giant scale questions

[koenejet]

Hello,
So I have been thinking a lot about giant scale and multi-blade heads. I am very interested in the physics behind helicopters and started crunching some numbers for a large heli. This is something that I would love to be able to build someday, and maybe someday I'll try, but for now, I'm just doing some preliminary calculations to see what kind of lift is possible and what kind of horsepower would be required.

At the moment, I am not asking anyone, nor expecting anyone, to double check my work; all I'm after is if someone (who has experience designing large helis) to say "yes, that sounds about right" or "no, you're way off".

So here it is: I figured that a heli with a nine foot rotor dia and 6 blades, with a head speed of 800rpm, would require about 20hp and each blade would produce about 50lbs lift.

Now 20hp isn't impossible to get using electric brushless motors, but would be extremely expensive. So If I slow the head speed down to about 600rpm, then each blade produces about 20lbs lift (which is still way more than enough), but now only requires about 13hp.

Does all of this sound right? Are these numbers probably pretty close?

Also, I was wondering how slow is 'too slow'. Obviously, the main rotor has to spin fast enough to get enough lift to fly the heli, which at 600rpm is plenty, but is that too slow for performance? Keep in mind that the intension is for scale flight, not 3D. Is it normal for people replacing a two blade head for a multiple blade head for scale purposes to slow the rpm way down?

Thanks in advance for any insight. And yes, I know that I still need to figure the hp needed for the tail. I'm just focusing on the main for now.

Thanks again,

[koenejet]

So I got a somewhat rude comment about my questions on another post. I don't know if it was intended to be rude or helpful, but because of that I just want to clarify my request.

First of all, yes I am fully aware that there are several factors involved. Blade length, cord, blade root cut out, twist, taper, blah, blah, blah... All of that is calculated into my original work. And again, I'm not asking any body to redo my work.

I do have a lot of experience with helicopters, but this is my first attempt at doing any actual calculations. The book I'm using has all of the formulas for lift and drag and horsepower, and etc. etc. etc.... However, I did notice a couple of minor typos in the book ( for example, one page would show a formula, and on another page the formula was slightly different), and I am pretty sure what the correct one is, And the book didn't perfectly explain how to calculate the lift to drag ratio, so I used the one the author used for his example model.

I just assumed that if anyone has any successful experience in designing large scale rc helicopters, it would be possible to look at the numbers I provided to either say "yes, that's probably pretty close" or "no there's no way that's right". Just like if someone were to say to me "I calculated that a Cessna 150 should be able to fly from New York to L.A. non stop", I would say "no. not even close. you did something wrong in your math."

If you don't know, you don't know, but please don't comment on your opinion about my lack of knowledge. I would appreciate any helpful, non-condescending comments.

Thanks again for any insight.

[npomeroy]

I tried to divert the argument away from "my" thread on bellcranks back to here where it belongs.

Most of us have been stung by extrapilot's dismissal of our inferior understanding and it isn't nice. However I still respect his insights and even seek them, hence using this sub-forum (which I persuaded Will to set up).

I come from a science discipline (see my avatar, but my research is not about their flight!) where the variables are so immense as to make helicopters look like child's play, although the penalties for error are higher for helicopters. Consequently I consider myself an empiricist. "Try it and see". There are lots of situations where we could do more of this: Shove in an engine and measure the lift - build really basic prototypes and test fly. I think the real industry does a lot more of this than most people realise.

Best wishes
Nelson

[npomeroy]

And I am interested to be clearer on where you are coming from. Are you primarily enjoying the calculation/design challenge, or do you have a vision of actually building a machine? If you are focused on the future machine, what would you like it to be like? Is it a beautiful true-scale model? I would have thought it would be instructive, rather than trying to calculate the parameters, to look at existing trends. Current RC scale helis on the market get pretty big. Plot their power / mass trend. Do the same for full scale helis. Can you interpolate between the two data sets?

[koenejet]

First of all, thank you for your comments, and I apologize for letting the other comments on your post go so far.

I have been dreaming for years about actually building a large heli. My vision is of a 1/8 scale of an existing helicopter. I'd rather not say publicly yet what it is, but this heli has a 72ft rotor span and 6 blades. At 1/8 scale that gives me a 9 ft rotor.

Before I started crunching the numbers, I expected the results to tell me it would take about 10hp. So when I saw over 20, I was staring at my work thinking "this can't be right. What did I do wrong". So I started researching other ways (comparison as you said) and it looks like I was pretty close with 20hp.

I would love to just put a motor in it and see what happens, but I can't afford to be wrong. So I was trying to make sure that what I put into this project would work.

At the moment, this whole project is just in it's daydream stage, slowly making it to paper. I have a few other projects I need to finish before I actually start building this one. If I actually start building this one. My thought was to post my build progress here once I got started, but I don't want to give away yet what the heli is.

And as I have said before, I am a builder. I am 100% positive I can build it. But if it's underpowered, and it doesn't get off the ground, then I would have to buy another powerplant, and hope that works.

And there are other things that I have not found any info on. Such as negative side effects of running slower head speeds. As I said earlier, if I could run it at 600rpm, the lift would still be fine, and this would take less horse power, so the motor would be cheaper, but is there a downside to running that slow? I need to find out how to calculate minimum head speed. Or does it even matter as long as it provides the needed lift to get off the ground? So there are a few aspects that I'm not sure of at this point. Hence the original post. If I had the time and money I would love to just try it and find out, but I don't think I will have that luxury.

Anyway, sorry for the long drawn out response, but thanks again. I do appreciate it.

[koenejet]

By the way, I am curios about your avatar. I'm not sure what I'm looking at.

[giiba]

I'm anything but an expert. But I do have simple observation on the minimum head speed.

The minimum would be the minimum to generate enough thrust to overcome gravity. Such that you need greater thrust than the weight of the craft to actually gain altitude. I know in the quadcopter world a 2:1 ratio (max thrust : weight) is considered the minimum for a flyable copter, and the acrobatic ones can be 4:1 or higher.

I'm curious what you find, so do keep us posted.

[npomeroy]

Originally Posted by koenejet

By the way, I am curios about your avatar. I'm not sure what I'm looking at.

Yeah, it was a bit unclear - it was a bumblebee hovering in front of a kiwi-fruit flower. I've changed it to a more obvious one! I've been into bumble bee domestication, pollination, and studying the biology of their colony growth and social interactions.

[koenejet]

Very cool about the bumble bees! My dad and I had about 70 honeybee hives many years ago, so yea, I realize there is a difference between bumble and honey, but still... very cool.

giiba... thanks for the input. That is what I was leaning toward assuming, but don't know if slower head speeds affect other aspects such as controllability or stability. I believe it does down to a certain point, but don't know where that point is. But yea, if I find out anything I'll be sure to post it.

[nbells]

You mentioned a book that you are using for the calcs and that some are different on other pages.

Can you tell us the name of the book you are using ?

[koenejet]

the book is title "RADIO CONTROL HELICOPTER MODELS" by John Drake. It's an older book first published in 1977. I only found a couple little typos that are easy enough to assume what they are. But still, I don't know if I should put 100% faith in the accuracy of any results.

[gorlandheli]

Hi koenejet - first off I am not a helicopter designer but do have an interest in the physics.

I have previously played with some maths targeted at Trex 450-600 just to gain a "feel" for what goes on... I have collected a couple of calculator spreadsheets, one of which goes into fair detail [not written by me but by someone who appears to know what they are doing].

So, I added your parameters with some guesstimates. I can only get near your 20hp at 1200rpm creating 241lb of thrust (total). At 600rpm, the same set up gives 60lb thrust (total) and 2.85hp.

My concern from this would be that there is a squared or cubed term that is not making its presence felt...

[gorlandheli]

It's not clear to me what mass your target helicopter has... if I assume 2:1 thrust to mass, then there is an indication that you are in the 80kg/160lb territory.

If that is the case I can see the 20hp being much nearer the mark based on some very sketchy input data. I wonder whether it might be feasible to use 3 motors driving a common main gear. This way you could buy 1 or 2 motors, see what happens. if it's not enough add the 3rd.

This would put you in 5kW territory per motor - in the realm of big motors for 2-blade "big" hobby helicopters.

Not sure that it will run off a 1s 200mAh battery though!!

[koenejet]

Thanks for the input. From what I have read, the horse-power curve is pretty great. According to my book, if it takes X amount of horsepower to spin a rotor of a specific dia a specific rpm, it could take up to about 30 times X to spin a rotor twice the dia at the same rpm.

And, part of the horsepower calculations include drag horsepower. The power needed to overcome the drag per blade. So going from 2 to 6 blades also ups the h.p. quite a bit.

As far as the mass goes for my heli, I'm not really that far along yet. I am just scaling from an actual heli. So I know that (IF I actually build it) the rotor dia will be 9ft and have 6 blades. I would say it would be somewhere around 15 to 20lb? I could be way off on that, but with the calculated lift, I don't see weight being an issue.

I was actually planning on using two motors to drive the gear box. I can get two 10hp motors for less than one 20hp motor. Plus, it would then be a twin engine heli, just like the one I'm modeling after. The motors I was looking at would run minimum 24v. Probably would use closer to 32 to 40 v.

I have a few other projects that are taking more time than I have, so at he moment this is just a pipe dream. IF I finish these other projects, then I'll actually think about starting this build. I'll post pics when it comes to that.

But I do appreciate any thoughts or helpful comments from anyone. Gives me more to think about.

[MariaFan]

Originally Posted by koenejet

Very cool about the bumble bees! My dad and I had about 70 honeybee hives many years ago, so yea, I realize there is a difference between bumble and honey, but still... very cool.

giiba... thanks for the input. That is what I was leaning toward assuming, but don't know if slower head speeds affect other aspects such as controllability or stability. I believe it does down to a certain point, but don't know where that point is. But yea, if I find out anything I'll be sure to post it.

I have a Blade nCPX that I have put longer blades on, and also did a brushless main motor conversion.
The Nano's are known for being "twitchy", and I have found that lower head speeds make the little thing fly much, much smoother.
I have also noticed increased flight times by running slower head speeds using more pitch.

Edit: Here is a link to a thread I started concerning my experiments about head speeds and pitch.

https://www.helifreak.com/showthread.php?t=699356

There is a csv file attached that lists the data I have gathered.

[koenejet]

Thanks for all the input. I have to put this project on hold for a while . Some other stuf came up. I do have a friend working on creating a calculator based on all the formulas from the book I mentioned earlier. If he gets that to work, then I'll try to pst it some where for people to play with and see if it's close to accurate.

[MariaFan]

Originally Posted by koenejet

Thanks for all the input. I have to put this project on hold for a while . Some other stuf came up. I do have a friend working on creating a calculator based on all the formulas from the book I mentioned earlier. If he gets that to work, then I'll try to pst it some where for people to play with and see if it's close to accurate.

This would be interesting.
I look forward to you posting this. Thanks!

[junkpilot]

Have you considered the following. If electric powered how big will the motor have to be? How big will the battery have to be? What will you make the blades out of? What do you project the all up weight to be?
Wouldn't it be easier to start with a homebuilt heli kit ou wouldn't have to reinvent every little thing need. The bonus is The homebuilt has a seat on it for those days you feel like flying. All home built helis I've seen use Two stroke gasoline motors,This is more desireable than a four min. flight time.
I'll bet you could score an unfinished Scorpion for free or almost free. Another idea would be to start the build on a autogyro airframe. Good luck!
Junkpilot

[seanh]

1. why the 600 rpm limit?
2. What is your realistic target weight of the scale heli?
3. What happens when power is lost...can you effectively auto with suck low head speed?

I've done some low head speed tests just for fun and can tell you that you can get a heli flying at even a very low HS. The issue was that you can't do much with it...hovering and light sport flying. On my 700 sized heli's the minimum HS was about 1200rpm...2blade, Rapter 90. I know I could have got it lower but everything happened so slow it just wasn't worth it. By slow, I mean input control...wait, wait for the result.

One issue I had was gearing vs engine RPM....just couldn't get the engine in the power band I wanted. But that was just working with gear sets I had sitting around- wasn't going to make a purely low head speed bird....but this may not an issue with a custom heli going electric

While you can scale the blades all you want what you can't scale is the air molecule...it takes a certain amount of resistance/mass off air for the blades to function. As you slow the blades you're limiting the amount of air you have to work with. So while you can get the heli in the air and can get the blades to perform light duty, there simply may not be enough to safely recover the heli should there be any problems....mass/time-- the more mass you can move, the shorter the time frame it takes to move the heli.

The first thing I'd be doing is looking at what head speeds the guys who have already built giant scale birds have been using then contact them directly about the issues found.

I would see if you can contact one of these guys

Kaufmann Roland, Jet Ranger, Big Scale Heli (6 min 7 sec)

LAMA - SA 315B - Scale 1/3.5 (7 min 1 sec)

"Blue Thunder" Giant Scale Helicopter (4 min 58 sec)

SA-319 ALOUETTE III WORLDS LARGEST RC TURBINE MODEL HELICOPTER / Pöting Turbine Meeting 2015 (9 min 9 sec)

Super Giant Bell 47 RC Heli (4 min 8 sec)

Giant Rc Mi-24 Hind (4 min 6 sec)

[seanh]

here is a link on low head speed that might be helpful

https://www.helifreak.com/showthread...ghlight=lowest