Inverted Full Size Helicopters?

[darren_uk]

"If a full-sized heli can't do negative pitch, how do they do autos?"

I've started a new thread, "How do full sized helicopter autorotate?"

There are several reason's why full-size helo's do not fly upside down. Yes the lynx and the Apache and other helicopters with fully articulated rotor systems can do momentary inverted flight such as loops and rolls. Underslung rotor systems can not go inverted do to a phenomena called "Mast Bump" This is when the rotor head comes in contact with the main mast which thens causes another phenomena called "Blew Blades." That when one blade blows off left and the other one blows off to the right.

Keep in mind the power to weight ratio which is always critical to develop lift is much greater in a R/C helo than in the actual aircraft. Some full-size helo's hover using anywhere from 65%-100% of their avalible power just to hover, depending on weight temperture and pressure altitude.

A Cessna can not sustain inverted flight no more than a helicopter can because of the wing design. A Cessna has a non-symetrical wing design meaning it only creates lift on the top of the wing. To sustain inverted flight it would need a symetrical blade. Most helicopters have a semi-symetrical blade designed for the least amount of drag and the most lift from the top of the blade.

Also a Cessna and most helicopters fuel and oil systems are gravity feed systems. The pumps are usually suction type, sucking fuel or oil from the gravity filled sumps to the top of the engine or gearboxes for lubrication. During inverted flight no oil is being pumped to the bearings for lubrication and cooling. To make these systems postive pressure would be costly and add unnecesary weight to the aircraft.

Lastly, what pratical purpose would it provide? Fix-wing use it to change there direction quickly, such as to get weapons on target quickly for ordance release. Helo's can do it quicker by kicking the pedals and yawing around there center axis. The only time I consider going near inverted is at high altitude, when my target is below and behind me and I want a rapid change in direction while maintaining my kinetic engergy (airspeed). Notice I say near inverted.

Keep this in mind. Do you really think you could hover very long with all the blood rushing to your head. I THINK NOT!

Darren_uk
NO full size helo's do not need negative pitch to do auto-rotation. Negative pitch is used in full size Navy aircraft to keep them firmly PLANTED on the deck of a pitching ship while the chains are secured. Negative pitch in a full size helo is bad for several reasons. In a full size helo say the Apache for maximum lift from the blades RPM is maintained at 298 +/- 20%. Above or below that amount the blades start loosing lift. If the rotor system overspeeds then the blades cone and stall. This can crack the blades also the overspeed can cause damage to the drivetrain components not design for those types of loads.

The maintenance test pilots adjust the pitch in the blades at flat pitch to obtain the optimum RPM for auto-rotations. Above or below this RPM rate of descent is increased. It is prefered to adjust rate of descent by airspeed in fullsize not by pitch. So if there was negative pitch in the blade during the auto the blades would overspeed, stall increasing rate of descent to an undesirable speed or blade failure would happen also undesirable.

Since in a R/C helo the blade loading is a lot lower and over/underspeeding of the rotors is not critical, there is a lot more leeway to be able to use both +/- 10 degrees for inflight manuevers.

I hope this answers your questions.

[colin]

Hi.. I beg to differ. A cessna *can* do inverted flight, so long as they can continue to pump fuel into the engine.

A typical 172 is gravity feed, so flying inverted will cause fuel stavation, which will then konk the engine. But a Cessna 152 is fully aerobatic with pressure pumps in the fuel system.

The wing cross section has nothing much to do with flying inverted.

Heck, even my RC plane with a typical aerofoil cross section wing flies inverted very well... except that it takes on a nose high attitude.

Originally posted by warrior29

A Cessna can not sustain inverted flight no more than a helicopter can because of the wing design. A Cessna has a non-symetrical wing design meaning it only creates lift on the top of the wing. To sustain inverted flight it would need a symetrical blade. Most helicopters have a semi-symetrical blade designed for the least amount of drag and the most lift from the top of the blade.

Also a Cessna and most helicopters fuel and oil systems are gravity feed systems. The pumps are usually suction type, sucking fuel or oil from the gravity filled sumps to the top of the engine or gearboxes for lubrication. During inverted flight no oil is being pumped to the bearings for lubrication and cooling. To make these systems postive pressure would be costly and add unnecesary weight to the aircraft.

Ok Colin,
To keep this discussion clear, we are discussing two seperate things aerobatics and sustained inverted flight.

Yes a cessna can do loops and rolls but it can not sustain inverted flight straight and level. You could attempt it, but as you mentioned it would require extreme pitch attitude and the aircraft would continue to lose altitude. Why? Because the wing is no longer producing lift in the (up) axis.

If your so sure of your facts, I invite you to go rent a Cessna and let us know how long you stayed inverted. But please let me know when your going to do it so that I can get the impact on tape. If you don't believe me call your local FBO and talk to one of their instructors. I sure they will give you the same answer just as soon as they stop laughing. You can roll a 747 but that does not qualify it as an aerobatic aircraft.

Wing design has EVERYTHING to do with flying sustained inverted flight or flying right side up for that matter. Where do you think the lift is derived from? Do you believe the engine is dragging the aircraft through the air? On a non-symetrical wing only the top of it is producing lift. This is a fact. http://www.aa.washington.edu/faculty/eberhardt/lift.htm

Please read the following link on flat bottom or symetrical wing design
http://www.quicktechhobby.com/articl...ing_to_fly.htm
Here is another source for you. http://www.iacvirar.com/flying_basics.htm
Yes the cessna is considered a aerobatic trainer as defined here http://www.airliners.net/info/stats.main?id=138 Because it is capable of spin and stall training.

Now to call the cessna aerobatic can be true by FAA definition;

FAR 91.303 defines aerobatic flight thusly: an intentional maneuver involving an abrupt change in an aircraft's attitude, an abnormal attitude, or abnormal acceleration, not necessary for normal flight. Note that aerobatic flight is not demarcated by a specific pitch attitude or bank angle. (Aerobatic flight is often mistakenly thought to occur only when an aircraft exceeds 30 degrees of pitch or 60 degrees of bank relative to the horizon. This 30/60 rule, which appears under FAR 91.307 (c), merely specifies the conditions under which parachutes must be worn by the occupants of an aircraft.) In the classical sense, the term aerobatics includes spinning, looping, and rolling an aircraft through 360 degrees of yaw, pitch, and roll.

But for a true aerobatic aircraft it must have symetrical or semi-symetrical wing design, postive pressure oil and fuel pumps. Research the CAP232, Pitts S-2 or S-3. Look at their wing design and let me know what you find out.

As far as your R/C inverted, it is the same as sticking your hand out of the window of a car, the angle of attack is such that you are producing lift only because you have an engine powerful enough to overcome the drag. Like F4 phantom pilots say, " You can make a rock fly, if you put a big enough engine on it."

Have another bong hit. A cessna 172 will fly all day inverted, providing power or a steep glide. A positively cambered wing will produce lift inverted. How is it that my slow stick park flyer with an undercambered wing will fly inverted? with or without power? Not the most efficient arrangement, but it works fine. I have flown inverted in many aircraft, full size & model, without symmetrical airfoils. You are wrong, go take a basic ground school.
A full size heli has a small amount of negative pitch available to autorotate, either at the head, or through twist on the blades, otherwise, the rotor would not spin due to air flowing upwards through the rotor disk, (unpowered decent) and autorotation would be impossible.
Full size heli's will not fly inverted for several reasons. If it has a teetering rotor head, it MUST remain loaded at positive G, or it will mast bump (look it up) as far as the rest, assuming the fuel & lubrication systems were designed for inverted,
the blades are too flexible to prevent hitting the tail boom from abrupt control inputs. Model heli's blades are relatively stiffer. If a full size heli were designed the same as the model and scaled up, everything else being equal, they would indeed be capable of inverted flight. Problem is, if it doesn't work or you screw up, you can't jump out. Who wants to try it first?
There has been an aerobatic heli team performing for years with 3 R-22 helis out of Salinas, Ca. called "the showcopters". They have been performing at airshows for years. (no inverted flight). They are remarkable to anyone aware of the limitations of a teetering rotor system. One of them was killed a few years ago when one of there students made an abrupt control input. Blades came off, struck fusalage, R-22 fell down, fire consumed everything.
Rest in peace Kent.

-Dan

Stratusfive,
Your all over it. Yes a postive cambered wing will produce lift inverted if you place it at a high enough incident angle, Which is why park flyer can do it combined with an incredible high power to weight ratio. Inverted flight means straight and level not a controlled descent which means it is using a reduced angle of attack to overcome the stall because it doesn't have enough power to sustain straight and level flight.
With that being said, I have been in a 172 inverted, no it is not capable of sustained inverted flight and I really don't recommend it anyway.
I don't mean to insult your great aerodynamic knowledge Stratusfive as far as I know you could be a aerodynamics engineer or could have possible slept in a Holiday Inn Express last night.No I don't smoke dope but I do fly for a living and I have been inverted in both fix wing and helo's. Sorry I also forgot to mention I teach guys like you how to fly and I have been doing it for 19 years. So for give my ramblings I am probably not qualified to speak on this subject. How about you?
" You can make a rock fly, if you put a big enough engine on it."

[quote=warrior29]Stratusfive,
Your all over it. Yes a postive cambered wing will produce lift inverted if you place it at a high enough incident angle, Which is why park flyer can do it combined with an incredible high power to weight ratio. Inverted flight means straight and level not a controlled descent which means it is using a reduced angle of attack to overcome the stall because it doesn't have enough power to sustain straight and level flight.
With that being said, I have been in a 172 inverted, no it is not capable of sustained inverted flight and I really don't recommend it anyway.
I don't mean to insult your great aerodynamic knowledge Stratusfive as far as I know you could be a aerodynamics engineer or could have possible slept in a Holiday Inn Express last night.No I don't smoke dope but I do fly for a living and I have been inverted in both fix wing and helo's. Sorry I also forgot to mention I teach guys like you how to fly and I have been doing it for 19 years. So for give my ramblings I am probably not qualified to speak on this subject. How about you?

First, what is an "incident angle"? You must mean incidence, which is the relationship between the main wing chord and the tailplane. I hope you don't use the wrong terminolgy when teaching, as Im sure you were referring to angle of attack, not incidence.
The park flyer does not need power to glide slowly downwards without power, upright or inverted. Neither does the 172. The 172 will not maintain altitude without power, right side up or inverted, unless you slope soar it or thermal it. Both which I have done.
I didn't stay at a holiday inn, but I am a professional pilot with ATP, I fly jets for a living, I am a CFIG with over 5000 hrs in sailplanes, I have towed gliders over 8000 times with various AG aircraft. I have over 5000 hours in hang gliders over the last 30 years. I fly full size sailplanes with positively cambered airfoils inverted all the time. Does this make me an expert? Not really, but I routinly do what you say is not possible, and Im sorry, you do not teach guys like me to fly. I have taught avaition for over 30 years, and have held an ATP for 18 years. How about you?
Try this next time you fly your bug smasher...get up high, shut off the engine and stop the prop. (bet you have never done that) Next, note how you are gliding downward in straight and level flight, without power. You are generating lift. You are flying. Downwards, but you are flying. Now, roll inverted, and keep your airspeed above about 80kts. Guess what? you are still flying! Yes, your glide angle is steeper, and your sink rate will increase, because you are not using your airfoil efficiently, but you are still flying.
Now explain to me why a cessna will not fly upside down? with, or without power?
Quote "Inverted flight means straight and level not a controlled descent "
Really? so that means you do not consider a controlled decent as flight? DUH!
So when you begin a decent, you are no longer creating lift and flying? I hope you don't teach this way. Guess you never flew a 150 aerobat, or a Citabria, both have a positively cambered wing wich will fly inverted, and they certainly do not have excess power.
http://www.pbs.org/wgbh/nova/nazipri...d_airfoil.html
http://airsports.fai.org/feb99/feb9904.html
http://www.av8n.com/how/htm/airfoils.html (pay close attention to 3.8)
Stop abusing Bernoulli ! If you believe a positively cambered airfoil cannot sustain inverted flight, you owe it to your *students* to learn the facts!
Power is required to create lift. It can come from engine thrust OR downward flight through the airmass. Lift is a result of displacing air downwards. A positively cambered airfoil can displace air downwards when inverted, given a positive AOA.
Even a frisbee will fly inverted.
Im not trying to assault you personnaly, but I encourage you to discuss this with with people who know more about it than you do. If you did not teach, I couldn't care what you believe, but you need to know the facts to be a good teacher.