PPU.

[birdy]

Theres alot understood bout the physics and dangers of PPO [power push over] caused mainly by a high thrust line tipping a machine over forwards if the rotor lift is lost for a certain amount of time.How much thought has been given to the possability of a PPU [power push under]???[}]Some of the new age machines you see get'n round now have the TL below the COM, and in some cases, along way below.Do the people fly'n these machines know that theres an equaly dangerous possability of a machine being flipped over backwards or are more susseptable to a torque roll over??[xx(]IF....... a machine started to tip over backwards, the point of no return will cum much quicker than in a forward PPO.[xx(][xx(]Just some food for thought, I'll leave the scinarios that could cause this to happen to your imagination.[]Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[asra]

The response of a HTL machine to diminished rotor thrust is to pitch nose down, reducing rotor angle of attack and farther diminishing rotor thrust which in turn increases the nose down pitching rate, etc. It is something that feeds on itself.The response of a LTL machine, where the rotor thrust vector must pass to the rear of the CG, is to pitch nose up, increasing rotor thrust which resists the pitch up.A LTL machine is stable inasmuch as a disturbance tends to return it to trim; a HTL machine is unstable inasmuch as a disturbance tends diverge it from trim.But too much stability is not always a good thing. The restoring force behaves exactly as a spring, increasing the frequency of short period oscillations.

[birdy]

So you'd agree that a LTL isn't as safe as CLT then Chuck???While the rotor thrust vector is in a faverable position in a LTL machine, effect of the rotor itself will not be enough to counter the prop thrust if it[prop] isn't decreased rapidly. Just out of interest Chuck, wot's the lowest thrust line you'v seen on a machine?? You would have seen a few over there with all them jackedup dominaters .Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[asra]

Having a low Thrust line is not all good.There are several problems associatted with this condition.The positives are1) the thrust forces the nose up, and positions the RTV further aft of the C of G.2) this aids in high speed flight stability, and wind insensitivity.The negotives are.1) at very low power setting and or loss of power at high RPM, could cause a bunt, just like a high thrust line machine. This is due to the low thrust line holding the nose up and RTV behind the CofG.During loss of power the nose drops and the CofG can move behind the RTV.2) In order to balance the Low Thrust line you require a positive pitched HS, and this does not provide static airspeed stability. So you have to fly the machine in an active condition, like a HTL machine.3) in order to trim and fly your machine at a higher power setting it requires you to apply a significant amount of forward stick, and if you couple this with the fact that your HS is also lifting your tail to balance your machine, then consider what would happen if your engine cut out. (BUNT)!!!!Image Insert: 3.44 KBImage Insert: 3.52 KBRegards Sam []

[asra]

[asra]

Image Insert: 6.96

[asra]

Image Insert: 4.74

[birdy]

The reason I started this thread is coz I'v dropped the TL on the ferel 5" by lowering the hole engine, prop and mount on the 912.It was bout 3" high before, and with the TL and engine weight lowered 5" I reckon it'd be pretty colse to CLT.At cruise and high speed theres no real difference except for the pitching with power changes.Its at low speed I noticed the most difference.Positives;Lower min speed before sink under full power coz of the more efficiant thrust.Lighter on the stick.More effective rudder coz of the increased airflow.Less chance of a PPO.Better power efficiancy .Negatives;Annoying pitching with power changes.Sluggish rotor responce.Can't 'loadup' the rotors as easly.Torque roll is more of a safty issue.Out running the rotors is easer.And lastly, I shoulda dun it years ago coz now I gota relearn to fly, but I gess I can live with that.Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[asra]

Image Insert: 6.7

[asra]

Sam, you said that a low thrust line machine has "Wind instability"? wrong, exactly the opposite. Also, on low thrustline machines ie Dominator styles the horizontal stabilizer has a negative angle of attack, (leading edge down) not positive, and does not lift the tail.As Chuck said when you "chop" the throttle the gyro dosn't wildly nose over killing all and sundry,properly set up gyros slowly nose down where you can trim for best glide speed.Unfortunatly it seems to me that the more people that are starting to "see the light" re low thrust line machines and horizontal stabilizers run the risk of not setting them up right and making the gyro more dangerous.If you are building a Dominator style, or copy of, put the engine level and tilt the Tall Tail one degree forward,its as simple as that.M Barker

[birdy]

Thanx Chuck B.The pitching, while I know its wot a stable machine is supposed to do, is annoying at low level and low speed [when on the shoulder of a critter].It never used to pitch before and its sumthn I'm guna have to get used to.Sluggish rotor responce, I'm put'n down to the lower rrpm, coz they ain't countering the HTL now.[less artificial weight.]Shorter rotors will fix that.Which is the same reason they're harder to loadup, they are starting at a lower rpm, and coz of the lower artificial weight at full throttle, it takes considerablely longer to load um up.[ which is annoying coz they still slow down just a fast as they used to.]Torque roll. Coz its lighter at min speed,[no artificial weight] and can creep at a lower AS, the rotors are spining slower[ wish i had a rotor tack], the prop is carring more of the weight and the nose is higher. Prop torque is much more noticable.Out running rotors.Simply coz of the lower relative weight and RRPM.Example; changing from a 90* left bank to a 90* right bank without changing from your starting 20' alt and on full power,thats a 180* virtical and 180*+ horisontal change in not much time. The rotors arn't going to respond as fast as they used to and may object to the high pitch inputs.But shorter, faster spinning rotors will fix that. Jeff, mate,.....help!!!!Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[asra]

Murray, the topic was "Power Push Up" so I took it as a considerable lower thrust line, not just a few inches.Having a CLF machine, aids in pitch stability, and if you place your thrust line even further bellow you CofG means you are moving your CofG further forward of your RTV.This will further enhance your machines pitch stability in higher speed flight, and windy conditions.You are right Murray, I did say wind instability, what I meant is wind insensitivity.This lower thrust line will aid in these conditions.But if your machine has a very low TL it is constantly trying to nose up, just like a HTL machine tries to nose down.To trim this in a low TL machine you have to set your HS at a negotive AofA, which is the opposite to a HTL machine.The only problem is having a positive pitched HS in a LTL machine, at high speed, a sudden reduction of power might initiate a transient pitch reaction beyond which the pilot skills are adapted or experienced.This is why if your machine has its C of G within +/- 2 inches of its thrust line,the pitching movement is minimal and Birdy wouldnt need to manually componsate for it.Regards Sam

[asra]

[asra]

Birdy.A very interesting thread.One question tho !Why did you decide to change the setup of the Ferrel after all this time ?Was it a safety or handling issue ?Did the bugger try to buck you off or smething ?Tar.Robert DunnMackay. Qld.Growing old is good while it lasts.

[birdy]

You may be right Chuck, afterall, I'm no flight tester or scientist, and it could all be in me head. It dose feel over rotored tho. Time will tell.[ I'v only done bout 3 hours init so far and no mustering.]Robert,I did it coz I had the time and wouldn't need the machine, not coz of a fright or for handeling.It's never scared me.............yet.I only scare meself.[]The reason I never did it before now was coz it was a low priority.IOW, it wasn't dangerously unstable.Also, I'v got a new hart for it in a box with more power, so I thought I mite as well frig around with the COM before I drop it in.That said tho, its obviosly less prone to PPO now.Anyone in the market for a very reliable 80hp 912.............just run in, never coughed,farted or used oil???[]Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.