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Ultra Low Headspeed - Nitro

Airwolf2009

Heliman

Hampshire, UK

Let me firstly state my set-up;

- Raptor 90 G4
- YS120
- Hatori 120 3D pipe

I have the engine running amazingly well on 15% nitro and the YS#2 plug, I can govern the headspeed at 2060rpm and almost nothing will bog the engine - fantastic.

Now over the past few weekends I have been playing with low headspeeds, I have the headspeed down to 1250rpm which gives an engine rpm of 9900rpm! Now at this headspeed I can fly very smooth sports type flying (loops, rolls, slow flips etc) and is actually great fun to see what can be done at this end of the envelope. There are two benefits; firstly the model is 'almost' as quiet as an electric one at this headspeed, and secondly I can get 18 minutes flying time on one tank. The reaction from guys at the field was very cool, I don't think any of us thought that a big bore 90 could fly like this.

The question I have is will this cause any harm to the engine? After each flight at 1250rpm the engine comes down barely warm, as you'd expect, however I am just wondering if I am actually operating at too low a temperature.

Any thoughts!

07-31-2012 Over year old.
marc8090

Elite Veteran

Long Island, N.Y.

I think the engine will be just fine as long as you're not bogging it at low speed.

07-31-2012 Over year old.
unclejane

Elite Veteran

santa fe, NM, USA

I was doing some "ULHS" flying with my 700N/OS 105 also (until its death a couple weeks ago) and kind of had the same question. I fly my 700 electrics the same way (on 6S) all the time now, but was reticent to do this too much with the nitro. That motor is expensive lol.....

So I can only guess at the problems that could arise, but I have no data either way... My guesses at potential problems are: low piston speeds, low operating temperature, lugging in general.

No idea if the situation is harsh enough to actually reduce the longevity of the engine.

OTOH, with my experimentations with this on my electrics, I've found that the efficiency of flight at HS's of 1000-1200 or so is much higher than at more typical HS's of 1900+. I.e. on a single batt, I get more flight time out of my Whippy than I do with 2 and the batt even comes down cooler than when I run it with 2 on 12S.
To rule out weight as a factor, I've run my trex 700E with both batts in it as usual but with them hooked up in parallel rather than series. Same weight, but flight time goes up to almost 15 mins, indicating a significantly higher efficiency at 1100 rpm HS than at 1600+ even with the same amount of weight.

So the same thing is probably true on a nitro, the engine is running slower and cooler, but is also not having to put out the same amount of power to maintain flight....

I guess someone would have to run an engine this way for a while to find out about the longevity lol....

LS

07-31-2012 Over year old.
jschenck

rrProfessor

La Vista, NE.

I find that piroflips are easier to control at 1400-1500 on the head. at 1200 it's just too close to stalling to have any margin for error. Lower headspeed does have some advantages, just can't do it with FBL - I assume your Raptor is the flybar'd version.

Team GrandRC

07-31-2012 Over year old.
F1 Rocket

Key Veteran

Melbourne, Florida USA

Lower headspeed does have some advantages, just can't do it with FBL -
Actually the FBL systems make low head speed flying easier. With FBL the system keeps the cyclic rates the same at low head speed as at high head speed. The reduced drag helps too.

Danny - DemonAero Support Team

07-31-2012 Over year old.
jschenck

rrProfessor

La Vista, NE.

When I've tried it I get head wobble and since the gyro doesn't know about the lower headspeed it throws more pitch in trying to get the commanded acceleration which can cause a bog fast. I think this is why I don't like auto's with a FBL heli, low headspeed landing don't "feel" right and can kill the remaining headspeed quick with subtle input where flybar just slows down the reaction. in other words you put in 2 deg of cyclic correction you get 2 deg. with FBL you ask for it to tilt the heli and FBL controller interprets that to "accelerate over this direction at this rate of acceleration". Low headspeed wobble would be a real problem with FBL heli's though since they tend to have much tighter dampening.

Team GrandRC

07-31-2012 Over year old.
gwright

Veteran

Champaign Il

Current setup 1850 and 1180 in my highest and lowest flight modes. Still can do smooth 3D even at the lowest rpm(piro flips/loops, sideways and tumbling stuff, rolling stuff,etc.) Yes, there is an extremely small margin but if you can manage collective properly it's fine. Flight times go up to 16 minutes possible if low rpm is used the entire time (Raptor G4 E720). I noticed that damping wobble was really a range, not a specific point,.. and you could actually get below it as well as above it so that's not an issue. As Danny said, fbl systems really allow it, as you get full cyclic. The above comment about auto's I can empathize with. I hated autoing fbl at first, then learned you don't have to bang the stick over like with a flybarr,.. plus cut my cyclic down in throttle hold to 70%,.. and autos, including aerobatics are no longer an issue,..actually better than flybar. When I started flying helis again last year and re-entered with flybarless I hated it for a few months, now I hate flybars <G>. Due to exactly what you state I had three dumb thumb crashes doing aerobatics in autos. The old reflexes for banging the stick over didn't really work, just stopped the head. When I got ESC's with auto-bailout functionality I started practicing, and learned the differences.

Gary Wright
Futaba
Hacker
Thunder Tiger
Flightpower

07-31-2012 Over year old.
AirWolfRC

rrProfessor

42½ N, 83½ W

Lower head speed WILL be more efficient.

A while back I estimated that the down wash through the rotor disk is good for about 4 of pitch. To that add the best efficiency blade Angle of Attack of about 8 and you have a blade pitch of about 12 degrees.

At the best efficiency AoA, you don't have the best response to control input so back off a couple of degrees and you have a hover pitch of about 10. Bring your head speed down to where that angle works and get noticeably longer flights with less power.

Note, If you want 3D like you're having a siezure, this won't work for you.

07-31-2012 Over year old.
HOMEPAGE  
gwright

Veteran

Champaign Il

Good way of measuring the power used (without telemetry) is flying an exact amount of time in the same style. Check what goes back int he battery. Change rpms, repeat. Do several times with each rpm to normalize the results. I haven't checked the 1180 setup like this but at 1250 I was averaging just a shade over 300mah per minute, and at 1600 it goes up to just under 400, at 1850 it's almost 600, and at 2000 it was just under 900.

Gary Wright
Futaba
Hacker
Thunder Tiger
Flightpower

07-31-2012 Over year old.
JasonJ

Key Veteran

North Idaho

I was playing around with low headspeed awhile back when I still had my Fbl Rappy. I had it down to 1100 rpms and the beastx still managed it just fine. Nothing weird happened except when I botched an auto bad, killing all the headspeed and the Rappy bounced after a 75 foot fall.

07-31-2012 Over year old.
unclejane

Elite Veteran

santa fe, NM, USA

I think this is why I don't like auto's with a FBL heli, low headspeed landing don't "feel" right and can kill the remaining headspeed quick with subtle input where flybar just slows down the reaction.
NOBAR actually gives this increased rotor stability with decaying RPM back - this was one of the reasons I tried so hard to get the CGY 750 to work on my trex's for weeks and weeks, far longer than I should have mucked with it.

With the 8FG radio, the 750 allows you to put NOBAR on a switch by turning a selected bank of gains down to 0 and then switching to it in flight. I quickly got heavily spoiled by being able to select NOBAR for autos. With the gyros off, you get the flybar behavior in the flare and touchdown, which is what I was used to.

Something like this may be doable with bank switching on the Vbar too, though my 700N went to the great helipad in the sky a couple weeks ago. So I haven't messed with that since.

I've gotten more or less used to the FBL behavior in the flare now, tho, so it's not too much of an issue anymore.

LS

07-31-2012 Over year old.
jschenck

rrProfessor

La Vista, NE.

my opinion is the FBL controllers are missing key information by not having a rotor speed indicator. Could be as simple as an optical sensor aimed at the main gear with a piece of flash tape attached or some other point. That would allow them to change the expected acceleration based on speed of the rotors.

Team GrandRC

07-31-2012 Over year old.
Airwolf2009

Heliman

Hampshire, UK

I am actually running a FBL setup, the CGY750 seems to handle the low headspeed without any issues. The cyclic response is a little slower and I am careful not to bog the headspeed too much, but overall it flies amazingly well and once in forward flight feels about the same as it does at almost twice the headspeed.

07-31-2012 Over year old.
JKos

rrProfessor

Redondo Beach, CA

my opinion is the FBL controllers are missing key information by not having a rotor speed indicator.
Most have built in governors which certainly know your rotor speed.

- John

07-31-2012 Over year old.
jschenck

rrProfessor

La Vista, NE.

they would know engine or motor speed - not HS. Specifically when shooting an auto.

Team GrandRC

07-31-2012 Over year old.
marc8090

Elite Veteran

Long Island, N.Y.

I have done a few less than perfect autos recently and got the head speed WAY lower flybarless than I did before with a flybar and every time it did nothing weird. I was actually quite impressed just how well it handled the low head speed. CGY 750 by the way.

07-31-2012 Over year old.
jschenck

rrProfessor

La Vista, NE.

it may not be an issue, I don't have a FBL 700 so I can't really compare 700 to 700. I just know that I can coast I'm my 700 FB on a really low head speed where my 500 does funky things when I try that. 3GX on the 500, flies great.

Team GrandRC

08-01-2012 Over year old.
KC

Elite Veteran

WA

'Will this cause harm to the engine?'...No

We did this with woodies for decades, many learned how to hover this way. Most f3c pilots still use a two stage setup with low rpm for hovering. It doesn't hurt the engine.

Most blades today are too stiff to really take advantage of low rpm.
Try a pair of semi-sym Spinblades. they are about a third wider than a normal blade and flexible enough to work at low rpm.

Heres a vid, not mine....I've done all the same stuff with these blades on a 600se, they really handle well at low rpm

Watch at YouTube

08-01-2012 Over year old.
Airwolf2009

Heliman

Hampshire, UK

Thanks for the video, that is exactly how I fly my nitro, albeit at a slightly higher head speed. What I love is the way the rotor makes a sound at times very similar to full size machines :-) I will post a video this weekend of it.

08-01-2012 Over year old.
Airwolf2009

Heliman

Hampshire, UK

Video

Here's a video of mine taken last weekend at the low head speed.

[url=

Watch at YouTube

]

08-22-2012 Over year old.
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Ultra Low Headspeed - Nitro

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