balloning?

[desertbird]

What is it when someone says a heli has a tendency to ballon or not ballon?

[darkchiild]

Ballooning up is when wind gets under the rotor disc, and makes the heli float upward. All helis do it, heavier ones do it less for obvious reasons.

[Skiddz]

It's not the wind getting "under" the rotor disc. It's the increased efficiency of the rotor disc due to lateral movement of air, be it by wind or movement of the helicopter. It's called Translational Lift should you care to google for more info (But I'd suggest you visit www.dynamicflight.com instead.)

[desertbird]

So balloning is tendency for a helicopter to translate transational lift more effieciently and want to "ballon"; or increase altitude without elevator input?

[TheBum]

Yes. Conversely, if the wind is blowing and suddenly dies down, translational lift is reduced and the heli will drop.

[desertbird]

What makes one design feature prevent this over another? Is it head speed?

[darkchiild]

Just guessing, but I'd say the two big factors are the size of the rotors, and the weight of the bird. Bigger rotors means more area giving lift, but usually bigger blades are on bigger (therefore heavier) birds. My understanding is the heavier the heli, the less you'll notice it.

afaik the only factor headspeed really plays is that you need decent headspeed w/ negative pitch to compensate for it.

[Skiddz]

Size of the rotor disc and the velocity of the lateral "wind".. Lateral flow of air at X mph will affect a small disc more than a larger one to a certain extent. It's also a matter of weight and inertia. The lighter heli will react more quickly to translational lift than the larger, heavier one so that momentary increase in efficiency might not be long enough for a visible change to occur in the larger heli.

Ever wonder why you need to pull back on the power a bit as you transition from a hover to FF to keep from climbing out too fast???

[spork]

Some helis have a greater tendency to pitch up in forward flight (or sideward or backward flight for that matter). I think this factors into the sense the heli is "ballooning" as well since a gust will create the same result. Gust hits heli, heli pitches up, this results in some climb.

There are a number of elements to the head geometry that will tend to increase or decrease this tendency. The Bell-Hiller mixing ratio I believe is a significant one, and I had a bad pitch up or "hockey stop" tendency when my dampers were dead tired out.

So I'm not 100% sure this is technically the same thing as "ballooning" but I think it is a significant factor.

[vera]

In the full size world, ballooning is a term that's applied to a pilot's mismanagement of elevator or cyclic control when terminating a descent, such that instead of nosing up enough to reduce the rate of descent the pilot over controls and gains altitude.

[terrybeb]

and I had a bad pitch up or "hockey stop" tendency when my dampers were dead tired out.



:clappp:Bliss

I've never heard that term before. Being Canadian that's kinda endearing to me.:D

[spork]

So - you're Canadian - eh? :YeaBaby:

[terrybeb]

:woohooBring on the Back Bacon and pancakes!!!

[Skiddz]

In the full size world, ballooning is a term that's applied to a pilot's mismanagement of elevator or cyclic control when terminating a descent, such that instead of nosing up enough to reduce the rate of descent the pilot over controls and gains altitude.

Um, "elevator" is a cyclic control. :) Ballooning is typically what happens when the pilot pooches an auto and instead of stopping the descent and and slowing the heli, he/she flares too hard and the heli climbs a bit.

Pitch ups in FF could be from wind, from aerodynamic forces on the canopy/body, blowback (caused by really fast FF) and even retreating blade stall. If your dampers are shagged, my vote would be for blowback - the rotor disc "tilting" backwards due to aerodynamic forces in FF.

[vera]

"Um, "elevator" is a cyclic control."

But not on a airplane, of course, which I was intending to include because "ballooning" is a term that applies to airplanes also... usually excessive elevator control during the landing flare, but could include rapid transition to slow flight.

"blowback - the rotor disc "tilting" backwards due to aerodynamic forces in FF."

I don't see why that would be the case. The rotor isn't along for the ride, it's the driving force. :) This "blowback" isn't something that happens in full scale helis except just below translational lift.

However, at the low speed just below where translational lift occurs, the transverse flow tends to pitch the heli nose up. But this wouldn't be happening at high speed.

"retreating blade stall"

Yep, that would do it. I haven't encountered any such thing though. It isn't something one does for full scale certification, not even for the instructor certificate. When I'm tired of my RC model I'll see if I can get that to happen. The RC contraptions turn such a high rpm, at least six times the full scale rpm, so I'm not sure it's possible. Has it ever happened to you? As long as my heli is a beloved member of my family I can't bring myself to try it. :)

[Skiddz]

I don't see why that would be the case. The rotor isn't along for the ride, it's the driving force. :) This "blowback" isn't something that happens in full scale helis except just below translational lift.

Incorrect.

Blowback is a factor when dealing with Dissymmetry of Lift. Due to the flapping action, the advancing blade reaches max flapping uplift over the nose, the retreating blade reaches max downward flap over the tail and as such, tilts the tip-path plane to the rear of the heli. Voila! Blowback.

Retreating blade stall will cause a pitch up due to the retreating blade losing lift. Rotor disc tilts backwards, (blowback) nose pitches up and the heli rolls towards the stalled blade. Kind of self correcting since to stop the retreating blade from stalling, you slow down. When the nose pitches up, you slow down. Magic! :)

At speeds below translational lift, there's not enough aerodynamic force to cause blowback due to Dissymmetry of Lift. Like you mention below, this is Transverse Flow Effect, not "blowback"

However, at the low speed just below where translational lift occurs, the transverse flow tends to pitch the heli nose up. But this wouldn't be happening at high speed.

Not trying to be condescending (and I apologize if I appear to be coming off like I am) look Blowback up in your favorite helicopter text. Blowback is mentioned in every one of mine when discussing DoL and forward flight at high speed. For giggles, I pulled out my Rotorcraft Flying handbook. Yep, page 3-8 under DoL. :) The ASA book Principals of Helicopter Flight mentions blowback on a half dozen pages as well.

Yep, that would do it. I haven't encountered any such thing though. It isn't something one does for full scale certification, not even for the instructor certificate.

True, but it IS simulated and you need to know how to recover. The DPE on both my private and commercial checkrides simulated it and had me perform the proper recovery procedure.


When I'm tired of my RC model I'll see if I can get that to happen. The RC contraptions turn such a high rpm, at least six times the full scale rpm, so I'm not sure it's possible. Has it ever happened to you? As long as my heli is a beloved member of my family I can't bring myself to try it. :)

Nope and I've got my helis absolutely haulin' ASS while RBS is in the back of my mind. Nothing so far, but the TR sure starts singing at high forward speeds. The tip speeds on our models are so high that I'd have to also believe it's not possible - or at least VERY unlikely.

[vera]

Skiddz - "Blowback is a factor when dealing with Dissymmetry of Lift."

Yes, I stand corrected. But blowback increases with increasing speed and the nose doesn't abruptly pop up abruptly as with ballooning, which is is pilot-caused and associated with loosing speed.

[desertbird]

Thanks for the feedback.

Here I was thinking it was a helicopters tendency in FF to want to lift without elevator input but was not sure what would cause this and was wondering exactly what is was.

I hear "balloning" in reviews of helicopters as "this Axe CP has tendency to ballon" I don't see how its a pilot induced tendency with that statement, its usually refererred in terms of a mechanical tendency.

[Skiddz]

It could be mechanical. Shagged dampers, sloppy head, sloppy links, sloppy swash, sloppy thumbs. :)

[mikelebron]

For the most part shouldnt forward flight be very close to 0 pitch.. too much positive pitch plus agressive elevator is usually what causes this correct?

[spork]

For the most part shouldnt forward flight be very close to 0 pitch..

Depending on your speed forward flight can be anywhere from about 3 or 4 degrees positive to 10+. I used to fly my X-Cell 60 around at max pitch quite commonly.

[mikelebron]

Depending on your speed forward flight can be anywhere from about 3 or 4 degrees positive to 10+. I used to fly my X-Cell 60 around at max pitch quite commonly.

I have a TREX 450 and find with too much pitch it will "baloon" up...not sure if this is the correct term.. What could cause this? I have found that if I keep it close to 0 pitch I have faster forward flight and the bird is kept level.