700 class gasoline heli tail shake

we had modify align 700 DFC with 26cc gasoline engine, and we also test it in loiter, the problem is that the tail seems shake with low frequency. I had changed P and D without any effect.

the tail in log

and the vibration

the loiter video

the flight log attached here
flight log

does anyone could help to figure out the issue?

My best estimation is that you are running the headspeed too low for a piston engine. So there’s not enough tail authority.

It’s somewhat hard to tell from the exhaust note, but it really sounds to me like the engine is not running much much above ~10,000 rpm. Those engines like to run at 11,000-11,400 with stock porting. A ported engine will want to run at 12,500-13,000 rpm. Electric to piston conversions are usually heavier than their electric counterpart, so can’t scrimp on headspeed because the rotor produces more torque with the piston engine. You have to have the tail authority to handle it.

Compare the tone of the exhaust note with one of mine running at 11,400 rpm


I would tweak up the 75% point on your throttle curve until she starts to run at something more than just above idle.

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The flight attitude shown in the video does not look very clear, judging from the speed of the main rotor, indeed, as @chris said, the head speed is a bit low. However, I suggest to find the installation condition of the tail mechanical parts first, to ensure sufficient mechanical smoothness, and then adjust the value of Yaw’s ATC_rate_P/D to see if it can be relieved!

thanks chris, I will try to modify the throttle curve to increase the headspeed

head speed maybe the point because the speed of gasoline engine is much less than piston engine

and another question is that we used gasoline engine but not piston, this engines is always run below 12000 rpm and much more heavier than piston

I am.not a heli guy but I believe when they are speaking of the " piston" engine they are referring in general to a non electric fuel operated engine. unless you are using a jet engine, you are either using electric or piston. Your engine does have a piston.

yes, gino, but actually the gasoline engine have much more lower speed than piston

The gasoline engine IS a piston engine. So is nitro. They just burn different fuel and have different ignition method. The engine’s porting determines how fast it likes to run, not the type of fuel it burns. Most RC and PUH format heli engines have milder port timing than nitro engines, and less specific power output per unit of displacement.

But unless your engine has some really different porting than I’ve seen before, it’s going to like to run around 11,000 rpm. G26 engines have idle speed 4,000 rpm, clutch engagement 6,000, no-load 19,500. They are usually ported for max hp at 12,000.

yes, we used the zenoah 26cc gas engine G260PU which have max power at 12000 rpm.

so should we loiter at so high speed at max power? I worried for the overheat.

and I also read in wiki (http://ardupilot.org/copter/docs/traditional-helicopter-tuning.html)

Important Note - UAV helicopters, as opposed to sport helicopters, will usually be running low headspeed and higher disc loading. With a mechanically driven tail this also means lower than normal tail speed and reduced tail authority. If your helicopter meets this description, it is recommended to set ATC_RAT_YAW_VFF to 0.05 before the first test hover.

so should I increase the VFF of YAW if I have low headspeed ?

That note is intended to warn people that the tendency is to run the headspeed too low for UAV helicopters with the mistaken idea this is going to provide a lot more flight time. We don’t want one spinning out of control on the first flight due to an ineffective tail rotor control. So turning up the VFF provides more tail control to help prevent this as tuning of the helicopter proceeds.

It would be expected after that first flight that the user would realize the tail control is marginal and turn up the headspeed to get it handling correctly. It is very difficult to compensate for bad mechanical setup with settings in software. Once you get into 700 class and larger with heavier helicopters, if the blades aren’t running at minimum 450fps blade tip velocity the machine is not going to handle correctly, it will not be stable and the tail will seem “loose”. I’m not sure where this low headspeed concept came from in RC but it is not a good idea.

In your log I noticed that to change the heading only 10 degrees requires the tail servo to go from 1400 pwm to 1700 pwm. Which tells me the tail rotor thrust is totally ineffective and barely managing the torque of the main rotor.

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thanks for your help, chris

I will try to use larger tail rotor blade and increase the headspeed for test recently.

While bigger tail rotor blades will help, I think I would try increasing the headspeed first and see if it tames it. You gas heli is heavier than a typical electric with a couple 5A 6S batteries. So it’s going to like headspeed because of the extra weight. The headspeed increase will increase the tail rotor speed ~4.5x more than headspeed increase, which also improves the tail rotor effectiveness.

So simply getting it up to speed so it hovers on ~4.5-5.0 deg of pitch will likely make it a whole different machine - more stable and better tail control. And it will make your engine happier.


You might want to check out the 3 blade tail rotor upgrade hub from Align for Trex 700/800 part number H70T008XXT.

Looking at your heli picture I noticed that you have the aero version of the Zenoah without the cooling fan shroud instaled. This version of the engine overheats even with the shroud, so the engine doesn’t stand a chance without it. The Trex original fan is designed for glow engine and it just can’t produce enough airflow to cool a gasser engine.

My Trex 700/800 gasser has the RC version of the Zenoah which has a huge fan and I still need to be careful not to HOGE for too long in summer time. My Trex is quite heavy.
If I were you I would monitor the engine temp closely.

Agree 100%. It’s pretty easy to overheat the engine in extended hovering OGE at high ambient temp.

I wish the engine could be cool by the down air flow by the main rotor blade, that’s why my heli without canopy.

so far, I had test hover with 10 minutes

Free-air cooling does not work all that well for helicopter engines. They can make 7 minute 3D sprints with what amounts to free-air cooling. But for commercial/UAV use they need a squirrel-cage fan aka the RC-format engines.

It’s hard to find good telemetry sensors for CHT. So I equip mine with a temp sensor on the base of the cylinder near the exhaust port. I like to see them run from 165 to 180F. 200F is approaching over-heat. In hot weather the engine temp will climb quite rapidly in hover and 200F is the limit before the helicopter either has to go into forward flight, or land.

You can certainly abuse your engine by not paying attention to operating temp, not doing proper warmup before takeoff, not doing proper cool-down on landing, but the engine won’t last. Especially auto-landings and sudden shutdown is bad for piston engines. The internal parts of the engine are much hotter than the external and without running at no load at flight idle on landing to normalize temps the internal heat will coke oil in the piston ring lands, cause the ring to stick, score the cylinder and result in an eventual in-flight engine seizure.

The care and feeding of piston helicopter engines in RC is no different from full-size. How you treat it will make the difference between getting 50 hours out of your engine, vs 500 hrs before overhaul.

Thanks for the heads up on temps and operational aspects

increase the headspeed help tail more stab than before. here is the automatic test video with takeoff and land, thanks chris