Compound Helicopter Design

Exactly. I think that was the primary design objective. Which is not quite the same as my objective. That Piasecki design looks very interesting.

Yes, for the stuff you are doing, it makes sense that you want the best range you can get. Iā€™m continuing to work on my new compound design. I am interested to see if I can squeeze out a higher range airspeed with the compound design. Iā€™m a little surprised that it is only 15 to 18 m/s for RC helis. The efficiency of the power plant plays a big part in the best range airspeed. Too bad we can get power readings from your gas heli variants. but I guess you have done similar trials with your gas powered helis to determine best range airspeed. Is the number very different?

The piston heliā€™s are different. A big gasser burns about 23-28cc / min and you can run them at higher power and speed without a big penalty in flight time because weā€™re looking at 2 hours. If it shortens the flight time by 20 minutes for a long flight it really doesnā€™t make much difference unless youā€™re pushing it right to the edge of the envelope. And the piston heliā€™s are most efficient at maximum torque - any internal combustion is. So theyā€™re inherently much faster to get into the best efficiency range of the engineā€™s torque curve.

The limitation with electric power is energy density of the fuel system. You start looking at carrying 8lbs of batteries vs 6 lbs of gas (one gallon) and the 6lbs of gas will take the heli on a 50 mile flight and bleed off weight as it flies. Where the 8lbs of batteries are dead at 20 miles and efficiency drops as the voltage sags and amp draw goes up for the same watts shaft power output. Unfortunately, electric cars have the same problem. Two different worlds.

Thanks for the info but did you determine your best range airspeed for piston heliā€™s. If so, what was it? I know it is probably more difficult to determine because you either need a fuel flow sensor or be able to measure fuel consumed for the same circuit flight. At some speed your range for a tank of gas must drop off dramatically.

@bnsgeyer let me put this a different way.

Electric power is most efficient at light loading, least efficient at full loading. So the best efficiency you see with electric is where the airframe has the best lift/drag ratio.

Piston power is a combination of the aircraftā€™s lift/drag ratio, plus the fact that piston engines achieve best efficiency at maximum torque. So if we fly the heli at the speeds that electrics fly at the engine is not very efficient. Since the conversion efficiency of piston engines is lower than electric motors, we need to push a piston heli harder to get into the best range vs fuel burn. Which happens to be ~double the speed of most electric drives in the same aircraft.

Yep. 44-50 mph is best speed vs range with the piston power.

Thanks. I appreciate the info.

Keep in mind that what I have tested for is best range with the fuel onboard. The reason the piston is faster is because of the conversion efficiency of a combustion engine vs the conversion efficiency of an electric drive. Not because of aircraft lift/drag. Thatā€™s why I look at power loading when comparing aircraft performance. Best efficiency in turbine and piston is at around 85% of maximum power. An electric drive wonā€™t last very long at 85% of itā€™s maximum power rating.

So with the electric drive weā€™re optimizing several things. Weā€™re finding where the best lift/drag ratio is for the best efficiency from the drive, and how fast the batteries can discharge to produce the least amount of heat internally in the battery to provide the power to go.

Weā€™re doing the same thing with piston or turbine power. But the ā€œsweet spotā€ for the drive is at much higher loading on the drive, at or near 85% of maximum horsepower, which is right around the peak of the engineā€™s torque curve.

So the same airframe gets two different results, depending on what drive is used.

@bnsgeyer I think you could probably relate this to your experience flying full size aircraft. Compare the same aircraft with a normally aspirated vs turbocharged (altitude compensated) engines. The turbocharged aircraft has a much higher cruise speed. The best cruise power setting for the normally aspirated engines will be 23 square - 23" of manifold pressure, 2,300 rpm. The best cruise for the turboā€™d engines will be 32/23. The altitude compensated engines have to operate at higher manifold pressure to achieve best range because of the boost in their torque curve. Even though they operate at the same rpm as normally aspirated, the props canā€™t be run quite as flat with turbos, so they take a bigger bite and go faster, and get best range at higher power, despite the drag on the airframe actually being higher at higher cruise speed.

The same thing applies to the helicopters. And thatā€™s the realization I came to looking at the X2 design, since it has the most published testing data. It may actually be a quite efficient helicopter despite the high power requirement to go 250 kts. And may very well achieve the same efficiency as a lower powered slower helicopter, with the only penalty in fuel burn being the additional energy expended to go faster. Donā€™t care what kind of aircraft it is, the faster you want to go, the more power it takes. With the full size compound designs weā€™re looking at additional power requirement due to compression ahead of the blade tips approaching Mach. We donā€™t have that problem with RC size (at least not yet).

I found an article regarding the airbus racer. It listed some details on the design which I thought was interesting. So the Racer will have the same engines as the X3 (RTM 322). However they plan to cruise on one engine with the Racer design for the very reason that you have stated, more efficiency at higher power setting. So they claim that the aircraft will consume 15% less fuel per distance at 180 kts compared to a helicopter at 130 kts. They stated that it will be a medium sized helicopter. So Iā€™m guessing probably about the same size as the X3 (~12,000 lbs).
So I mention this because for industry its not speed at any cost (and I donā€™t think that is the military motto either but I do think speed to the objective is higher on their priority). It appears Airbus will attempt to provide a marketable aircraft that will offer more speed with more efficiency. And from reading the article they are doing a lot to the design to be able to achieve their goal.

This article supports your statement. Something like a tiltrotor or tilting props where cruise flight is accomplished thru wing borne lift can offer huge improvements in efficiency in cruise but they lack in performance in hover. Always a tradeoff :slight_smile:

Edit: So just to clarify the Racer will have two engines but one will be shut down in flight and they will cruise on one engine.

There I think weā€™re getting to the meat of the issue. And thatā€™s what makes this project so interesting because RC modelers have for many years been at the forefront of experimenting with different designs. I think to design a successful RC compound we need to design for a target cruise speed. Not a ā€œone fits allā€ design. So we establish a baseline that a particular design of helicopter and drive achieves best efficiency at 15 m/s. Now, we want to design to go 20 m/s instead at best efficiency. That doesnā€™t mean that the faster compound will have the same power requirement as the slower conventional. It will have a higher power requirement. But it will go the same distance as the conventional on the same, or less, fuel. And do it in less time.

Correct. I got that. What theyā€™re doing is optimizing the torque curve of one engine for slower speed flight. Fire ā€˜em both up if we want to kick the tire, light the fire and get outaā€™ Dodge. Actually makes perfect sense.

Iā€™m with you. I was just curious what piston powered heliā€™s could do. So to add to your statement. For a particular design and drive, we want it to achieve higher best range airspeed with as good or better efficiency for the same usable payload.

Yep. I think that would be a suitable design goal. I think it could be done with piston power too, and probably use electric thruster drive. The Zenoah engines can have a Jewel or PowerBug generator fitted to charge batteries so electric drives could be used in combination without a big penalty in battery weight.

@bnsgeyer I have kept written logs of my flights so was looking thru them. Nitro vs gas fuel is not much different. The nitro burns a lot more fuel due to it being an alcohol based fuel and lower energy content per gallon. But due to the ideal air/fuel ratio for gasoline vs nitro, the nitro engine can burn more fuel, and create more power per unit of displacement. But the loading on the engine is about the same, just thatā€™s itā€™s smaller displacement than a comparable gas.

For the electrics, it appears that they like to be loaded at about 20% of the motorā€™s maximum continuous kW rating. Three different helicopters, same result on where I get best range - right at about 20% of the max kW rating.

Note, Iā€™m not talking about ā€œthrottle settingā€ with the electric. Iā€™m looking at watts, which is the equivalent of horsepower in the Imperial measurement system.

I had not actually ever looked at that before, from that standpoint, for the electrics. According to industry standards, electric three-phase motors should be most efficient at about 75% of rated load, very similar to internal combustion engines. But in these RC drives I think weā€™re looking at a combination of the motor plus how much is converted to heat in the ESC (basically a very inefficient three-phase square sine inverter), and what the batteries are capable of. I found it interesting that despite using different ESCā€™s and different battery combinations, it came out to about 20% of the motorā€™s maximum continuous rated output on all three, at the point where I get best range.

Edit: I think what weā€™re looking at here is I^2R losses in the entire electric drive system, vs what can be use to efficiently make the heli go. Iā€™m not familiar enough with electrical engineering to explain it. Give me pistons, volumetric efficiency, turbine bypass ratios, and Iā€™m at home. But electrical stuff is not my forte when it comes to drives.

Sorry for a dumb question
What if you use a smaller motor?
Ignoring the weight difference, it is going to work at above 20% for the same watt.

That, I donā€™t know. When I build the heli I put in a motor sized to the maximum power requirement of the heli. Then size the ESC to match the motor. Iā€™ve tried different battery configurations from 8S to 12S, and different mAh capacity. But donā€™t have any spare motors or ESCā€™s laying around to try underpowering or over-powering.

Like my 696, I went with over-power because its intended use is high speed survey carrying maximum battery capacity. And it is a faster helicopter. But it still figures out to about 20% of the motorā€™s peak continous power where itā€™s the most efficient.

Edit:
On this bold above, the 12S configurations have tested out to have best flight time and efficiency. And way more available power than 10S or 8S.

I wanted to determine what changes I need to make to my design to increase my best range speed for a given power plant type. So I developed a simple level flight performance model that can be used to look at design variables to determine what affects best range speed aside from power plant type. The model looks at the contributions of induced power (power due to rotor producing lift), profile power (power due to dragging the rotor thru the air), and parasite power (fuselage drag).
So the design variable that makes the biggest change to best range speed is the coefficient of drag on the fuselage. So the more you can reduce drag on the fuselage, the faster your best range speed will be. I also looked at the effect of lower rotor speed and as many of you probably already know that has a big effect on specific range. But didnā€™t really change the speed for best range.
So there is nothing unique about the compound design that increases your best range speed. I think where it will help with my design is in offloading the rotor both from propulsive thrust and lifting the weight of the vehicle. Getting a wing to reduce the weight the rotor carries will allow me to slow the rotor down to reduce the profile power. I just have to be careful about getting too close to RBS. It is likely that I will have to have two rotorspeeds set. One for hover which will be higher and then a slower one for forward flight.
In addition I will try and significantly reduce fuselage drag to help increase the best range speed. So weā€™ll see how well this works.

It will be interesting to see how that works. I typically run higher headspeed when flying over 20m/s and go to my idleup 2 setting. Otherwise the heli wants to roll to the right. 60.5 inches main rotor diameter @ 1,650 rpm is about 550fps blade tip speed. About in the same range as most light utility helicopters like the Mosquito, R-22, etcā€¦ Good to about 80 mph airspeed. My 626 wonā€™t go that fast unless I speed the head up to 1,950 rpm and itā€™s main rotor is only a little bit smaller - 55". Otherwise at its idleup 2 setting @ 1,700 rpm itā€™s good to about 60 mph, but have to watch it flying upwind and turn it down a little bit if the wind is strong or it starts rolling to the right and end up with half stick in it to keep it wings level.

If you ramp a 700 up to 3D headspeeds theyā€™ll hit 135 mph no problem. A 600 wonā€™t.

I think this makes sense that aerodynamic drag would be the limiting factor.

I was looking at the specs on my Synergy 806 build and the maximum headspeed is 1900 rpm. Thatā€™s a blade tip speed of 865 fps, or 590 mph. In the world of full-size they typically max out at 750 fps.

These helicopters, with enough power, and using the ā€œspeedā€ fuselage, are capable of 190mph airspeed. That puts the advancing blade tip well over Mach 1. At 171 mph airspeed the advancing blade tip hits Mach at sea level. I didnā€™t think it was possible to hit Mach speed with the blade tips on a model RC helicopter, but it is.

It also takes a lot of power to go that fast - right around 7-8 kW for a typical 7kg heli with a 800-class rotor. Observing the 800-class machines in the speed runs, Iā€™ve never seen one where the blade tips go supersonic because the motor lugs down due to the expodential increase in power required to push the blade tip past the speed of sound. But pushing some numbers here, I think experimenting with a larger 800-class machine for a compound might yield results very similar to the real-world Sikorsky X2 and AirBus x3 demonstrators.

I have become convinced you will not beat a conventional in fuel consumption vs range with a compound. Thereā€™s too many additive losses in a compound design. But you may be able to push it beyond that ~195 mph ā€œbarrierā€ that limits the speed of helicopters due to the blade tips exceeding the speed of sound. But itā€™s going to take a LOT of power to do it. In RC neither piston nor electric power is practical for this. The piston engine simply canā€™t produce enough power. The electric can only do it for a very few seconds. You would need turbine power to do it, and a Wren 50 would not be big enough. Youā€™d have to use a converted JetCat SPT-15 with a bevel gearbox on it. And the SPT-15 sucks JetA at a burn of ~8oz/min at 100% torque.

I think this all boils down to a fundamental rule in aviation since the very beginnings - the faster you want to go with a particlar airframe design, beyond itā€™s best lift/drag ratio, the more power it takes. And there is no free lunch. Improving the lift/drag ratio of the airframe always yields higher efficiency than bolting on more horsepower.