Compound Helicopter Design

ArduCopter Traditional Helicopter
61

I think what would be really cool for a compound heli is a flight mode where when the heli goes into dynamic flight it flies like a regular helicopter. That is, if you push the nose down 3-4 degrees and recenter the cyclic, it stays there. Or if you roll 10 degrees right and recenter the cyclic it stays there. With normal collective response. But when it drops out of dynamic flight it goes to self-leveling with the ability to bring it to a stop in hover and it stays there with GPS position holding. And the collective goes back to the ā€œdeadzoneā€ like Alt Hold has where it commands a change in altitude instead of an instant change in collective pitch.

Probably not even possible to make such a flight mode. But it sure would be cool.

62

This is effectively acro mode for Copter. It is a rate command system with attitude hold. Add in turn coordination and it would be really good for high speed flight. For me though I like flying using an althold or stabilize mode in copter because I donā€™t trust myself in determining orientation especially if it is flying fast and at some distance.

So this automatic change over to a different mode obviously is not programmed in copter but probably could be. Most pilots of manned aircraft donā€™t like it when the aircraft changes flight modes on them automatically. This is the whole reason that 53K has pilot selectable modes.

63

It kind of is with Pos Hold. I think it switches back and forth between Alt Hold and Loiter, depending on if the pilot is moving the cyclic stick. I never flew with it because I crashed a 600 with it back in 3.4.2 when it had a problem. But I tried it maybe a week or so ago and I really like it. So thatā€™s a mode where if the dynamic flight flag could turn off the braking when you center the cyclic in dynamic flight, and the braking isnā€™t active unless it drops out of dynamic flight, would also be pretty cool. According to the wiki the minimum angle for the braking is 20 degrees. And that donā€™t work at all for a heli. I set it to 8 degrees and forced the change and it works good. The only problem is, in higher speed flight if you center the cyclic it goes to 8 degrees nose up, which is a little radical if the heli is going 20 m/s. I would like it just to coast. And then it would work perfect for a compound.

Iā€™ll bet it wouldnā€™t be too hard to disable the braking with the dynamic flight flag. I might take a look at that.

64

I made one thruster for my test heli. With the frame brackets, wing spar, motor mounts, motor, prop, ESC it looks like two drives with DDFP would come in at 760 grams, or 1.67lbs of additional weight with 10" CF props. Basically bolting half of a quadrotor to the helicopter.

So one immediate disadvantage of the compound design will be shorter hover time and reduced payload capacity. The DDFP drives using a conventional tail wonā€™t be running in hover. I suspect the tail rotor will absorb less power for yaw control than running twin drives. So wonā€™t be able to get good test data on hover performance with this setup.

The wing and spar weight is negligible. Most of the weight is in the drive hardware. I ran the DDFP drive on a static thrust test to see how much power it draws @ 40% throttle and I got 155 watts. Two of them would be 310 watts. I donā€™t know how much thrust it will take to push the heli @ 20 m/s but Iā€™m pretty sure it will take at least that.

I already have testing data on this heli at various payload weights.

My payload testing data shows that with 2.4 additional lbs in useful payload I get a reduction in power requirement of 181 watts from hover to best cruise @ 31 mph. At minimum takeoff weight I get a power reduction of 130 watts from hover to best cruise @ 31 mph. At minimum takeoff weight I run the main rotor at 1,550 rpm. With 2.4 lbs additional payload I run it at 1,630 rpm. So for whatever reason I get a higher efficiency rise going from hover to translational lift with the rotor loaded slightly heavier, and running at slightly higher speed. I suspect this is due to decreased efficiency in hover at the higher disc loading, so the net gain in translational lift is higher. Regardless, the required cruise power is still 104 watts more with 2.4lbs of payload than it is at min takeoff weight.

I do not have exact numbers for 1.67 lbs of payload because thatā€™s not one of the steps I tested for in the past.

So just extrapolating some numbers here, I can see the electrical and prop tip losses in the thruster drives will no way be recovered in efficiency gains in the main rotor from not having to provide the horizontal thrust vector to make the heli go. It cruises with the 2.4lbs of payload at only 6-7 deg nose down pitch @ 20 m/s. Two things make that heli go horizontal - a very slight amount of tilt on the main rotor disc. And the cyclic pitch operating the advancing blade at quite high efficiency, and the retreating blade at a higher lift/drag ratio. I found one place in an old log where the heli was cruising @ 11 m/s and I put it into a nose on the horizon attitude to let it coast and slow down. The change in power while it was coasting was only 65 watts. And this actually agrees with my experience in full-size helis - in translational lift the engine torque changes very little from steady state cruise to nose on the horizon bleedoff of speed for an approach. Basically only whatā€™s being absorbed in the cyclic pitch cycle due to dissymmetry of lift on the disc, which happens to be very, very efficient in the big picture.

So Iā€™m not liking the numbers on this, without even testing it. I think Sikorsky might have a better idea with the X2 concept. But I think it could be modified and not use a coaxial head. Why? Because we have already proven with TriCopters that we can provide very adequate (more positive than differential torque) yaw control with a tilting tail rotor. I have a 1 meter Tri and there is just no quad/hex/octo that can match it for positive and very powerful yaw control. So letā€™s apply that concept to a compound heli. But instead of tilting the tail rotor laterally, tilt it longitudinally. Use it for yaw control in hover and slow speed flight, transition it to longitudinal thrust in high speed flight and let conventional tail control surfaces take over yaw control with a steerable rudder like most coax designs use.

I havenā€™t worked out the engineering details yet on how to do this. Itā€™s a concept that popped into my head. No additional drive losses over a conventional. Switch the tail rotor configuration to get the performance of a X2/S-97 Raider in high speed flight, but with a single-rotor head. Could be done with a DDVP drive.

65

So you looked at the weight of one side which included the motor, ESC and structure. Iā€™m not saying that the net weight change will be a decrease but you didnā€™t account for the fact that the motor for the main rotor does not need to be as powerful and thus as big (heavier) as what you have now since it no longer needs to drive the tail rotor or provide the added power for fast forward flight. Plus the weight of the tail drive train and hub is also not zero. So removing the tail rotor hub and drive train will help too. I donā€™t think the additional weight is going to be as much as you are indicating here. Again I donā€™t think it will be a wash but maybe not as bad as you think.

From your data Iā€™m pretty sure we can develop a performance model to help extrapolate or interpolate to the weight you are interested. Iā€™ll need to get a little more info.

This does not account for the fact that you want to offload the main rotor thrust with the wing. There is drag associated with the production of lift on the blades which is required to increase the thrust vector to go faster. The wing and thrusters both reduce the thrust required on the main rotor and subsequently the power associated with that. The difference in power between constant speed attitude and coasting attitude as you talk about above is interesting but the example for 11 m/s is probably not very representative since is such a slow forward speed. Power increase with speed beyond the bucket speed is more a function of (velcocity)^2. So I think you will see much larger differences in power between the attitude for maintaining forward speed and level attitude plus if you had a wing, your power can further be reduced because the rotor doesnā€™t have to produce as much thrust.

In addition, you can slow the rotor to reduce the profile drag of the rotor. power required by the rotor is a function of (rotor speed)^3. So reducing the rotor speed has big benefits performance wise.

As you mentioned before, this will probably not be a very efficient hovering machine due to the fact that the wing is causing a download. However I donā€™t think the picture you paint for forward flight is as bad as you think.

Interesting idea.

66

Well, Iā€™m not locked into anything yet. Iā€™m exploring different ideas. When it comes to drives, for instance, the mechanical tail drive absorbs roughly the same power as an electric motor does running with no prop on it.

The data I have gathered during performance testing comes from the most efficient cruise speed for the heli, which is 31 mph. And the heli has to be able to fly at that speed just as well as at 60 mph because otherwise on survey flights @ 75ft over terrain it will overfly the camera. Iā€™m not convinced that stub wings are going to offload the main rotor very much at the speeds weā€™re looking at with RC heliā€™s. I played with this on my 1 meter Tri and it was marginally successful, but there weā€™re looking at a 1 meter (motor shaft to motor shaft) wingspan with a 2,750gr aircraft. That one meter wing was able to produce only about 10% of the total lift at the speeds and AoA it could be flown at. So Iā€™m borrowing on that experience as to how effective stub wings will be on a heli at anything but very high speed flight. I fear they will simply become sources of drag and dead weight for the majority of flying.

Thereā€™s a probably a lot of tradeoffs. But I do want to explore this design a bit. Sikorsky has a long history of getting it ā€œrightā€ out-of-the-box so I think itā€™s worth looking at.

67

Hah, fair point. I would be pretty lost without stabilize in Copter and know a few people who would be lost without FBWx in Plane.

That sounds heavy, especially for the amount of power. You can get more than that from a mini-quad drive train, which wont weigh more than ~60gram (prop, esc, motor). Issue will be power from the motors, but that could always be regulated down (or get itā€™s own lipo for testing purposes).
If you give me some mounting points measurement, I can try to put a light weight setup together.

68

Itā€™s a Tiger 3510-630 but it has to be representative of an eventual DDVP drive anyway, if this is to work.

Bill brings up an interesting point:

The motor in the heli is a 460gr unit:
https://www.scorpionsystem.com/catalog/helicopter/motors_4/hkiii-40_1/HKIII_4035_530/

By going to this one I could save 95 grams and it would likely have enough power.
https://www.scorpionsystem.com/catalog/helicopter/motors_4/hk-35/HK_3536_510/

What is alarming is the amount of power these multi-rotor motors pull to create very little thrust. For the typical 4kg quadcopter each drive has to produce 1000gr of thrust to hover it. Thatā€™s about what I was getting out of it and it took 155 watts to produce that? Thatā€™s 155W/kg of thrust, which is not even close to the efficiency of a heli drive.

So anyway, Iā€™m just playing with things to decide what I want to test.

69

I was thinking mounting points on your heli, not for the motor :).

Having said that, I played around with some numbers and while you can certainly find smaller motors that can deliver the power, the problem is finding 6S capable motors. Closest I get is the T-Motor F80 and thatā€™s still 1900kv.,

70

And to futher complicate it, Iā€™m running the heli on 12S. I planned on building a splitter harness to run each drive off respective 6S halves of the 12S packs.

I got a bunch of Trex 600 and 700 parts here. I could pretty easily build belt drives for the thrusters too, and just order special-length belts from Bando Industrial. Based on hours and hours of flying belt tails Iā€™ll bet the power efficiency is higher on the belt drives than it is on DDVP. Belt drives kind of went out of vogue because the manufacturers came out with torque tube/gears. And the main reason they did it is because the heli is easier to pull the tail off for packing when the pilots travel to the different 3D meets and so on. But in my experience the torque tube drives are not as power efficient, nor as they as reliable as belt.

71

Some things are notable about the compounds compared to the venerable Bell 206B, of where there has been over 7,000 examples built and flown in just about every application imaginableā€¦

The service ceiling of the compounds is not any better than the old 206.

The PL of the JetRanger is about 10. It is 3.33 (actual tested at 3.66 @ 250kts) for the X2. It is 2.53 lb-hp based on specs for the X3, but no actual data was ever released for the X3 that I can find as to what maximum speed performance was.

The disc loading of the 206B is 4 lb/ft^2. It is 11 lb/ft^2 for the X2, 8.5 lb/ft^2 for the x3.

So some conclusions I come to; neither of the compound designs are as efficient as any conventional helicopter in their rotor size class that has much lower disc loading. The high disc loading is being used in the compounds to offset main rotor drag in high speed flight, and probably for a greater level of stability. Of the two compound designs, the X2 design is likely more power efficient, although without the actual tested engine torque data from the x3 at maximum cruise speed, that would be just a guess based on the PL.

Since a lot of engineering resources went into designing these things it become apparent that it is probably not wise to scrimp on shaft power. If they couldā€™ve done it with the X2 on 900 shp they wouldā€™ve. But the engine was producing 1,640 shp @ 250kts. Doesnā€™t take that much shp to turn a pusher prop on a 6,000lb aircraft @ 250kts. But it does to push blade tips thru the air at very high relative airspeeds :grinning:

What Iā€™m learning is that thereā€™s more to designing a successful compound helicopter than simply bolting on some thrusters.

72

Hello!
Havenā€™t visited here for a while.

Iā€™m also designing a compound heli.
My concept is a ā€œsimplifiedā€ X2. Since various sources Iā€™ve read said that the lower rotor beating disturbed/dirty air is inefficient, Iā€™m making the upper rotor generating only drag.
Basically change the tail blades(of a tradheli) into dumb paddles and move it to the top, no dual swashplate required.
The idea is that a rotor can have finite lift-to-drag but infinite drag-to-lift so this rotor can be ~100% efficienct at yawing.
Also with its horizontal plane of rotation, moment of inertia also helps, making DDFP more likely to work.

Any thought?

73

It sounds like a reasonable concept. Iā€™m not convinced yet that the compound designs are based on efficiency, but more on high-speed performance to migitage the effects of RBS in high speed flight. Both of the successful full-size designs are very high power to weight ratio helicopters. In fact both are higher power to weight ratio than the largest and most powerful helicopter on earth, the Mil MI-26, with 123,450lb takeoff weight and 22,800 shaft horsepower. The Mi-26 holds 17 world records in weight lifting that have stood for better than 30 years. When theyā€™re throwing that kind shaft power per pound of aircraft weight at going faster than 190 kts, it raises some alarms as to how power efficient these designs really are

I donā€™t know how the induced flow works for a compound as compared to a conventional that operates with the TPP slightly tipped towards direction of flight.

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At least it is the most efficient form of VTOL I know of and we are just trying to give it more speed (and range).

BTW has anyone compared compound helis to a tiltrotor like V22?

75

Speed is not a problem with RC helicopters

But something that can give them longer range and better efficiency is worth looking at.

I would guess the tiltrotor aircraft is probably more efficient in cruise flight. The helicopter probably provides greater flexibility in itā€™s operational range, far greater stability in slow speed flight, and is more practical for applications requiring VTOL. In the world of full-size the tilt-rotor V-22 has been one of the most expensive maintenance nightmares in the history of military aircraft, outside of the Apache helicopter. So practicality is a huge issue when looking at various designs.

The reason I brought up the induced flow issue is because the change in induced flow thru the main rotor is what makes the tail rotor become very efficient on a conventional heli in ETL. If itā€™s the same this should apply to a tail-mounted thruster prop as well. And might be the reason the X2 design achieved the same flight speed on less shaft power consumed per kg of aircraft weight vs the x3 design, despite the fact it is using coaxial rotors. Iā€™m fairly convinced it would be possible in a RC design, with the proper mixing, to build one that uses the tail rotor as a conventional in hover and slow speed. And transistions to the tail rotor providing thrust in high speed flight. And do it with a single main rotor.

And there are other things that could be done to improve aerodynamic efficiency such as using a ā€œspeedā€ fuselage like the Daiblo in the video. Or using a ducted fan-type tail rotor aka the Kamov Ka-62. And using more efficient main rotor blade designs, which I have already played with. At the quite low disc loading our RC heliā€™s fly at, they are already quite efficient, very high-performance machines. Iā€™m convinced it will take a combination of things that improve the aerodynamic efficiency of the airframe and main rotor beyond simply bolting on thrusters.

76

So the upper rotor (if you want to cal it that) is only making drag. And you arenā€™t going to be able to change the drag it makes? So this upper rotor will just provide the anti-torque moment to counter the lower lifting rotor?

It may be, but you now have this upper rotor producing drag with no lift benefit. Overall IMO this would not be a very efficient aircraft.

77

This is an interesting comparison. You may be right. The selling point of the X3 and X2 designs may not be efficiency but cruise speeds well above most conventional helicopters. Certainly much more than a jet ranger. It cruises at maybe 130 kts? However for the aircraft to be marketable, the efficiency has to be comparable to conventional aircraft. I guess it depends on how much the buyer values speed.

Currently there is research being put into multi speed transmissions for these designs. They want to slow the rotor for forward flight and then speed it up again for hover and low airspeed. I believe that slowing the rotor is being done to reduce the profile power requirements (power due to dragging the rotor thru the air). There are some drawbacks to that for conventional helicopters and the X3 which is the retreating blades stall and potentially flying qualities. So I plan to look at this.

There is a manned version of this called the Piasecki Speed Hawk. They took a black hawk and put a ducted propeller on it that the thrust could be vectored to provide anti-torque and directional control in a hover and vectored rearward to provide propulsion for forward flight.
http://www.piasecki.com/x49a.php

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My plan (for now) is to use another motor varying its RPM similar to DDFP.

Yes, it wonā€™t be much more efficient.
As I have found, lift benefit in coaxial is offset by the interference loss so IMO it will be at least as bad as convetional tail rotor with the benefits of infinite drag to lift ratio and moment of inertia.
Also, dual-swashplate is a pain to work with.

79

Yes, the reason I compared to the old 206B is because it has about the same size rotor and is a very popular, efficient, quite proven utility helicopter. Not that fast, but fast enough. Lifts a good payload. Very economical aircraft. So how do these super high-speed designs compare? Thatā€™s what I was looking at. I would expect the power loading to be a little higher like on the X2, which is a heavier helicopter but certainly not any bigger than a JetRanger. But when weā€™re pulling 1,680shp to go 250kts the question arises as to why when the old JetRanger can do what it does on 317shp from itā€™s Allison 250 :grinning:

80

Because the the Armed forces want a Vertical lift capable aircraft that can get to the objective faster than a conventional heli.