16 feet is lost in the next second. So if you're at 25 feet with a scale model, that first second will be lost in reaction time if power is lost. By that time the rotor is stalled and no way to get the headspeed back up in the next second before ground contact is made.
 

F106DeltaDart

Well-known member
#31
Added a few more details. Flare dispenser, antennas, and wire cutters for the most part. Also started hollowing out a few parts on the nose to make rooms for a movable TADS unit. Getting close to painting!
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PsyBorg

Wake up! Time to fly!
#32
Man that is really taking shape. Great work with all them little details. Can't wait to see her done. these are one of my favorite Heli's
 

F106DeltaDart

Well-known member
#33
Thanks man, I'm definitely getting close.. Cockpit, TADS, gun, paint, and the tail rotor pushrod connections are the next couple of major items. Spent last night reconfiguring a servo to do the vertical tilt/pan on the TADS sensors. The servo was slightly too long to fit inside the sensor, so I ended up deconstructing the servo and attaching the drive motor on the side. Shortened the servo just barely enough for a proper fit :D. Hoping to have the mechanism set up and working tonight or tomorrow.
 

F106DeltaDart

Well-known member
#34
After a long night of rewiring and getting the swivels to work properly, the TADS can finally tilt up and down successfully! Now I have to get it installed in the nose, and set up the servo for panning.
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F106DeltaDart

Well-known member
#39
A bit more progress, weapons pylons have been attached and the rocket pods and hellfires have been dry fitted. I'll glue on the ordinance after the aircraft is painted.
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