Headspeed, blade size, big helis

[trillian]

Just a few thoughts here on things I'd love to see but have not as yet. - in short: bigger petrol helis and low headspeeds with IC engines.

First of all, just a little context. I started flying when it was dominated by nitro and flybars. On a flybarred heli you need a decent amount of headspeed for the flybar paddles to do their thing. So on a 700 size machine generally you might hover at around 1500-1600 ish and do the aero stuff at 1800-1900. This was the mindset that I always had and carried over when I started to move to bigger electric helis. My first electric 700 still had a flybar.

Then I moved to flybarless. This is when I began to realised I no longer needed to run my headspeeds nearly as high. I found I could setup my 700 size helis to hover around 1000-1100 and do aerobatics at 1300-1400 (12s electric). Not smack 3D, but then I was never interested in smack 3D.

The power required to spin the blades at 1400 is much less than it takes to spin them at 1800, even with zero pitch. The power required goes up exponentially as the headspeed increases. For electric the benefit is longer flight times, which suits me very well.

And the whole LHS (low headspeed) flying thing has become a bit of a 'scene' now. But it has been pretty much entirely relegated to electric power. Partly because you can so easily change the speed of an electric motor by using a lower cell count and partly because electric helis usually also allow flexibility of gear ratios and motors come in all sorts of Kv ratings. On mine I'm still running them 12s but have them setup with choices of gearing and motors to give me low ish headspeeds even on 12s.

Check out this video of the Velos 880 running on 12s. It is basically flying on the same power (from the battery standpoint anyway) as a normal 700, but it weighs 8.5kg! It's running 850mm blades at 1500 rpm. Now, I think the same or better performance could be had with one of our typical Hanson /TRM 300s geared to be in the powerband at a similar headspeed. And this is a really big heavy machine as it's made to run dual 12s / 14s setups.

Velos 880 Efficiency (6 min 53 sec)

I'm seriously going to have a think about building up an 800 starting with a Robbe Futura SE drivetrain, which I have a bunch of already. The main reason is that it's a two stage gear setup that's very flexible on ratios and it's very light. But that's as far as I've thought so far.

[gassed]

FWIW
I had plans to stretch a g20 Century Radl using one of my modded 20cc engines out to 800 size. I currently have a g20 stretched to 700 that has the same power (maybe more?) as an aftermarket 231. it flies very nice.

when the new condor came out a long time back I liked the idea, but the price was ridiculous, but they are discontinued. the GSR was a nice heli too but was very heavy IMO.

I can stretch my already stretched g20 further out using a 11T or 10T pinion (but still run the engine in its powerband) and larger tail drive gear so that I can have a HS of about 1200 but still have good tail authority. the issue is the CG change. on my streched g20 to 700, I had to move all the electronics and the RX pack up front to center the CG... if I go to 800 using a longer boom, the CG will move back again. adding dead weight to the nose would not be good option. there is also the issue of strength in the boom block that needs to be addressed that would need to be improved due to the additional leverage that a longer boom has upon the frame mounting points. A new boom mount made from aluminum that incorporates the main shaft mount would likely be the best solution. other issues that rear their heads are the main shaft and bearings... they would likely need to go up to 12mm or even 15mm diameter which changes alot of other components too like the OWB rating....I figure I have enough power for a LHS gasser that weighs less than 12lbs.

just seems lately that interest in going into such a project is low for me though. alot of work for little return.

[rbort]

Where is that, looks like greece to me. Been there years back.

Are you running around 1790-1800 head speed there?

-=>Raja.

[jharkin]

You are on the right track... but its complex and I dont pretend to understand all the theory.

Yes, as headspeed goes up, the amount of power required increases exponentially as the faster speed generates more profile drag just to spin the blades at zero pitch.

So in theory you get better efficiency and can put in more pitch to compensate for hte lower RPM. But there are tradeoffs:

#1 - you can only put in so much extra pitch until you hit the point of blade stall and efficiency falls off a cliff (I think I've read this is around 14 degrees????)

#2 - Some of the efficiency gains you got from lower RPM (less profile drag) you give back in the form of increased induced drag from the larger pitch angles required to generate lift.

Overall though, the net effet is positive as seen by the run time increases of low RPM setups.


Now, go up to a longer blade and you have other problems. The longer blade means that you are generating more profile drag rpm for rpm. Just because the 850 is turning 200-300rpm less than a 700 doesn't necessarily mean its generating less drag. We would have to do some math to confirm.

The tricky part is that I have no idea how to calculate this. I do have some basic aerodynamic books that have thrust formulas for props but nothing for helicopter rotors.

Just to get some ballpark ideas I tried running this through a prop calculator tool using 60in @1800rpm and 72in @ 1500 rpm setups to mimic our two rotor sizes. I just experimented with pitch until I got a value that absorbed around 4hp for the smaller size and then used that. I would not read much into the results since propeller profiles are radically different than constant pitch rotor blades, but whats interesting is that at the same pitch the longer blade is absorbing 30% more power than the shorter one even at lower RPMs. However, if you adjusts the pitches to match thrust then the larger disk requires LESS power and only needs about half the pitch angle. The tradeoff is that predicted airspeed is cut in half.

[trillian]

gassed - yeah this is the difficulty of stretching a smaller airframe, even though everything on the boom is relatively light, it is placed so far out from the c of g that it becomes difficult to balance. So it really requires an airframe designed with the longer blades and boom in mind.

rbort - I think it might be Greece. They say the headspeed is 1500. With 850mm blades it should fly quite well even a bit lower. I fly my TDR at 1260 (and we're talking fully aerobatic and not really a big change in my approach, just a little mindful of collective management). I will say though, the TDR is about half the weight of that Velos.

jharkin - I don't think we really have to worry about stalling the blades. The 3D guys push things way beyond the range I'm talking about when they do those really crazy direction changes and overspeeds etc. I know some of them run 14 degrees and beyond. Of course you will have more drag with longer blades, but you can also run them proportionally slower. I don't think you lose in the area of induced drag (or if so it's extremely minor) in the kind of differences in speed we're talking about. In any case the bottom line is that it should be possible to fly a bigger disc / airframe with the same engines we're using in 700 size helis because we just don't 'need' as high headspeeds as we're used to running. Obviously if you want the manic smack type performance you'll need the smallest disc running the highest headspeed the engine will handle in order to have the inertia and overall low mass. That's already being done. What I'm talking about I have yet to see anyone do (except with electric power).

[trillian]

Originally Posted by jharkin

However, if you adjusts the pitches to match thrust then the larger disk requires LESS power and only needs about half the pitch angle. The tradeoff is that predicted airspeed is cut in half.

Just to expand on this - there's something that happens in efficiency when you increase the blade size or wing area. I can only guess this is due to the density of air being (more or less) a constant. To see this principle demonstrated the most profoundly, just look at the size of these human powered aircraft. They're friggin huge! You'd think the drag would outstrip any gains but this is not so.

[gassed]

exactly... just look at full scale examples... like the AH-1 Cobra... the large blade width is obvious and it turns much lower rpms compared to an Apache.

if you really want less drag, you gotta go semi-symmetrical for the blade profile or full asymmetrical to get the most out of it.

the rest comes down to weight of the airframe and efficiency of the drive train/engine... this is where you build light as possible yet maintain strength where needed. aerospace metals and carbon fiber, short wires and all sorts of weight reduction techniques.

but there has to be a point of diminishing returns... you either build for performance or efficiency... everything has a compromise at some point. so what do you want to achieve? 800 class gasser thats efficient (fuel wise) or 800 class gasser that has excellent flight performance?

cause honestly, the Velos880 is a big waste if you fly LHS... that feat could have been done with a Trex600 airframe stretched and a 6S setup and it would have been at or under 11lbs and likely have flown longer given the same mAh... the velos drive train has alot more Metal to deal with thus more loss due to the two motors, belts, gears, etc. for me the Velos880 was designed for high performance in an 800 and the two motors sold as being redundancy feature... not efficiency.

[jharkin]

Yes, there are efficiency gains generating lift with longer sections at lower speed. Interestingly the total area is not as important as span, which is why gliders use such long thin wings. Mathematically an infinitely long wing is the most efficient.

This is what is going on with those man powered aircraft, very long wings optimized to generate lift at extremely slow speeds, operating in very different Re regimes than heli blades. Trivia - the MIT professor who designed the airfoils for the Daedalus aircraft is a member of my RC club. He also designed many RC glider airfoils... He gave a presentation once on how they designed and built it and a lot of the construction techniques where taken from models - Foam, composts and mylar covering.

The point I'm trying to make is there is no free lunch in physics. Will it fly, certainly ... It will likely feel floatier in autos due to the lower disk loading. But we cant change the laws of physics so assuming the same engine and higher airframe weight you are gong to give up something, i.e. gear it for more thrust to carry the extra weight and you will give up speed, or vice versa.

This seems to be borne out by experience - every time this subject has come up the experienced guys who have flown gas powered 800s report that they fly slower and less snappy than a 700, even when geared down. (Raja, Carey, Tom etc feel free to chime in and correct anything)

[trillian]

Originally Posted by gassed

cause honestly, the Velos880 is a big waste if you fly LHS... that feat could have been done with a Trex600 airframe stretched and a 6S setup and it would have been at or under 11lbs and likely have flown longer given the same mAh...

Going big is the whole point! Why fly a 700 when a 450 will fly on a single 3s LiPo. :-)

[jharkin]

Originally Posted by gassed

exactly... just look at full scale examples... like the AH-1 Cobra... the large blade width is obvious and it turns much lower rpms compared to an Apache.

Again, not so simple. I looked it up....

AH-1Z Cobra rpm = 311
Apache rpm = 289

The Apache looks faster visually because its 4 bladed.

There is an upper limit to rotor speed because of the need to keep the tips from going supersonic. Interestingly if we look at these examples:

Apache = 48ft diameter rotor. 289 rpm. Tip speed 726ft/s
Cobra = 44ft diameter rotor. 311 rpm. Tip speed = 716ft/s
Velos 880 = 72inch (approx), 1500rpm = tip speed 470ft/s
700 gasser = 59in (approx), 1800rpm = tip speed 463ft/s

Notice that both full size helis are in the same range (and interestingly both are in te transonic region), and both model helis are in the same range. And we actually have a lot of headroom to go higher in the model helis (Mach1 is 1125ft/s. Transonic range starts around 675)

Originally Posted by gassed

if you really want less drag, you gotta go semi-symmetrical for the blade profile or full asymmetrical to get the most out of it.

Airfoils with more camber (semi-symetrical and assymetric) generate more lift for the same amount of induced drag, but at the expense of a steeper increase in drag as angle of attack and lift increases. Generally the slower you fly the more of an advantage gained from utilizing a highly cambered airfoil, but at higher speed the symmetrical section becomes optimal due to the shallower curve of its Cl/Cd ratio. This is why classic prop airliners of the 30s used such think asymmetric wing sections but modern jet airlines and all military jets universally use thin, near or fully symmetric, sections.

Our rotor blades are working in airflow much faster than what a glider or man powered aircraft experiences so I wouldn't assume any similarity.

Just for interest here is one page talking about lift vs. drag for different sections from one of my Aerodynamics books. The graph at the bottom right illustrates the point I was making above:


(c) Martin Simons - Model Aircraft Aerodynamics. Special Interest Books Ltd.1999


As with everything else discussed, this is an oversimplification looking at aircraft wings. Rotors are complex and the blade airfoil is working at very different speeds from the root to the tip. So again I dont pretend to be an expert on this stuff. I would reach out to Extrapilot if you want a real in depth analysis.

[jharkin]

Originally Posted by trillian

Going big is the whole point! Why fly a 700 when a 450 will fly on a single 3s LiPo. :-)

Maybe Im not understanding what you are trying to do. If you just want to fly an 800 slow and sport - sure that can be done. It already has been done, people have put the 800 stretch kit on TRex conversions, and Carey sells the G770 and the Synergy 766.

What am I missing?

[trillian]

Originally Posted by jharkin

Maybe Im not understanding what you are trying to do. If you just want to fly an 800 slow and sport - sure that can be done. It already has been done, people have put the 800 stretch kit on TRex conversions, and Carey sells the G770 and the Synergy 766.

What am I missing?

No one is doing proper low headspeed flying with an IC engine. I've never seen any heli geared for low headspeeds other than electric. Maybe it has been done but I've certainly never heard about it.

I'm not talking about only petrol. I've never seen someone, for example, gear a .91 nitro to run at 1400 headspeed with the engine in the powerband (around 15K for a .91 nitro). Never seen this done anywhere and, as I said before, the main reason is that the option to change the gearing just isn't there on most IC airframes either nitro or gas. (and just before anyone mentions it. yeah I know there are big scale helis running nitro and petrol engines, but that's only poodling around, circuits and hovering in a scale-like manner).

My other point is that there is no question about 'can this engine handle 800mm blades', the answer is YES it can... IF you can gear it properly.

But if you just take a heli geared for 1800 and fly it at half throttle all you'll be able to do is poodle around and hover :-) That's not the same thing.

The point about going bigger than 710/720 is that, unlike electric, with a gas engine you have limits on how light you can go. My TDR weighs less than 5kg with two 4000mah 6s packs (something like 4.8kg if memory serves) so it flies the way it does partly due to the low disc loading. You just can't get that with a gas engine in there, so if you want that type of feel you have to go up a bit, 750 or 800mm to compensate for a 6kg takeoff weight.

At the end of the day it's all personal preference. No one 'needed' to manufacture the Velos 880, our 700 size helis are perfectly fine and arguably fly as good as most of us could ever want. But... if you're one of those crazy folks that thinks an over-the-top massive machine is kinda cool...then... my point is I think it can be done with a gas engine as well, not just electric, but who knows because it has not been done.