Heli vs. multirotor considerations


I started a similar topic at RCG with mixed success. I’m hoping that a few experienced APM-Tradheli users could provide some more insight…

My ultimate goal is to build a hover-capable UAS with highest possible airtime, ideally in the range of 60+ minutes. Maneuverability is not an issue. Flying like a “pregnant watermelon” would be just fine. The UAS should be able to carry about 500-750g of payload which would be a small camera (Boscam HD19/similar or FLIR PathfindIR) on a gimbal with a small laser pointer and OSD/VTX. Primary missions would be wildlife observation and SAR (volunteer work).

AUW below 10lbs. would be a soft requirement because with the coming EU regulations, >10lbs. would probably put me automatically in the “professional user” category with the need for all kinds of paperwork and permissions.

I have read several times on the net that a helicopter - in principle - is more efficient than a multirotor. On RCG, I was advised that helos are more dangerous, which I don’t see in this case because to get what I want with an X4/X8 or Y6, the design would be about 4ft across with 28/29" props and something like 15lbs AUW… Also, on RCG, I was advised that helos would be less efficient than multirotors, which contradicts what I have read in other places.

So, the all-deciding question is: Is a helicopter really less efficient than a multirotor? And - what would be the ideal solution? Electric? Petrol? 550? 600? 700? 800? And could an APM/Pixhawk handle such a big Tradheli anyways?

Let’s keep this discussion on a technical level, please :slight_smile:.


A petrol helicopter running a gas motor (not nitro) will get you far longer flight times than LiPo powered electric. Your biggest issue with petrol, from experience, is vibration dampening. A petrol motor is basically a jackhammer…

If you can get a helicopter frame that will allow you to run a 15-20cc gas motor You’d be pretty set, and probably able to do it in your weight range too.

Helicopters are very dangerous, especially in the 600-800 size you would be looking at. The issue is the amount of inertia in the blades, as well as the tip speed. With a 600mm radius going 2000rpm - the blade tip is going 450km/h. You don’t want to be getting hit by several hundred grams of blade at 450km/h. Multicopters running 8-14" props will give you stitches, Helicopters have, and will continue to kill people and require amputation of limbs.

That being said, I consider it in the same category as LiPos. Many new users are told how a lipo can burn your house down, and how they have in the past and will continue to do so. Be careful, be sensible and nothing bad will happen. Dont fly next to someone, dont fly over a crowd, dont land next to vehicles… keep the helicopter clear of everything and you’ll be fine.

The simple fact that a helicopter can run a petrol motor gives you flight time you cant even begin to imagine on a multicopter. Check out Photoship One for some ideas of camera mounting booms which will isolate the camera nicely.

I do see the dangers of the blades but looking at a multi with 28/29" blades, also rotating at 2000+RPM, especially 4, 6 or 8 of them, I don’t think, that would be particularly less dangerous.

For a first “cheap” test-/experimental platform, would an electric 550 suffice? Given all possible optimizations, what airtimes could I reach with an E-550?

Where’s the RCG thread? I’m curious to see what happened. Probably a bunch of misinformed people.

This is a contentious issue, but there are a lot of misconceptions about helicopters.

One of the biggest ones is efficiency. I’ve kind of gone back and forth on this a few times, and here is my latest position:

  • A helicopter is in fact more efficient than a multirotor. This is due to very basic aerodynamics, particularly the momentum theory of lift. That being: it is more efficient to produce lift by accelerating a large amount of air to a low speed, than a smaller volume of air to a high speed. So basically, the load/area of the disk. Helicopters excel at this, just due to the layout.

  • However, this isn’t the common perception these days. But that is because people typically compare a purpose-built long-duration multirotor, to an extremely high performance aerobatic helicopter.

First of all, let’s ignore all the 1+ hour duration-special multirotors people have built. These typically are designed to do only one thing: Hover (or fly very slowly) in perfect weather conditions. The frames are barely airworth, and might break-up in difficult conditions. They cannot carry a payload at all, any addition of payload would very quickly destroy their duration number. Even with all this, these machines typically achieve 80-90 minutes of flight time.

Compare that to the world-record holding electric helicopter, which flew for 150 minutes. And this was actually not a very radical build. It was basically a fairly standard Logo 600 helicopter frame, with 800 size blades installed, and the same special batteries the duration special multirotors use. It would easily be capable of lifting a small camera, probably with minimal flight time loss. It’s also easily capable of flying in typical winds of 20-40 km/h.

The longest duration “working multirotor” I have seen were built by Ferdinand K. He has a Quad and an Octo that can fly (at a moderate speed) with a small camera payload for 40 minutes.

However, when one compares that to a typical helicopter that can only fly 4-5 minutes, it looks pretty fantastic. But looking at number for helicopters, you must consider that they are set up for extreme aerobatics. They have extremely high power/weight ratios, which require extremely high head speeds. Much, much higher than needed for a “scale-like” flight. They also typically carry a small battery load, on the order of 1/5th the AUW. This is far shy of the 1/3-1/2 battery weight of a common multi-rotor.

By slowing down the rotor blades, you can easily double the flight time. I have achieved 12 minutes out of my big heli when it was 600 size. And that was simply running 1800 rpm, vs. a typical 2200.

I then stretched this to 700 size, using Spinblade Asymmetric blades, and doubled the battery capacity. I also added a fairly substantial subframe designed for payload management. AUW was 16.5 lbs, including a double-load of batteries. So this was not a lightweight machine at all. Running 1400 rpm, it flew for 24 minutes. And that was with a battery/AUW ratio of only 1/4. I don’t think this was “the sweet spot”. It could take more battery, or even lower rotor speed.

I’m actually shrinking this machine back to 600 size, as I think the 700 size is a bit overkill for the payload and battery load I plan to carry. While it’s true that all-else-being-equal, you can put longer blades on the machine and just slow down the RPM and be more efficient, there are other considerations. Most of it has to do with control. As the rotor speed drops, the control authority also drops. A helicopter has a discrete control frequency which basically has to do with the rate that blades pass a particular spot on the swashplate. For example, pitch control is limited to the rate at which blades pass the pitch servo location. At 1200 rpm, this frequency is 40hz for a 2-blade head. At 600 rpm, this frequency is 20hz which is getting pretty low.

So while a 600 size machine, with 800 size blades might be able to fly at 600 rpm in ideal conditions, it would probably not fly very well. You also have to worry about retreating blade stall in fast forward flight. This also limits how low you can run the head.

So, IMO, there is a “sweet spot” of practical disk loading and blade size that you have to find. I’m still looking for it. I think it’s going to occur at a similar 1/3-1/2 ratio of battery to AUW.

Now all that being said, trying to lift 500-750g of payload, fly for 1 hour, and have it come in at under 10lbs, that is a tall order. Not saying it’s impossible, but it’s that much more difficult.

I am just finishing up an MSH Protos 500 build, and have similar requirements to you. I want to use it for mapping, with a fixed downward pointing P&S camera. But I’m only aiming for 30 minutes flight time. IMO, professional use beyond that time for mapping is kind of pointless, as you need to fly VLOS anyway. 30 minutes at 40 km/h is going to get you 20km of flight path which is quite a lot of mapping in a 500x500m area. That’s 40 passes which means a 12.5m strip width which seems quite good to me? Say you want to map a 1km square area, with the operator in the center, that is still 20 passes, for a 50m wide strip.

Anyway, so the Protos is nice because it is very, very light weight. Typically 1200g without the battery. Add 800g of payload, and you’re at 2kg. That leaves you with a potential for 2kg of battery while staying under 10lbs total. That’s a lot of a battery! I’m going to do my first flights with a 4S 5000 battery, weighing 535g. Targeting 1800 rpm head speed. I won’t guess at the flight time as I have not found a calculator which properly replicates the effect of using Asymmetric rotor blades. If I “build” my 700 setup it says 11 minutes, but I know I flew it for 24. Quite a difference! If I build the Protos as described in the calculator, it says 18 minutes… so who knows. Guess we’ll find out!

It would be downright trivial to get a gas helicopter to fly 2 hours. But it’ll be way over 10lbs.

I wouldn’t recommend a 550, as it’s an inefficient size. It’s basically a 600 with clipped blades. Look at a 600 instead. Though one nice option is a 550 heli, and just put 600 size blades on it. All you need to do to make this work is a 600 tail boom. The 550 frame will be a bit lighter than a 600, but generally all the components are the same strength. My big heli is a 550 frame, but I’ve only ever used 600 and 700mm blades, never 550.

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Here’s the RCG thread.

I remember that we talked about this before and that was why I was looking at the 550 size frame, considering the option of “stretching” it a bit in the future.
Another consideration is the price - which seems to raise exponentially with the frame size, so starting with a 550 and having the option of “stretching” it would be some kind of a investment protection.
Originally looking at the HK550GT, I am now looking at the KDS Innova 550V2, because being a competition 3D-model, it’s probably a bit more durable than necessary and should handle the stretching without trouble.

The 500-750g is kinda generous estimation, coming from a FLIR PathfindIR (320g) + a gimbal of unknown weight + VTX and OSD… Also, the 1h time is the perfect dream - I would be happy to have 30+ minutes for my first test setup. 30 minutes with a thermal cam over a lake with about 50 small islands goes a long way for finding a person in danger at night, maybe even in adverse weather conditions… To cover the whole southern lake area here by boat in manual search mode at night takes well over 3,5h…

Besides, there’s actually now a number of multis with “off the shelf” parts which can achieve impressive flight times. On RCG, the user Chuenwe has built a stretched Talon for FPV which flies 50+ minutes and his latest project is a Tarot 650 X4 and a Tarot 650 X8 which fly well over 30 minutes - all FPV.

So, bottom line, you would agree that a helo would be the better choice for an everyday-use observation platform with long endurance?

By the way, with “same special batteries”, do you mean those self-made Li-Ion packs? I thought, those cells are rated for like 2C. I wouldn’t have thought that they could handle a helo…

Anyways, maybe, it’s time for some long-endurance helo-thread on RCG? :wink: I think, more than a few people would be interested to follow your build of the mapping helo!

Hi Stefan (and Robert),
I feel compelled to weigh in on this as well.
First there is no doubt that Robert is the expert on traditional heli here, he literally wrote the code we are using now and has a lot of experience with them.
And he has a lot of experience with multicopters as well.
And regarding ultimate efficiency what he says about a large (propeller) moving slowly being the most efficient is “true”.
But the overall equation isn’t quite so simple as to lead you to the conclusion that for electric powered vehicles at least that a conventional heli is the best long endurance solution.

From a practical standpoint even for a 15 pound Multicopter you wouldn’t really have to use 26" to 28" propellers (although they would be very efficient). For a hex, 16 to 18" propellers would be plenty and for an Octo 14" would probably be fine.

Which does bring up a serious advantage of multis, with a hex or better an octo, you have a really good chance of saving yourself in the event of a motor / ESC failure.

Basically in a conventional helicopter if anything goes wrong your toast.

Also if you want a copter to do actual “work” with you don’t really want the longest endurance possible, you give up practicality to get it.

Panasonic Li Ion batteries are not good for general purpose use for copters.

Multis are easy to build, easy to fly and mechanically incredibly simple, helicopters are not.

One of the problems with multi’s right now is that they don’t really have propellers optimized for their normal state (hover) but they are getting better.

I think right now maximum endurance wise and maximum efficiency wise it is probably possible to build either a trad heli or multi that will accomplish roughly the same goals.

But the mechanical and operational simplicity and ultimately the long term reliability of the multis is going to cause them more and more to become the vehicles of choice for real world applications.

The HK550GT would actually be pretty comparable to the KDS. I really don’t think anybody could say that they would perform any different. I like using Align kits, as you get a lot of options on the aftermarket. Lots of upgrade potential, some of which is very nice. My 550 is basically just assembled from all upgrade parts.

And see, that’s the thing… talking about flight duration and comparing to multirotors is one thing. But if the requirement actually involved a search, in adverse conditions, the argument swings more heavily towards helis. There’s really no comparison. A multirotor uses more and more power to fly with higher airspeeds. Helis are actually more efficient at moderate airspeeds than they are in a hover. And flying in wind is even worse. Most typical multirotors max out at about 15 m/s, unless they are very overpowered. If you have a 40 km/h wind, and you’re trying to fly upwind, that become a serious problem. However helis, 15 m/s is just getting started, even for a tiny 450. I’ve had mine up to 30 m/s with little trouble. That leaves a lot more headroom for fighting a headwind.

Bigger helis handle wind even better. I’ve done 35 m/s, the only thing stopping me from going higher is because I’m flying LOS, and the heli covers a lot of ground at those speeds, I have to keep turning around.

That’s actually Wesley Chen. He worked on Arducopter for a while. Smart guy. Too bad he got frustrated at left. He basically got out just as Arducopter was starting to get really good. He also flies helis. Or used to anyway.

Sure, they could handle a heli with the right setup. You’d have the same considerations as a multirotor. You basically have to get past 30 minutes with standard batteries, then theoretically your current draw is below 2C. At that point, you can jump to the Panasonic cells and double your flight time.

The reality of the situation is that you need to be sure that you NEVER draw more than 2C. This is probably easier done on a multirotor, as the maximum current draw if fairly deterministic based on propeller and motor selection. Heli’s however, assuming you’re using an ESC with governor, you would really have to be careful of the collective pitch control to avoid bogging and having the ESC really hammer the throttle.

As far as an in-depth thread at RCG, I dunno. I’ve been very disappointed with the level of discussion there on other threads I’ve been involved in. It’s not moderated very well, and most of the community would prefer to see the wrong info presented in easy-to-understand words, than to have the correct info.

[quote]Which does bring up a serious advantage of multis, with a hex or better an octo, you have a really good chance of saving yourself in the event of a motor / ESC failure.

Basically in a conventional helicopter if anything goes wrong your toast.[/quote]

Yes and no. While there’s no argument that a helicopter has many more single points of failure than an Octocopter, helicopters have benefited from many more years of engineering than multirotors. A properly maintained heli is extremely unlikely to suffer a sudden mechanical failure. I can say that from my experience… I’ve got probably 3-4 times as much flight time on helis than on multirotors. Yet I can only recall a single in-flight failure on a heli, and it was my cheap little 450. I’ve had 3 with multirotors, plus one close call/miracle.

Mechanically simple yes, but electrically complex.

I’m as yet unconvinced. I don’t think heli’s have gotten a fair chance in this field.

I don’t argue that multirotors are mechanically simple and easy for a beginner to build. They have a massive advantage in vibration control for AP applications. Cost-wise, a multi can actually be more expensive than a heli with the same performance. As much as 2-3X as much in fact. They excel as flying tripods. They are great to learn on because if you have a light crash, the worst case damage might be a couple broken props. And the dynamics are a bit easier to write a program for.

But if the use-case involves actually flying, in a search pattern, in adverse conditions, etc, a heli blows away a multirotor. In fact, I dare say that helicopter flight performance might be even better than an airplane that is capable of performing in the same conditions. Yes, I know many people are flying well over an hour with Tech Pod’s etc. But in what kind of wind conditions? Factor in launch and landing complexity… catapults, parachutes… all the sudden helis don’t look so bad. :wink:

I think you misunderstood me. I meant that if I would build a multirotor that would be able to carry my payload and fly 45-60 minutes in other than perfect weather conditions, I would end up with a 4ft, 15lbs monster with T-Motor U8 and 28/29" props.

By the way, does APM: Copter-Tradheli know how to autorotate? Wouldn’t that be a nice failsafe feature?

No, it can’t auto-autorotate, but it’s on my to-do list.

The first problem is that it needs to know the rotor speed…

Glue a magnet to the upper swashplate and a reed sensor somewhere connected to an analog input :slight_smile:.
Or - easier - an IR reflex sensor.

I don’t disagree Robert,

And probably where more dynamic movement is required helis may have a serious edge in endurance.

But these days, electrical complexity is generally superior to (and more reliable) than mechanical complexity and is definitely the trend.

And when it comes to production efficiencies mechanical simplicity and electronic complexity will win every time.

Nonetheless, they build some dynamite helis these days and I am sure both copter types will establish their own utility envelopes.

Of course there are going to be some intriguing multicopter fixed wing hybrids that may eclipse them all.

Robert, have you ever considered using a low-kV pancake motor as direct drive or with a smaller gear ratio?
Just thinking, looking at non-Hobbyking helo motor prices, a T-Motor U8 with 105kV isn’t that much more expensive but might be more efficient - still giving you 1500 +/- x RPM at 4S.

[quote]Glue a magnet to the upper swashplate and a reed sensor somewhere connected to an analog input :slight_smile:.
Or - easier - an IR reflex sensor.[/quote]

I’ve actually got something working already. Magnet on one of the rotating parts (anywhere) and a Hall effect sensor, with an Arduino Pro Mini. It works and I can measure RPM reliably. From here, I just need to set up an I2C interface.

Absolutely! This is where the future lies.

[quote]Robert, have you ever considered using a low-kV pancake motor as direct drive or with a smaller gear ratio?
Just thinking, looking at non-Hobbyking helo motor prices, a T-Motor U8 with 105kV isn’t that much more expensive but might be more efficient - still giving you 1500 +/- x RPM at 4S.[/quote]

Yeah, I have. Even direct drive. But, basically it doesn’t work. Low RPM motors have much, much lower hp and torque density than a high speed motor. It would take a MASSIVE motor to be able to turn a helicopter disk directly. I have a 480kV motor that I have used with 10:1 gearing, and then a 600kV. So, fairly low.

I’m also using a direct drive tail rotor to eliminte the tail drive geartrain. This goes to what Gary is talking about with electrical complexity winning out over mechanical. I only have a single gear mesh on the entire machine, and less shafts.

Now I’m really torn between the models…
I’m bouncing between the HK550GT, a KDS 600 or 700 or that HK Assault 700 which looks pretty ok but I haven’t found much out about quality. I’d prefer flybarless rotor (less mechanics - hello Gary :smiley: ) but belt drive as to what I have read, belt drive is more forgiving when you graze something with the tail.
Generally, I’d like to stay with HK, because chances are always to get the stuff I want from the EU warehouse and even if not, they’re pretty cheap - much cheaper than dealers in the US - not to mention the ridiculous shipping rates from the US to EU…

So, the all-deciding question now to you, Robert:
Of the large helos which Hobby King stocks, which one would you think would be a good starting point for my first endurance helo project? :slight_smile:

Just saw this now. Within the bound of the model you suggest, I would say the HK550 would be the best, largely due availability of Align and other upgrade parts.


you might find that post on RCG interesting. It mainly quotes a number of sources and shows some videos - among others a Logo 600SE running Spinblades symmetric (can fly inverted, so no asymmetric blades) blades at 1k RPM headspeed. And he’s doing stunts with that!!! I have been following the discussion of that Assault 700 and there were a number of experience reports stating that it’s very high quality and not a typical HK helo. Given the price, I think, I found the frame for my first helo project :slight_smile:. Oh, and all spare parts are available from Gartt directly via eBay, so I wouldn’t even be dependent on HK :slight_smile:. Now I have to do some thinking and some math to figure out how to get the optimal efficiency out of this helo. I talked with Spinblades on the phone (yay, German company :smiley:) and they were really helpful!

This is a very old thread, and maybe not important anymore. But I would like to add a comment.

Saying a medium speed 1,900 rpm 700hp diesel engine does not have the hp and torque density of a 7,500 rpm 700hp NASCAR engine is somewhat not right. The SAE hp forumula is hp = rpm x torque / 5252. That basic forumla is used by all brake type dynamometers, which are speed and torque measuring devices, to calculate horsepower.

When I look at helicopters I see the high rpm low torque motors using gearing as being similar to a NASCAR engine. Very high performance, but also quite short lived. All for the purpose of 3D flying. However, when I look at a helicopter for lifting and work, I have different thoughts more along the lines of the 700hp medium speed diesel.

I have built a new 600 size helicopter using a Trex 600 head and mainshaft, which is isn’t complete yet, nor has it been flown. But I decided to part from the norm and use a Tiger U8-10 170kV motor direct driving the bottom of the mainshaft with a custom coupler I built.

That big Tiger produces tremendous torque. My initial static testing of the machine today with it tied down indicated it is going to perform very well on a 1.3m rotor at ~90% power efficiency. It is quite impressive - It produced 5.7kg of thrust @ 2,200 rpm headspeed and 4 degrees of collective pitch. At 1,850 rpm headspeed going from 0 to 11 degree collective slowed headspeed by 110 rpm with no governor. It just leans into it and draws more amps, but it doesn’t lose much speed.

It does jerk the helicopter on intital start and I should put a clutch on it for smoother spool-up of the main rotor. Or find a different ESC with a soft-start. But my expectations are that it will work quite well, although the motor weighs 240 grams.

As far as the gist of this thread, I built a fixed wing/TriCopter hybrid with 60" wingspan that flies for 45 minutes on a 10A 4S battery carrying a GoPro. But it only flies most efficiently at 20 mph and has a few problems in the wind. I fully expect my newest helicopter will outperform it without trying too hard. The helicopter is simply more mechanically efficient and does a better job of turning shaft power into thrust. There’s not a lot you can do to a multi-rotor to make it have the same speed, payload capacity and efficiency of a helicopter at the same shaft power.

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