RunRyder RC
 5  Topic Subscribe
WATCH
 1 page 877 views POST REPLY
Scorpion Power Scorpion Power
HelicopterMain Discussion › Blade flapping
11-27-2007 09:25 PM  9 years agoPost 1
wlfk

rrVeteran

uk

My Posts: All  Forum  Topic

So, I have hinges on my blades so they can flap forwards and backwards during flight. To what degree (in terms of angles...) does this actually happen? Is it one of those tiny adjustments that is imperceptible, but nevertheless important. Or do they move by a substantial amount?

If the latter, do you lose any energy through blade flapping?

I have a T-Rex 450, and my understanding was you should adjust the grips until the blades were just free enough to fall if you turned the heli on its side. But if I move the blade tips fore and aft by a centimetre, this takes a reasonable amount of force. Work done = force x distance moved, so if the heli is doing this 50 times a second, then I'd have thought it might take a substantial amount of energy?

K

A bit like a kite, but 500 times more expensive

PM  EMAIL  Attn:RR  Quote
11-27-2007 09:53 PM  9 years agoPost 2
Leif

rrElite Veteran

USA

My Posts: All  Forum  Topic

What you're referring to is lead/lag, not flapping. Flapping is where the blade moves up and down, as a bird does when flapping it's wings. There were a number of head designs that allowed the blades to flap independent from each-other. The Kyosho Concept helis were an example of such a design.

The blades will lead or lag depending on the amount of centrifugal force from the spinning rotor head offset by the drag of the blades. Naturally both of these forces change during flight, as changes in collective and cyclic pitch angles will have a major influence on drag, and the headspeed changes have an influence on the centrifugal force. Unless your blade bolts are extremely tight these forces will adjust the lead/lag without too much difficulty. If you have the blades too tight in the grips, you will be able to observe the resulting imbalance as major vibrations in the heli.

Leif

PM  EMAIL  GALLERY  Attn:RR  Quote
11-27-2007 11:04 PM  9 years agoPost 3
ZXXflyer

rrKey Veteran

stone mountain, georgia, US

My Posts: All  Forum  Topic

I adjust all my blade bolts before every flight so that I can tip the heli on it's side, and they stay straight out.. but if I give just a gentle bump while the heli is on it's side, they will fall.

Too loose, and you will end up with a boom strike on a rough landing/auto.

Believer in Weston motors!

PM  EMAIL  HOMEPAGE  GALLERY  Attn:RR  Quote
11-28-2007 12:13 AM  9 years agoPost 4
wlfk

rrVeteran

uk

My Posts: All  Forum  Topic

What you're referring to is lead/lag, not flapping.
Good! I'd thought the terminology seemed a bit strange.

K

A bit like a kite, but 500 times more expensive

PM  EMAIL  Attn:RR  Quote
11-28-2007 12:18 AM  9 years agoPost 5
AirWolfRC

rrProfessor

42½ N, 83½ W

My Posts: All  Forum  Topic

Too loose, and you will end up with a boom strike on a rough landing/auto.
That's not the mechanism that contributes to a boom strike.
Low head speed and high vertical accel/decell does that.

PM  EMAIL  HOMEPAGE  GALLERY  Attn:RR  Quote
11-28-2007 12:25 AM  9 years agoPost 6
wlfk

rrVeteran

uk

My Posts: All  Forum  Topic

That's not the mechanism that contributes to a boom strike.
Low head speed and high vertical accel/decell does that.
If the blades are at even a small angle to the blade grips, then the blades can be bent down to touch the boom just by swivelling the feathering shaft. If the blades are in line with the grips, then you need to push quite hard on them both in order to bend them low enough to touch the boom.

My vote would be a bit of both.

K

A bit like a kite, but 500 times more expensive

PM  EMAIL  Attn:RR  Quote
11-28-2007 12:33 AM  9 years agoPost 7
AirWolfRC

rrProfessor

42½ N, 83½ W

My Posts: All  Forum  Topic

If the blades are at even a small angle to the blade grips, then the blades can be bent down to touch the boom just by swivelling the feathering shaft.
True . . . . but think about it . . . . centrifugal force prevents that from happening when the blades are turning. When they slow down or stop is when the problems happen.

PM  EMAIL  HOMEPAGE  GALLERY  Attn:RR  Quote
11-28-2007 12:50 AM  9 years agoPost 8
wlfk

rrVeteran

uk

My Posts: All  Forum  Topic

centrifugal force prevents that from happening when the blades are turning.
The same forces should keep the blades from bending in any direction away from where they're meant to be, including down towards the boom.

Assuming the angle of deviation is small, the magnitude of the restorative force won't be that much different whether the pivot is in the rotor housing (to bend the blades towards the boom) or the blade grip.

K

A bit like a kite, but 500 times more expensive

PM  EMAIL  Attn:RR  Quote
11-28-2007 01:00 AM  9 years agoPost 9
AirWolfRC

rrProfessor

42½ N, 83½ W

My Posts: All  Forum  Topic

On a 90 size heli with 710 blades at 1800rpm, the centrigugal force is about 650lbs trying to remove the blades from the head. The heli weighs about 12lbs.

Do a bit of trig on those numbers and you have a very large restorative force at a very small angle. The coning angle in hover is only about ½º

PM  EMAIL  HOMEPAGE  GALLERY  Attn:RR  Quote
11-28-2007 01:46 AM  9 years agoPost 10
wlfk

rrVeteran

uk

My Posts: All  Forum  Topic

Do a bit of trig on those numbers and you have a very large restorative force.
For a deviation of 10mm:

(10/710)*650 = 9.15 lb per 10mm deviation

For small angles this will be roughly constant: 10lb/cm deviation to overcome this force - significant.

But in any case, that's missing my point. Imagine you have a 650lb weight hanging from a string. It doesn't make any difference whether you try to push it 10cm north (flap) or 10cm west(lead/lag). The restorative force is the same.

As you pointed out, boom strikes mostly occur at low headspeeds anyway (e.g. autos) so this restorative force is likely to be much less significant and the blade's natural springiness will be the major factor in keeping it from hitting the boom - provided the blades are at 180 degrees. In other words, you need to overcome both the restorative force and blade resilience.

OTOH if a blade is even a few degrees out, you can easily feather it to the point where it will strike the boom. In this case you need to overcome the restorative force but not blade resilience. The restorative force will be larger in this instance as it has a lead/lag component and a vertical component. But say you're feathering at 30 degrees (easily possible on my 450) it's only by a factor of 2 or so.

It might be that blade stiffness is less significant on 90's. On my T-Rex it's quite hard to push the blades down far enough to get a boom strike unless you feather them. For most of my boom strikes, I'm pretty sure the blades hit the ground before they hit the boom. And then they mostly don't do more than ding the boom anyway, which shows the headspeed wasn't too high when they happened.

K

A bit like a kite, but 500 times more expensive

PM  EMAIL  Attn:RR  Quote
11-28-2007 02:04 AM  9 years agoPost 11
AirWolfRC

rrProfessor

42½ N, 83½ W

My Posts: All  Forum  Topic

I don't follow your math. Your results are in the ball park but I don't agree with the form.

All numbers aside, if you come in for a hard auto with low head speed, you will most likely have a boom strike and the blades will still be fully extended untill the blades actually strike something.

PM  EMAIL  HOMEPAGE  GALLERY  Attn:RR  Quote
11-28-2007 02:14 AM  9 years agoPost 12
wlfk

rrVeteran

uk

My Posts: All  Forum  Topic

I don't follow your math.
Draw a triangle with the blade length (710mm) as the hypotenuse and the deviation (10mm) as the opposite angle. The length of the adjacent will be 710.07mm long, though I didn't bother working this out the first time.

The ratio 710:10 (adj/opp) will be the same as the ratio of 650lb:the restorative force. So to get the restorative force:
(10/710)x650.
All numbers aside, if you come in for a hard auto with low head speed, you will most likely have a boom strike and the blades will still be fully extended untill the blades actually strike something.
I disagree, but truth is what we really need is a high-speed video camera and a willing volunteer. Or... a randomised controlled trial of failed autos with different blade tightnesses. Only problem with the latter is we'd get through lots of helicopters.

K

A bit like a kite, but 500 times more expensive

PM  EMAIL  Attn:RR  Quote
11-28-2007 02:26 AM  9 years agoPost 13
ZXXflyer

rrKey Veteran

stone mountain, georgia, US

My Posts: All  Forum  Topic

Don't forget about the TAN of your damper durometer...
Just kiddin.

Believer in Weston motors!

PM  EMAIL  HOMEPAGE  GALLERY  Attn:RR  Quote
11-28-2007 03:19 AM  9 years agoPost 14
"Cam"

rrKey Veteran

UK

My Posts: All  Forum  Topic

Two Types

I Have seen several boom strikes where the heli is doing a backflip into backwards, inverted flight. The blades are at full rpm as the heli was at zero pitch a moment before, the pilot is putting in negative pitch and pulling back on the cyclic. I've seen boom strikes this way with TRex450's, TRex600's Raptor 90's and even the Aurora.
I've seen helis explode in the air from just doing a backwards 'pull-up'

Surely the cyclic is not enough to overcome the dampers? Is it more likely a link has failed or something yields?

I've had boom strikes from bad autos but see that as a phasing effect caused by the low headspeed, (started by the bump), that is different to what happens above.

DSX9
EMax V2
http://runryder.com/helicopter/gallery/37491/

PM  EMAIL  GALLERY  Attn:RR  Quote
11-28-2007 03:27 AM  9 years agoPost 15
ZXXflyer

rrKey Veteran

stone mountain, georgia, US

My Posts: All  Forum  Topic

Man those in-flight, inverted, boom strikes are amazing to see!!!

Believer in Weston motors!

PM  EMAIL  HOMEPAGE  GALLERY  Attn:RR  Quote
11-28-2007 12:54 PM  9 years agoPost 16
NORTHERNLIGHTS

rrVeteran

Riverview FL

My Posts: All  Forum  Topic

I think one would also consider the quality, and durometer of the Dampers. As mentioned above boomstrikes can occur at full extended RPM`s.

B.O.A.T. Break Out Another Thousand!

PM  EMAIL  HOMEPAGE  GALLERY  Attn:RR  Quote
11-28-2007 01:15 PM  9 years agoPost 17
Phrogger

rrNovice

Gloucestershire, UK

My Posts: All  Forum  Topic

Vario multiblade heads

Hi guys,

Returning to the question of lead/lag for a moment, I attended the UK Vario scale weekend earlier this year and was surprised to see that many of the Vario multi blade heads have the blades attached to the grips with two bolts rather than one.

With two bolts holding the blade, it presumably cannot rotate in the grip for lead/lag. Is the lead/lag less on multi-blade heads to the extent that it doesn't matter? Or do they achieve lead/lag some other way?

PM  EMAIL  Attn:RR  Quote
11-28-2007 03:08 PM  9 years agoPost 18
AirWolfRC

rrProfessor

42½ N, 83½ W

My Posts: All  Forum  Topic

Nobody seems to realize just how much force there is to cause those boom strikes, WITHOUT any lead/lag.

As far as deflection goes, using 710 blades at 1800rpm,
the CG is about 18", the center of lift is at about 23.4" with the rotor tip radius at 31.2".

Using 6lbs of lift at 75% radius gives 6 x 23.4 / 18 = effective 7.8lbs at CG for determining the cone angle.

Centrifugal force is about 650lbs, atan(7.8/650) = 0.69º
31.2 x (7.8/650) = 9.5mm rise at the tip.

The blade will have to deviate about 190mm below level to strike the boom.

For a boom strike to happen at 1800rpm, that's a VERY high G load for a hard auto or inverted pull out. Maybe about 190/9.5 = 20 Gs or more.

Lower head speed reduces centrifugal force and lets the blades get at the tail boom even quicker.

Again, the lead/lag you see after the boom strike is exactly that . . . . AFTER the strike. The blade stops but the hub keeps turning for a bit.

PM  EMAIL  HOMEPAGE  GALLERY  Attn:RR  Quote
11-29-2007 02:51 AM  9 years agoPost 19
wlfk

rrVeteran

uk

My Posts: All  Forum  Topic

For a boom strike to happen at 1800rpm, that's a VERY high G load for a hard auto or inverted pull out. Maybe about 190/9.5 = 20 Gs or more.
Nobody on this thread suggested the lead-lag mechanism might to blame when the headspeed's high - only during autos.

The formual for centripetal acceleration is w^2r, which means that if you reduce your headspeed by 50% acceleration is reduced by a factor of 4. I don't know what an autorotation headspeed is, but by this reasoning + your calculations, you might only need 4-5 gs of acceleration on landing to get a boom strike.

Say one blade touches a bit of grass (not the earth) or you land with some forward speed and a bump. Doesn't seem impossible to get lead-lag as a result. Lead-lag can also lead to boomstrikes in ground-resonance, though obviously this is a very different condition.

My experience is that since my blades have been tighter I've had very few boom-strikes, though I admit this is not Science.

K

A bit like a kite, but 500 times more expensive

PM  EMAIL  Attn:RR  Quote
WATCH
 1 page 877 views POST REPLY
Scorpion Power Scorpion Power
HelicopterMain Discussion › Blade flapping
 Print TOPIC  Make Suggestion 

 5  Topic Subscribe

Friday, October 20 - 11:47 am - Copyright © 2000-2017 RunRyder   EMAILEnable Cookies

Login Here
 New Subscriptions 
 Buddies Online