Why are twin engine a/c poor safety...

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Avion, Jun 11, 2004.

Help Support HomeBuiltAirplanes Forum by donating:

  1. Jun 11, 2004 #1

    Avion

    Avion

    Avion

    Active Member

    Joined:
    Feb 9, 2004
    Messages:
    27
    Likes Received:
    0
    Location:
    Brisbane, Australia
    I've read somewhere that the safety history of twin engined a/c is on par with the single engine a/c. Why is it so? Has this something to do with the fact that if one engine is out it is impossible to control the a/c? And why are the engines spaced so far out and the nose is protruding between the props so much. Would not be better to cut the nose and bring the engines/props closer together and reduce the effect of yaw with one engine out?

    Also what is the rotation of props/engine. Are they counter rotating as the should be?

    Twin Mustang had counter rotating props...

    Regards
    Avion
     
  2. Jun 11, 2004 #2

    orion

    orion

    orion

    Well-Known Member

    Joined:
    Mar 3, 2003
    Messages:
    5,800
    Likes Received:
    135
    Location:
    Western Washington
    Considering the millions of hours flown every year by the general aviation crowd, I'd say the overall safety record is an enviable one - in other words, you seem to indicate that the industry has a poor safety record when in fact it is actually one of the best when compared to just about any other endeavor.

    Regarding twins and singles, I think what the data shows is not so much the lack of safety in a twin as opposed to a single engine airplane, but that the accidents that do happen are most likely a function of the pilot and his decisions rather than that of the airframe itself. As a result, the number of engines is not a significant factor.

    Airframe failures are extremely rare nowadays. This is true for certified airplanes, as well as those built by the "Experimental" crowd. If there is a structural or engine failure of some sort, it seems to be usually a factor or result of some decision made by the pilot, and so again, it is not a function of how many engines the airplane has or how it was designed or built, but more so of who sits behind the stick.

    There are two attitudes to single and multi-engined airplanes. The first (tongue-in-cheek) says that if you have two engines then there are more systems and just plainly more things that can go wrong. They rarely do though - I think this is just a function of the owners' not wishing to incur the added expense of the second engine.

    The second attitude is as was expressed by the owner of British Airways about fifty years ago (I don't recall his name) - when asked why he only flies four engined airplanes he simply replied "Because there are no five engine airplanes."

    Regarding control, yes, there have been a few airplanes built decades ago where the single engine performance was poor, as were the airplane's control characteristics if one went out. On more modern airplanes though this is not so much the case as all are required now to be fully controllable on a single engine. Yes, you do have issues of control forces and the pilot does have to watch the speeds and trim, but overall, all multi-engine aircraft in order to be certified and/or meet today's customer requirements must have full contol authority if one engine goes out. The only issue then again boils down to pilot training and his subsequent decisions.

    As far as the positioning of engines is concerned, all aspects of airplane design are a compromise. The designer has to balance issues of control and safety with component position, useful load envelopes and of course, aesthetics. The engines are the single heaviest component of an airplane so their position drives many of the airplane's design variables. You position them somewhere else and now you have to change the interior loading requirements, you may have to change the position of the wing, you may have to limit how much load you can take or where it is placed, etc.

    Cut the nose off? Yuk!!!

    As far as the prop spin is concerned, no they do not have to be counter-rotating. Yes, it does help and can be beneficial but in general this is not necessary except in the cases where the egines develop a substantial amount of torque in relation to the size of the airframe. Actually the example you list is a good one in that fighters, due to their extremely large engines and props (huge torque loads), do have to have the props couter-rotating in order to aid the controllability of the airframe on the ground and during high rate power changes.
     
  3. Jun 11, 2004 #3

    Midniteoyl

    Midniteoyl

    Midniteoyl

    Well-Known Member

    Joined:
    Sep 4, 2003
    Messages:
    2,406
    Likes Received:
    500
    Location:
    Indiana
    Some are counter-rotating, others not.

    I think the main reason twins arent 'safer' than they should be is due to lack of pilot training and lack of control during an engine outage. For some reason, some pilots forget every single thing they learned about controlling an aircraft with only one engine and make stupid mistakes.

    The other is seems to be that pilots like to 'overload' the plane - thinking it can carry more than gross, just cuase it has two engines.

    Quick trip through the NTSB reports show these two factors more than most.
     
  4. Jun 12, 2004 #4

    HeliDev

    HeliDev

    HeliDev

    Well-Known Member

    Joined:
    Jul 7, 2003
    Messages:
    108
    Likes Received:
    0
    And some times your just plain outa luck.
    We had a flight test officer who was one of the most experienced pilots you were likely to meet. Top guy, Veitnam Vet, over 30,000hrs in everything, from helos to big jets.
    While doing a check ride at night with a CPL practicing engine out on take off, the other donkey quit, and they crashed into a nearby feild. Saddly both pilots were killed.
    The final report on the accident hasnt comeout yet.

    The interesting thing about helos is that ask just about any helo pilot how many engines he would like and the answer will always be similar to that of the British airways guy, howmany can I have, though this probably comes down to the fact that helos perfom the same on one or two engines, the only difference will be the power available.
     
  5. Jun 12, 2004 #5

    wsimpso1

    wsimpso1

    wsimpso1

    Super Moderator Staff Member

    Joined:
    Oct 19, 2003
    Messages:
    6,011
    Likes Received:
    3,266
    Location:
    Saline Michigan
    A well trained and metally prepared pilot of a twin will do just fine when one quits, even if it happens on takeoff. The biggest problem with conventional twin engined airplanes is that they go from easy to fly to being a test of mental preparedness and then physical endurance. With one engine dead and the other making max power (assume it died shortly after takeoff), you have to carry aileron and rudder forces to keep it coordinated, and you must stay above blue line speed to prevent it from departing controlled flight. Fly below blue line speed and you have to reduce power on the good engine and drop the nose to regain blue line speed, then push the power back up again... And this is once you have transisitoned to single engine flight.

    The transistion to single engine flight looks like this - The pilot must recognize that one engine has gone soft (which can take a little time to get through both "this is not normal" and then through the disbelief), push all of the power controls forward, keep the airplane above blue line speed and in coordinated flight, identify which one is dying, keep control of the airplane, feather the dead engine, keep control of the airplane, retract the gear and flaps, keep control of the airplane, close the cowl flaps on the dead engine, keep control of the airplane, open the cowl flaps on the full power engine, keep control of the airplane, etc.

    Which is why Burt Rutan, the owner of a Beachcraft Duchess, designed and built the Boomerang. Nothing is symetrical at all about the ship, except the way in which it flies, regardless of the number of engines that are running. Look it up in Sport Aviation - there was quite an article on it a few years back. Two fuselages one big for five people, one small for baggage, engines on the front of the fuselages and close to each other, one prop forward of the other, forward swept wing with dihedral originating in one fuselage and sweep in the other, one main gear forward of the other, one swings forward, one back, nose gear in only the big fuselage, wing lengths different on each side, same with the horizontal tail... Quite the ship, but it flies on one about the same as on both except for climb rate.

    Burt flies it to OshKosh non-stop from Mojave, you should check it out.
     
  6. Jun 13, 2004 #6

    Avion

    Avion

    Avion

    Active Member

    Joined:
    Feb 9, 2004
    Messages:
    27
    Likes Received:
    0
    Location:
    Brisbane, Australia
    Thank you guys for all the info.

    Sorry Orion, i never meant to say the industry is not the safe one, i just meant exclusively in regards to the configuration of twin engine aircraft. My understanding after reading a number of the books about the design is that due to the P-effect, torque of the engine, propeller slipstream and a few other things that make the symetric aircraft fly not so symetric why not use two engine configuration where all of those things cancel each other. That means counter-rotating engines/props. I am well aware of Rutans Boomerang and how assymetric it is but even before Rutan there was a Blohm & Voss BV141 which was quite assymetric.

    Really, i am only exploring how feasible would be to build a scaled down replica of Twin Mustang. As small as possible. With two counter-rotating engines and close props i am hoping to increase the safety of twin engine configuration. I know that there is penalty to pay for extra drag due to the wetted surface area and a few other things but the price to pay for a bit lesser performance would be small in exchange for added safety.

    Anyway any more thoughts would be greatly appreciated.

    Regards
    Avion
     
  7. Sep 2, 2004 #7

    nickofhf

    nickofhf

    nickofhf

    Member

    Joined:
    Sep 2, 2004
    Messages:
    6
    Likes Received:
    0
    Location:
    Harpers Ferry WV
    Twin

    Avion,
    I am going to build a 4 place twin engine all metal plane in about 3 or 4 years from now and am looking to modify an existing design or if I have to I will design my own.I am no engineer but I know one who might help me.Please let me know about any progress you make on yours.
     
  8. Sep 3, 2004 #8

    Avion

    Avion

    Avion

    Active Member

    Joined:
    Feb 9, 2004
    Messages:
    27
    Likes Received:
    0
    Location:
    Brisbane, Australia
    Hi Nick

    Good to hear from you and good luck with your project. I am just an amateur like yourself and after countless number of aviation books, i am sold on the idea of twin fuselage a/c ala twin mustang. Rutan has taken the concept further and i love his plane. But on anything with more than one seat i strongly recommend to you to speak to a pro.

    Currently i am looking into rotary engines, like Mazda conversions. I wonder how hard would it be to make it run the other way. Maybe if you flip some housings...

    Also here is an aircraft trivia for you: look into Jean de la Farge modification of the Flying Flea... outstanding aircraft, super STOL, parachutal descent (meaning safe!) and many other advantages that conventional aviation does not even know about!

    Regards
    Avion
     

Share This Page



arrow_white