iwfudia Posted January 19, 2014 Posted January 19, 2014 Hi everyone, Just wondering if anyone could educate me as to the purpose of having 2 props/ engine on the lazair ultralights? I could understand if they were 90o offset but they are one behind the other.
dazza 38 Posted January 19, 2014 Posted January 19, 2014 Ozzie on this site owns one . He will fill you in when he sees your thread
biggles Posted January 19, 2014 Posted January 19, 2014 Hi everyone,Just wondering if anyone could educate me as to the purpose of having 2 props/ engine on the lazair ultralights? I could understand if they were 90o offset but they are one behind the other. Are they really ?
dazza 38 Posted January 19, 2014 Posted January 19, 2014 Hi everyone,Just wondering if anyone could educate me as to the purpose of having 2 props/ engine on the lazair ultralights? I could understand if they were 90o offset but they are one behind the other. The engines are mounted on the leading edge of each wing like a normal twin set up.
Guernsey Posted January 19, 2014 Posted January 19, 2014 Hi everyone,Just wondering if anyone could educate me as to the purpose of having 2 props/ engine on the lazair ultralights? I could understand if they were 90o offset but they are one behind the other. Only when they are sideslipping. Alan.
SDQDI Posted January 19, 2014 Posted January 19, 2014 What the original poster was referring to was a picture of a lazair that looked to have twin props on each engine but they were mounted one right behind the other with no offset he put a pic in another thread. He wasn't referring to the engines but to the fact that it looked like four propellers and two engines Edit looks like he beat me too it
iwfudia Posted January 19, 2014 Author Posted January 19, 2014 thought i better start my own thread so as not to hijack the other one
Guernsey Posted January 19, 2014 Posted January 19, 2014 2 props on each engine..... Must be a spare prop or a large washer. Alan.
Guest Nobody Posted January 19, 2014 Posted January 19, 2014 Details here: http://lazairinfo.com/additional_lazair_info/rotax-185/
Guest Maj Millard Posted January 19, 2014 Posted January 19, 2014 I was told a different story, and I was not aware of the woodruff key problem even though I owned and flew a Lazair for about a year. The story I heard was that as the Lazair really is a powered glider, everything is set up so that you can shut the engines down and set them up for a restart , and it's all covered in the pilots operating handbook that I still have. With the biplane props stopped there is only half the drag, than if the prop was twice as long, whilst you soar away. The props by the way are very efficient in the biplane configuration. When sitting in the aircraft on the ground, if you bring one side up to full Rpms that wing will lift up about one foot, as the wing chord behind the prop has to be around five foot wide.........Maj....
cscotthendry Posted January 19, 2014 Posted January 19, 2014 I've seen many racing sailboats with two gibs. Apparently, the outside gib accelerates or compresses the airflow over thw lifting surface of the inner gib generating more thrust. The closely spaced biplane props may be doing something similar.
Guest Maj Millard Posted January 20, 2014 Posted January 20, 2014 I've seen many racing sailboats with two gibs. Apparently, the outside gib accelerates or compresses the airflow over thw lifting surface of the inner gib generating more thrust. The closely spaced biplane props may be doing something similar. The little carbon props on the Rotax 185s are actually 'staggered' like a biplanes wings, and also tapered in planform. It must work because they do put out an amazing amount of thrust for the horsepower. (Around 18 HP total both engines) The nicest set of biplane wings I ever saw were on a Waco taper wing at Reno one year. Flew superbly also, the bipe props are are a bit like those wings but longer relatively...........................Maj.....
Bob Llewellyn Posted January 22, 2014 Posted January 22, 2014 Hi everyone,Just wondering if anyone could educate me as to the purpose of having 2 props/ engine on the lazair ultralights? I could understand if they were 90o offset but they are one behind the other. My 2 cents - when matching a propellor to an aeroplane, the diameter has to be limited so that the tip speed stays below the speed of sound (at the rpm for max engine power); and there has to be enough blade area ("solidity") to stop the engine exceeding its redline most anywhere in the flight envelope. Now, the only magic about a biplane wing cellule is that it allows an extremely light structure, thanks to bracing wires - see DH 84 Dragon. The Lazair should gain a little prop efficiency by putting the blades at right angles, but this would only be measurable as a few more revs. As someone said, the biplane configuration would give much less drag when shut down. The hidden benefit the Lazair gains by using small diameter props, is that the slipstreams only affect a small area of wing, so in the single engine case the adverse roll is small. Who will be the first to put 6 engines on the wing of a Jab 160? :o) -Bob.
Guest Maj Millard Posted February 12, 2014 Posted February 12, 2014 Bob, I have flown the Lazair on one engine, and I can tell you that there is minimal to no 'adverse roll' encountered. Stick about 1" toward the good engine takes care of it fine. And you could fly along all day like that, except you won't be going up much on the remaining 9 HP. The operators manual states " If one engine is lost just consider you have lost one half of total thrust, it is safer and easier, to shut the other off and land dead stick". Or words to that effect. I have also done many dead stick power off landings, and they are the best way to land the Lazair, as they are just a powered glider after all. The little engines are so close into the root on the 36 foot span wing that there is no adverse anything really......Maj.....
Bob Llewellyn Posted February 12, 2014 Posted February 12, 2014 Bob, I have flown the Lazair on one engine, and I can tell you that there is minimal to no 'adverse roll' encountered. Stick about 1" toward the good engine takes care of it fine. And you could fly along all day like that, except you won't be going up much on the remaining 9 HP.The operators manual states " If one engine is lost just consider you have lost one half of total thrust, it is safer and easier, to shut the other off and land dead stick". Or words to that effect. I have also done many dead stick power off landings, and they are the best way to land the Lazair, as they are just a powered glider after all. The little engines are so close into the root on the 36 foot span wing that there is no adverse anything really......Maj..... Ok, ta...so it's like a Cessna T-37(?) Dragonfly, except slightly less ROC :o)... That puts me to mind of a story told me by a GA CFI, about a bloke doing a twin endorsement in GA after about 800 hours un-endorsed twinning (naughty...); when the instructor reached down and turned the fuel off to one engine, the pilot reached down and turned it back on... the instructor spoke to him, and turned in back off. When the donk quit, the pilot shut down the other donk and dead-sticked it... he later asked the instructor to show him the reg that said he shouldn't have done that...
facthunter Posted February 13, 2014 Posted February 13, 2014 It's not the optimum course of action when the plane can fly on the remaining engine, but people have died by losing control by not reducing the power on the remaining engine, when they are too slow and airborne, to have enough rudder effect to maintain control.. The Airline TAA lost a Viscount at Mangalore (Vic) by being below VMC (a) and an Ansett F-27. did not make the runway at Launceston landed short (loss of control) for the same reason. It's often called Blue Line speed and is well above stall speed on just about all aircraft. Most aircraft have excess power over min required except at absolute ceiling for instance. If the aircraft normally has two engines running and loses one it may have virtually no surplus power so has no ability to climb even when it is at the optimum speed and flown correctly, which is slight wing down on the good engine. That might raise some comment because it is not generally put out there. Nev
Bob Llewellyn Posted February 13, 2014 Posted February 13, 2014 It's not the optimum course of action when the plane can fly on the remaining engine, but people have died by losing control by not reducing the power on the remaining engine, when they are too slow and airborne, to have enough rudder effect to maintain control..The Airline TAA lost a Viscount at Mangalore (Vic) by being below VMC (a) and an Ansett F-27. did not make the runway at Launceston landed short (loss of control) for the same reason. It's often called Blue Line speed and is well above stall speed on just about all aircraft. Most aircraft have excess power over min required except at absolute ceiling for instance. If the aircraft normally has two engines running and loses one it may have virtually no surplus power so has no ability to climb even when it is at the optimum speed and flown correctly, which is slight wing down on the good engine. That might raise some comment because it is not generally put out there. Nev FAR 23.67(b): "The steady gradient rate of climb at an altitude of 400ft above the takeoff must be measurably positive... TO power; must be not less than 0.75% at an altitude of 1,500 feet above takeoff... max continuous" these are the engine-out requirements for piston multis above 6,000lb TOW, and all Turbine multis, that fit FAR 23 (i.e. less than 12,500lb / 5,700kg MTOW). The Dornier -27, -28 twins blue line below stall...
facthunter Posted February 13, 2014 Posted February 13, 2014 Earlier twins Miles Gemini DH Dragon etc did not meet these requirements. as for one thing you could not feather the dead engine. the Avro Anson too had fixed pitch props.. For recreational aircraft I can't see a problem with not being able to maintain height on one engine. You at least extend the glide but you have two times the chance of having an engine fail, and unless the engines are centreline thrust or compensated by skewing the thrustline and/or running the engines in opposite directions, there has to be some residual asymmetry associated with eng. fail. The usual method of compensating is by rudder as less performance is lost that way. Nev
Bob Llewellyn Posted February 13, 2014 Posted February 13, 2014 Earlier twins Miles Gemini DH Dragon etc did not meet these requirements. as for one thing you could not feather the dead engine. the Avro Anson too had fixed pitch props.. For recreational aircraft I can't see a problem with not being able to maintain height on one engine. You at least extend the glide but you have two times the chance of having an engine fail, and unless the engines are centreline thrust or compensated by skewing the thrustline and/or running the engines in opposite directions, there has to be some residual asymmetry associated with eng. fail. The usual method of compensating is by rudder as less performance is lost that way. Nev Erm, the development of twin-engined design standards occurred in the light of pre-existing big Bellancas, Lockheed Sirius/Vega/etc; as the twins carried more metal, they had smaller payloads, so the design standards had to impose the minimum requirements - remember, the FAA must foster aviation... such things as flying with so much yaw and so much roll on one engine, tongue against the molars and hat off come to mind... There is no reason that a non-commercial twin shouldn't have a better power-to-weight at MTOW, so giving the extra power required to overcome the corrective drag in the engine-out situation. The Anson was a mongrel for getting separation off the back of the nacelles, esp after the fan stopped on that side (and the 336/337 is a mongrel for separation off the underside of the rear fuse if the rear donk stops, too). The Speed Twin is an interesting study for life without feathering. The main problem I see, is that a mutli engine rating is the holy grail for commercial pilots, so the ones in CASA aren't going to let any recreational pilots share their grandeur, no matter what... The Cri Cri seems to work...
facthunter Posted February 13, 2014 Posted February 13, 2014 It's VERY small (Cri-Cri) and the thrust lines aren't far apart. I think you would still have to treat it with respect and put in the effort. Bob, some of the small twins require to be flown very precisely. While they are quite lovely gadgets. they bite and kill people who don't fly regularly and practice assy fairly often. While everything is going fine they are just another aeroplane. Single Pilot IFR in a Chieftain, you probably earn your money. Nev 1
Bob Llewellyn Posted February 13, 2014 Posted February 13, 2014 It's VERY small (Cri-Cri) and the thrust lines aren't far apart. I think you would still have to treat it with respect and put in the effort.Bob, some of the small twins require to be flown very precisely. While they are quite lovely gadgets. they bite and kill people who don't fly regularly and practice assy fairly often. While everything is going fine they are just another aeroplane. Single Pilot IFR in a Chieftain, you probably earn your money. Nev As you may have gathered, I've been considering design issues for recreational twins, on and off, for a while - but reading the POH etc is not a substitute for flying them! I do feel that there is considerable potential to make smaller twins rather less touchy than they are - from a design aspect, certification in a commercial role has limited the design options chosen. The improvement in payload given a 402 via a VG kit is an example, to my mind, of the prosaic design choices made. I read from your comments, that quite a considerable improvement in the engine-out handling would need to be made - is this correct? Perhaps a tandem 336 layout would be the preferred option...
facthunter Posted February 13, 2014 Posted February 13, 2014 You could use the propwash directed onto the fin by the two methods I mentioned earlier. If the aircraft is not to close coupled low powered engines shouldn't be much of a problem. I'm not much of a push-pull fan myself. If the engines are well forward they get the thrustlines much closer. My area of interest is a pressurised plenum and blown wing and pressurised air propulsion. Keeps all the mass near the centre no prop to kill dogs and people and little stress from the drive on the engine which could be a high revving motorcycle engine or such. All the cooling and exhaust heat could be utilised. Not sure about the efficiency but it has control possibilities and would suit a glider. too. Nev 1
pmccarthy Posted February 13, 2014 Posted February 13, 2014 The Piper Apache had a rate of descent on one engine.
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now