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HelicopterMain Discussion › Why is autorotation easier in forward flight?
12-02-2007 04:49 AM  9 years agoPost 1
wlfk

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uk

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My understanding of the standard explanation is that you have 2 ways of storing energy: the momentum of the rotor blades + the forward momentum of the helicopter.

In the final phase you convert rotor rpm to lift, and flare to convert forwards momentum to lift.

The problem with that is that it fails to explain why autos work so much better if you descend vertically into a stiff breeze. If you flare, then you'll find yourself going backwards re. the ground, which is not a good idea.

I'm assuming it's just down to translational lift? Though I thought this wasn't a big issue during autorotation.

K

A bit like a kite, but 500 times more expensive

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12-02-2007 04:58 AM  9 years agoPost 2
corey11

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Bay Area, California

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you wont go backwards if you control it..but its simple..your airspeed will be faster...note this tho..

ground speed, and airspeed are different..ground speed is how fast you move over the ground.airspeed is how fast you are flying through the air..

soo saying that, auto'ing into a head wind means you have a faster airspeed so theres more potential for more lift..say you have a ground speed of 10mph, but theres a 5mph head wind..your airspeed would actually be 15mph(10mph ground speed+5mph head wind=15mph airspeed)

the only reason you are drifting backwards is because your not using enough negative pitch, and forward cyclic..its all practice

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12-02-2007 05:00 AM  9 years agoPost 3
wlfk

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I get the difference between airspeed and groundspeed.

I talked about descending vertically into a stiff breeze. By this I meant that you have 0 groundspeed but significant airspeed. Sorry if this wasn't clear.

In this condition I don't see how you can do a flare without being blown backwards.

K

A bit like a kite, but 500 times more expensive

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12-02-2007 05:22 AM  9 years agoPost 4
jschenck

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La Vista, NE.

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you are converting some of the energy of the wind into rotational energy in the blades, in other words the wind is helping to keep the main blades spinning. imagine holding a pinwheel into a breeze vs. holding a pinwheel in calm air. or if you want to really feel what the heli is experiencing take the motor out of a BladeCP and hold it flat in some wind while moving the collective. you'll note that you can speed up the blades a bit just standing there. pull it back with some slight negative and I'll bet it'll spin right up.

With a good headwind you can come in with a really flat final approach. auto's in wind are a lot of fun.

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12-02-2007 05:30 AM  9 years agoPost 5
wlfk

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imagine holding a pinwheel into a breeze vs. holding a pinwheel in calm air.
The difference there is that you're holding it relative to the ground. More like an autogyro where you have a propellor giving thrust than an unpowered autorotation in a heli.

You can do this for a while during a flare, as you bleed off forward momentum, but if you keep at it then the heli will start to move backwards. What I'm talking about is a steady-state descent, where both airspeed and groundspeed remain constant (I chose 0 groundspeed to keep things simple).

K

A bit like a kite, but 500 times more expensive

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12-02-2007 05:36 AM  9 years agoPost 6
jschenck

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La Vista, NE.

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take that pinwheel and angle it like a descending heli, you'll notice more lift with a breeze than in calm air if the angle if the 'blades' is right. I think the exercise with the freewheeling BladeCP would help illustrate the point better.

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12-02-2007 05:44 AM  9 years agoPost 7
wlfk

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uk

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What do you mean by 'angling it like a descending heli'? Do you mean in terms of collective pitch, or the angle of the rotor to the wind? Or both?

Not quite sure how to tilt the rotor to best simulate the effect. Effectively the wind is blowing from below (1-2 M/S) and in-front (5-10 M/S) so you would have the rotor pitching up about 20 degrees relative to a horizontal wind. But then you need to angle the rotor forwards to gain some horizontal thrust to maintain airspeed... Don't know how much this would be. So about horizontal sounds about right...

I don't have a blade CP but I imagine my T-rex would react the same.

K

A bit like a kite, but 500 times more expensive

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12-02-2007 05:47 AM  9 years agoPost 8
TMoore

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In this condition I don't see how you can do a flare without being blown backwards.
Go out and try some and your questions will be answered.

In a stiff wind, you can auto straight down with little or no flare and land safely every time. I can put them where I want them just about every time too.

TM

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12-02-2007 05:49 AM  9 years agoPost 9
jschenck

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La Vista, NE.

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both. I think the freewheeling trex will work also, just don't let the blades spin up too much or they might bite you

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12-02-2007 05:52 AM  9 years agoPost 10
wlfk

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Go out and try some and your questions will be answered.
It's been windy today and yesterday, and I've done lots. That's where the question came from. I'm not asking whether it works, but how/why.

K

A bit like a kite, but 500 times more expensive

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12-02-2007 06:01 AM  9 years agoPost 11
TMoore

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Like corey11 said, it's all about forward speed. The heli thinks it's flying fast due to a fast moving headwind.

TM

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12-02-2007 06:10 AM  9 years agoPost 12
wlfk

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Like corey11 said, it's all about forward speed. The heli thinks it's flying fast due to a fast moving headwind.
I completely get this, and did from the start. I've flown hang-gliders where if you don't get the difference between airspeed and groundspeed you won't live long. What I'm not sure that I understand is why a heli should autorotate better if it has airspeed.

I thought I was making things simple by specifying no groundspeed, because in this condition it's obvious that you can't reduce your rate of descent at the last moment by making a big flare.

So, to rephrase the question: why do you have a lower rate of descent if you autorotate in FFF compared to a vertical autorotation with no airspeed.

K

A bit like a kite, but 500 times more expensive

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12-02-2007 06:30 AM  9 years agoPost 13
TMoore

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So, to rephrase the question: why do you have a lower rate of descent if you autorotate in FFF compared to a vertical autorotation with no airspeed.
It's a function of how much energy you can store in the disk and how much wind you have to help you. If you do autos with little wind, you will need to store more energy in the disk in order to have enough headspeed at the end of the auto to catch it. You can store energy by using forward speed or use a lot of negative on the way down, building approach speed and then burning off energy at the end of the auto by flaring a lot to stop the forward speed and then using collective to finish it off. This is what I call a "screaming eagle auto".

If there is no wind or a little wind and you auto straight down you can balance the approach by lifting the nose and using a lot of negative collective to maintain headspeed. By doing it this way you will end up using very little collective at the end of the auto and having very little forward speed to burn off with a flare. You also won't have much inertia stored in the disk but you won't need it either.

Depending on how you do the auto, you may not necessarily have a lower rate of descent if you initiate the auto out of FFF as opposed to out of a hover up high because it depends on the angle of attack of the heli on the way down as much as anything.

TM

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12-02-2007 08:02 AM  9 years agoPost 14
spork

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Mountain View, CA

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wlfk,

I fly HG too (not that that has anything to do with anything - but small world and all).

With regard to your questions I think there are two answers. As you clearly know, the heli doesn't know a thing about a steady wind. As with hang gliding the wind only comes into play in reference to the ground - which is obviously pertinent to doing autos. So, part 1... as you suggested you're better off having some airspeed during your auto for the same reason a bit of airspeed helps in powered flight - namely transitional lift. Basically you're just getting a cut at fresh new air with each rotation. You're not working as much on the air above and below your disk that you're already disturbing. Ever been waked in your HG (I'm sure you have)? Descending with no horizontal airspeed in an auto basically has you getting affected by your own wake. But as you are aware, you can get this airspeed by flying forward, holding your position in a head wind, or any combination. It makes absolutely no difference until you're transitioning to flare.

Part 2 - the flare...

Imagine you're doing your auto on a no-wind day. You've got airspeed (and therefore groundspeed) because you generally get a better approach that way (i.e. more blade speed for your descent rate). In the end you have to zero out both your forward and vertical speed for a successful auto. Zeroing out your forward speed requires some of the energy stored in your spinning blades. If instead you land into a 10 mph head wind, that's 10 mph less deceleration you need at the end of your flare. That's free energy you can use to arrest your vertical velocity. That's all there is to it.

If you take a bird like the T-Rex 450, it's somewhat challenging to auto well. But if you put the training gear on, and auto onto asphalt, you can skid your autos in all day with no trouble at all. But if you have a headwind you don't need to skid in because you don't have to arrest that forward speed.
So, to rephrase the question: why do you have a lower rate of descent if you autorotate in FFF compared to a vertical autorotation with no airspeed.
You really don't want fast forward flight in an auto. Typically the best a heli can manage is about a 4:1 L/D in an auto. This is not where you want to be to maximize blade speed for a given descent rate. It seems most of us find a 1:1 glide to be somewhere near the optimum for the steady state portion of the auto. Obviously you can (and should) adopt a steeper angle if you're landing into a headwind.

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12-02-2007 08:42 AM  9 years agoPost 15
Itsindilas

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Greece-Athens

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Heli aerodynamics says that "as soon as the first molecule of air, passes from 12 o'clock to 6 o' clock of the heli disk, then aerodynamic lift is generated". This lift is additional to the lift generated by the rotation of the blades and eventulally represents the lift generated from a disk plate, same diameter as your heli's one, when acting as typical wing. For full sized helis, over 30knots, the heli is considered as a fixed wing flying machine.

So, whenever you have your heli is in forward flight, regardless you are under power on not, your heli needs to spend less power to maintain it's height. The same happens as soon as you lose your power plant and you glide. As far as you maintain a forward speed, then you take the advantage of the additional lift generated by the heli disk plate.

Moreover, don't forget that the "disk plate" behaves exactly like a wing. Consequently, whenever you increase angle of attack you get more lift but in the other side you get more drag. In the same time, the higher the angle of attack is, more air pass through the blades, contributing to head speed increase, but you lose forward speed.
Concluding, that due to the fact that you flight is without any power but gravity, don't forget that every available resource is limited (height, forward speed, head speed etc).

In this point, I would like to mention a technic used by full sized helis under heavy gross weight conditions, when the flight manual charts says that the heli is not able to take off conventioanly and an available runway is needed in order to take off as a fixed wing heli.

The same happens to your RC heli whenever you convert from hovering to forward flight without any pitch change. At first the heli loses a small amount of height but then it tends to gain height.

Caution, a very high forward speed in conjuction with a very low AoA for the disk plate, during an auto, leads to a head speed loss. This happens due to the high drag generated and due to the fact that eventually the air doen't have the available time to pass through the blades.

Cheers

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12-02-2007 08:50 AM  9 years agoPost 16
spork

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Heli aerodynamics says that "as soon as the first molecule of air, passes from 12 o'clock to 6 o' clock of the heli disk, then aerodynamic lift is generated".
This sounds like something from an "aero for pilots" type of manual. Can you give us the reference? I wouldn't call it "wrong". It just sounds like an oversimplification of the principle of translational lift.

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12-02-2007 09:01 AM  9 years agoPost 17
Itsindilas

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Greece-Athens

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This is the way translational lift is applied in heli theory. As far as, every molecule of air passes from 12 to 6 o' clock, then you have the additional "translational" lift. When you hover, you have only the translational lift generated from the blade's spining.
It's not a simplification, it's just aerodynamic's theory.

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12-02-2007 11:57 AM  9 years agoPost 18
RayJayJohnsonJr

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Midwest

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In this condition I don't see how you can do a flare without being blown backwards.
The only reason for the flare in the first place is to achieve zero ground speed at touch down. (Unless you're doing running autos). Autoing in a stiff breeze requires no flare. Don't worry about it.

-Mark

There, their and they're. It's really that simple.

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12-02-2007 02:03 PM  9 years agoPost 19
wlfk

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uk

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Thanks for your replies - it's a lot clearer now.

I'd been flying autos by gaining altitude in windy conditions, adjusting collective to get a steady rate of descent with no groundspeed, then killing power. When I get it right it's pure poetry. My T-Rex just floats down and I barely even need to add any collective at the bottom. But in nil-wind conditions the rate of descent is 2-3x higher and I normally chicken out.

Today - a still air day - was spent mostly practicing 45° descents under power, trying to level out over a coke can some litterbug left in the field. A few weeks ago when I last tried, I was lucky to end up with the heli close enough to land. But today, to my delight and surprise I found I could do it fairly well. Then I crashed.

K

A bit like a kite, but 500 times more expensive

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12-02-2007 02:59 PM  9 years agoPost 20
Thomas L Erb

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Alliance ohio

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If i understand you pattern of thought here the only thing relitive to a successful auto is having enough rotor rpm to stop the heli and land safely. this is achived by the airflow passing through the rotor. the trick is finding the finding the sweet spot for your perticular model as far as neg pitch and airspeed that maitains the maxium rpm during decent. so just as an example if your model needs -3 pitch and 10mph airspeed to be successful then on a clam day you would need to be traveling at 10mph to land but in a 10mph wind you can land verticaly. I don recomend vertical assents in wind because it is never a perfect constant. This example only considers all other factors being constant. the in wind auto would require a flare to stop the forward ground speed where the vertical would not.

The best way to pracitce auots in my opinion is to shoot approaches in normal to learn how the model reacts to conditions . you should be able to auto all the way to the flare with out adding power and when you flare there is no need to be hunting for a switch to bail out. The other method i have used is setting your hold to a fast idle that just engages the clutch and use that power to maintain rpm to start and slowly decrease power as you get more comfortable with the flare and landing part of the auto.

The next big factor to consider is how well your machine will auto. A trex with wood blades will not auto with much success as it has so little blade mass that the flywheel effect of the head is very low.Where a 90 size ship with 220 gram blades can flare ,stop and hover for a time before running out of head speed.

The only thing the head knows is rpm and airspeed . the pilot is the only one that has to deal with ground speed. Hope this helps

Tom

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