Hello DIY Drones,

I am new to this forum. I am planning a build an UAV (Helicopter) that can carry a payload of 20 Kg. The UAV with this payload might weigh between 30 to 40 Kg (of course with very good Engg.). Also, the UAV has to travel at least to 20 or 30 km distance in one recharge. Hence I believe, gasoline powered UAV is cheapest option. However, I have tried various Engine options like Zenoah, DLE. Not able to find a suitable engine. Could someone suggest me a suitable engine or some other suitable option to fly the UAV?


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Ok, so here's another question, do you want to go direct drive, meaning basically one of the large traditional RC gas helicopters or my though is an engine/generator and some of the current large quadrocopters with a battery buffer?

4strokes are heavier, less power to weight ratio but far longer lived and tons more efficient. That said a Honda EU1000i is a 4 stroke 50cc weedeater engine and a dc alternator. Strip out the inverter and all the stuff you don't need-and there is a really nice DC power source. You would be very hard pressed to build your own equivalent coupling of an engine and generator thus the cost is fully justified and you could sell the parts like the inverter board which you don't need.


Again, it's 13KG but's that's with all the housing, electronics and stuff you don't need. For example, you can browse the parts diagrams and see what I mean here:



Once you strip it down, it's this


16pounds plus whatever the generator weighs. (the EU1000I complete was 28.66 pounds)

Or this much smaller 35cc engine


Otherwise, your into regular gas RC helis like this (note that price is for the kit, minus engine, servos and everything else)


Which takes up to the 26cc 2 stroke.


Not sure of payload but barring some really expensive options like a much bigger Yamaha http://www.gizmag.com/go/2440/


Sikorshy solved this problem about 20 years ago, with special designed wankle engine, should be stuff on the WEB about it.



I think the bigger problem is the platform, not the engine. I mean building your own rotorhead isn't exactly easy, matching blades, tuning it, and then the rest of the mechanics are far from trivial. Tail rotor is just as complex.

Unless you really want to build something from scratch and have the time and engineering/machining capabilities, the engine is the least of your worries. For example, I have a Yamaha KT100 from an old teratorn ultralight ~15 HP and pretty reliable for a 2stroke. If that's not enough oomph, 28hp Yamaha bravo 250cc single cylinder would fit the bill, but the airframe around these is the hard part. Or if you need to just go all the way, stuff the Kawasaki TA440 twin into it 48HP. Oh wait, you need turbine power? Then my Solar t41 nets you 50HP at 98lbs but then there is my GTP70-52 making 160hp at just under 200lbs.


The point here is, find your heli first, then worry about the power because likely, they solved that in the kit. I would not go from the engine first, then build a heli around it.


I was inspired by the German team who built the giant quad and flew a person on it. I saw no reason why one of the even larger engine driven generators couldn't power it instead of the 16 or more massive LiPo packs.


Ups. I forgot to mention mention my first couple of questions

How to find the required engine capacity (in CC) or the power (in KW) to lift an helicopter of x kg mass? any basic equations available? I am still working on it.

Secondly, whats the difference between a helicopter engine and a plane engine. If there are no major diff. why can't I employ the DLE engines to the propeller through a bevel gear mechanism?

Also, I just want to (atleast try to build) build this copter within a budget of 5k $


The spirit here in these friendly confines is one of generous help, but do you really expect the members of this forum to design a heavy lift multicopter for you?  Seriously, you have some learning to do.  If you're a mechanical engineer, then you should be no stranger to graduate-level engineering text books.  If I were you, I'd get this book and read at least the first 4 chapters:

Principles of Helicopter Aerodynamics - Leishman

Yes, it has just the equations you're looking for in Chapter 2.

It doesn't have the answer to "difference" question #2, however.  There isn't much difference between a helicopter and a fixed-wing engine, except allowances are made for the former due to the fact that the powerplant spends much more time in the high power ranges.  An aircraft engine is often "derated" for helicopter use (a governor is added to keep the maximum throttle setting below the absolute capability of the engine) to insure longer-term mechanical integrity.

Well! Seriously lots of learning is required and indeed trying it out. However, Avionics is again totally new at least to me. Btw i am trying to understand the mechanics so that I can go deeper once understood. Sorry, had I been looking for ready to eat questions.

and thanks everyone for the replies and links


Probably 2 stroke is better on account of its weight. Just was confused when reading some pages where the use of normal engines was not recommended for aerial vehicles. Just looking into the issue. However, till now have found only DLE and Zenoah being is the lightest. Incase of electric motors Scorpion is available. However, battery storage capacity and the escalating costs is yet another issue

I went through the articles of RMax. Thats one awesome product. However, I read that the RMAX variant is not for sale to civilians and the price is approx. 80K $

btw thnx for the links

Using Turbine power is indeed a better option when cost is not an issue. And the human flying quad is awesome.

Thnx for the links

Well, one difference between engines is cooling. Most are air cooled but forced cooling in a heli is mandatory and further, most heli engines have a larger head heatsink since that's where the heat is most concentrated. This is why he was having trouble looking at engines because what works for an airplane Zenoah G26 is highly modfied to use in a gas heli. The final kicker is you must couple via a clutch to get direct drive and it must not have runout or introduce vibration. Even worse, because often the clutch is further out from the bearings, now you have bearing and crank stress points. None of this is trivial and requires some serious testing, engineering and materials knowledge.

Sorry, we are not trying to crush your project, just you need to look at already flying kits and spend less time on engineering a heli and more on integrating your payload and control system. I think we all agree the payload you were talking about is at the limits of existing tech without getting REALLY expensive.


Look at it this way, I've been flying RC helis for a long time. I've never built my own from scratch even though there are tons of kits that you could adapt. Why, because it's first DANGEROUS (1-2 hp or more spinning blades nearly 1 meter in span or more). Read this forum and look at some pics. The worst ones are not cuts, but bruises so bad they look like they got hit with a bat.


You don't want to be the test pilot of an aircraft that is not proven under autopilot.

So here is the perfect bird for the weight you are trying to lift.


Yes, it's $5.5k but known tested and backed up by a good company. This is pro level stuff and when talking about a heli with 8, yes 8 HP, and nearly 2 meter rotor span, safety is a requirement. No way should anyone attempt to build something like this from scratch without considerable experience, time and money, and likey, you'll be pushing those numbers just to duplicate one of these. Key point is that they couldn't find a suitable gas engine and had to make their own twin from 2 Zenoah G26 cylinders. Yes, you might be able to use a Desert Aircraft Engine, but you still have to modify it extensively in order to be able to add the required clutch and fan.


 And being that the Bergen takes 8 HP to lift 25lbs reliably, I would say that's a good target for the power system. You can do the calcs but it's far better to take proven tested work someone has already done and build off that before reinventing the wheel.


That Bergen ship has been around for a while now, and just for grins, I decided to run the numbers.

Converting to Imperial units ('cause we Americans love imperial...oh, wait...the irony):

810mm blade ~ 2.66 feet or 22.23 sq. ft. disk

Gross weight = 43 lbs.

And solving for ideal power: induced velocity sqrt (43/2 * 0.00238 * 22.23).  The answer is 20.16 feet/second ^-1.  So 43 times the induced velocity is 866.9 pound feet/sec ^-1.  Dividing by 550 yields an ideal power of 1.576 horsepower.  To convert to watts, 746 * 1.576 = 1.176 KW

Bergen didn't spec out a payload of 25 lbs for an 18 lb ship to barely hover, so it's a safe assumption that that it really only uses 6 out of the 8 horsepower to do so.  The figure of merit (or FM - the ratio of ideal to actual induced power) would be 26.2.  That's with the symmetrical airfoil rotors, of course.  They do offer an asymmetrical rotor option, and it would seem if you're going to lifting a lot of weight, it would be a compelling choice.

Full size helicopters' average FM is around 75 and yes, the vast majority use symmetrical airfoils.  The difference boils down to Reynolds number effects, even if you account for the fact that most model helicopter rotors are, shall we say, less than precise.  In all fairness, you can't spin up the headspeed of small choppers because the blades are actually weighted to make autorotation possible, plus the range of airfoil section options is limited to those that don't have much in the way of pitch moment forces (which would place quite a load on the swashplate system to say the least).  So high-lift, low Re (thin) airfoils won't work.

Consider this: here's an application (in this payload range), where a series hybrid motor-generator-powered multicopter might be just the solution.  When equipped with fixed-pitch blades, designed for this Re range, such a craft can achieve a FM in the 60-70 range.  I think the FAA statutory guidelines reach a certain strata at a gross weight of 50 lbs.

In all fairness, the motor and generator losses in the 40% range would more than offset a conventional helicopter's transmission and tail rotor energy wastes.  But I really believe the higher rotor FM potential alone would more than make up for this.  Comments?

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