Fun factor for twin concept

I have been flying all kinds of planes and been kind of figuring slowly out what is the optimum for power loading. It turns out like 9 lbs/hp produces the “fun” experience. That is the “RV-grin” I would say.

So what comes together is:
– Optimum aircraft would consist of 2 x 100 hp engine
– Very low drag fuselage
– Very low drag wings
– High aspect ratio
– High wing loading, 22 lbs/sqft.
– Double slotted flaps
– Power loading 9 lbs/hp
-> mtow 1800 lbs = 818 kg
Empty weight should be under 450 kg to have enough useful load (368 kg, includes fuel).
=> wing area = 81 sqft.

For more general purpose use, it could be written:
– for high performance use, mtow limited to 818 kg.
– for long range use, mtow limited to 950 kg.

This becomes:
– the wing loading limit of 24 lbs/sqft can not be exceeded for the 950 kg because otherwise the stall speed gets too high
=> this becomes:
– 2090 lbs / 24 lbs/sqft
The wing area can be then assumed to be 87 sqft. 7 sqft more than on the case of high performance case.
– Wing loading calculation for the high performance case becomes:
87*22 = 1914 lbs MTOW.
1800/87.0 = 20.6 lbs / sqft

This would cause the airframe to gross weight ratio to be 0.47. This is very low and may not be realistic without special structure. A more realistic figure would be 0.55 ratio. This becomes: 450.0/0.55. Guess what, we get the 818 kg = 1800 lbs gross weight from that. So structurally the 450 kg empty weight and 818 kg gross weight should be feasible. Dynaero MCR-01 is 0.53; 260 kg / 490 kg = 0.53). The LH-Aviation LH10 is 260 kg/500 kg = 0.52. Both of these are carbon fiber structures. With lower cost materials, this may not be even nearly feasible.

If we take a pessimistic value for airframe to gross weight ratio – 0.6 and we have set the gross weight to 830 kg (based on optimizing the power loading), this gives 498 kg empty weight. This should be easily feasible if turbos and pressurization is not taken into account.

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    • dodlithr
    • January 5th, 2009

    Some comments:

    1) If you want to have twin engine, you maybe have to increase the empty weight factor. Remember that all engine parts, supporting structures, fuel lines, propellers, gear boxes are doubled and that will add weight.

    2) The second funny fact is that twin engines have more accidents than single engines. Even if they are better against engine failures.
    (Must be the more complex handling?)

    3) I have chosen a different approach: add parachutes or similar devices and you can live with the single engine concept. The fatalities happen mostly flying high and there you might as well use the parachutes, since there is more time.

    4) Yes, sure carbon fiber adds less weight. But at least I have very little experience with it. It needs vacum bagging and everything.. ?

    5) I agree with the idea to use high lift devices and higher wing loading. After all any plane is primaly made for flying and the take off and landing situations are only transition phases. So the wing should be designed primaly for flying. I would first design the plane for flying and then add the landing gears and high lift devices and what ever additional it needs.

    ***

    I have a similar twin propeller design, but I use there twin propellers with single engine and transmissions.

    Lately I have consentrated on the strucutures mostly (I have made some styrox models of the structures and tested their strength just to be able to understand where the loads will be and how to handle them). I have some nice FEA programs available, which should be able to calculate laminates also, but I need time to learn how to use them to do that.

    Janne.

    • dodlithr
    • January 5th, 2009

    Some comments:

    1) If you want to have twin engine, you maybe have to increase the empty weight factor. Remember that all engine parts, supporting structures, fuel lines, propellers, gear boxes are doubled and that will add weight.

    2) The second funny fact is that twin engines have more accidents than single engines. Even if they are better against engine failures.
    (Must be the more complex handling?)

    3) I have chosen a different approach: add parachutes or similar devices and you can live with the single engine concept. The fatalities happen mostly flying high and there you might as well use the parachutes, since there is more time.

    4) Yes, sure carbon fiber adds less weight. But at least I have very little experience with it. It needs vacum bagging and everything.. ?

    5) I agree with the idea to use high lift devices and higher wing loading. After all any plane is primaly made for flying and the take off and landing situations are only transition phases. So the wing should be designed primaly for flying. I would first design the plane for flying and then add the landing gears and high lift devices and what ever additional it needs.

    ***

    I have a similar twin propeller design, but I use there twin propellers with single engine and transmissions.

    Lately I have consentrated on the strucutures mostly (I have made some styrox models of the structures and tested their strength just to be able to understand where the loads will be and how to handle them). I have some nice FEA programs available, which should be able to calculate laminates also, but I need time to learn how to use them to do that.

    Janne.

    • Karoliina Salminen
    • January 6th, 2009

    1) Yes, that is quite likely. Basically I have calculated to the 450 kg already those additions. The gearbox is included in the Rotax and don’t add any extra weight. According to statistics, 0.55 weight ratio is quite common figure for twin engine aircraft [Raymer]. Using more pessimistic 0.6 and 500 kg empty weight gives: 500/0.6 = 833 kg MTOW. For power loading that means 9.16 lbs/hp which is still very good figure. The plane with wing loading 22 lbs/sqft can still take of succesfully with single Rotax with 12 lbs/hp power loading per two engines and 24 lbs/hp power loading for single engine situation.

    Reference for low weight single engine is Dynaero MCR-01ULC which weights around 260 kg empty. The higher aspect ratio will add weight, one more Rotax will add 75 kg more weight plus additional duplicate equipment some more. Fuselage is heavier because pressurization was planned, that can’t be escaped. Two turbos, one turbo compounding, 2 electric motors, their piping, fuel injection, intercooler, aftercooler per engine etc. will add quite a bit of weight. It is not very easy to estimate because there are no such aircraft out there. For basic Rotax 912ULS and Woodcomp SR3000 the 450 kg empty weight might be realistic with no pressurization. With the turbos added, MT propeller instead of the Woodcomp etc. it will be more most likely unfortunately.

    2) That is true, somehow people manage to do more failures with twins, but it doesn’t change the concept which needs redundancy. The handling is not very difficult on a modern twin, I have flown Diamond DA42 with one engine out and the rudder trim makes the handling to feel almost normal in single engine operation.

    3) Parachute is going to be there. However, parachute does not save lives when there is 1000 kilometers of ocean to nearest land or when the engine fails on takeoff. The engine fails quite often on takeoff and with two engines the impossible turn back to runway becomes a possible turn. E.g. takeoff from Malmi airport to about any direction, and you are potentially dead if only the engine quits. Some have succeeded to land to road etc., but there is very high likelihood to hit rocks, cars or buildings nowadays and no safety cockpit is safe enough to withstand that kind of crash. Ballistic parachute is not usable at low altitude.

    4) Yes, it needs vacuum bagging or RTM infusion. I recently learned how to do that and I plan to use what I learned. Hand lamination without any kind of vacuum is very outdated technique.

    5) That is the way it is designed, but the high lift devices are required considerations from the very beginning because it affects the needed wing size radically (through wing loading). Stall speed shall not exceed 55 kts and high lift devices can not be bolted on but are part of the consideration with optimization of wing profile. At no circumstances stall speed can exceed 61 kts, if that happens, it requires complete redesign for the wings and its high lift devices.

    6) I don’t see the point of using two propellers for single engine. I would see the point of using two engines and one propeller. Two propellers for single engine don’t add redundancy, they add complexity and more parts which can fail and makes the reliability worse than with single propeller. The reduncancy of twin engine is achieved when the two engines have completely individual systems and one failing does not affect the other engine in any way.

    7) Cool if you have FEA programs. They are very useful in designing structures. The use of NISA FEA program is described in Martin Hollman’s books, but it is quite apparent that these programs are very complicated and hard to use and they require long learning curve.

    • Karoliina Salminen
    • January 6th, 2009

    1) Yes, that is quite likely. Basically I have calculated to the 450 kg already those additions. The gearbox is included in the Rotax and don’t add any extra weight. According to statistics, 0.55 weight ratio is quite common figure for twin engine aircraft [Raymer]. Using more pessimistic 0.6 and 500 kg empty weight gives: 500/0.6 = 833 kg MTOW. For power loading that means 9.16 lbs/hp which is still very good figure. The plane with wing loading 22 lbs/sqft can still take of succesfully with single Rotax with 12 lbs/hp power loading per two engines and 24 lbs/hp power loading for single engine situation.

    Reference for low weight single engine is Dynaero MCR-01ULC which weights around 260 kg empty. The higher aspect ratio will add weight, one more Rotax will add 75 kg more weight plus additional duplicate equipment some more. Fuselage is heavier because pressurization was planned, that can’t be escaped. Two turbos, one turbo compounding, 2 electric motors, their piping, fuel injection, intercooler, aftercooler per engine etc. will add quite a bit of weight. It is not very easy to estimate because there are no such aircraft out there. For basic Rotax 912ULS and Woodcomp SR3000 the 450 kg empty weight might be realistic with no pressurization. With the turbos added, MT propeller instead of the Woodcomp etc. it will be more most likely unfortunately.

    2) That is true, somehow people manage to do more failures with twins, but it doesn’t change the concept which needs redundancy. The handling is not very difficult on a modern twin, I have flown Diamond DA42 with one engine out and the rudder trim makes the handling to feel almost normal in single engine operation.

    3) Parachute is going to be there. However, parachute does not save lives when there is 1000 kilometers of ocean to nearest land or when the engine fails on takeoff. The engine fails quite often on takeoff and with two engines the impossible turn back to runway becomes a possible turn. E.g. takeoff from Malmi airport to about any direction, and you are potentially dead if only the engine quits. Some have succeeded to land to road etc., but there is very high likelihood to hit rocks, cars or buildings nowadays and no safety cockpit is safe enough to withstand that kind of crash. Ballistic parachute is not usable at low altitude.

    4) Yes, it needs vacuum bagging or RTM infusion. I recently learned how to do that and I plan to use what I learned. Hand lamination without any kind of vacuum is very outdated technique.

    5) That is the way it is designed, but the high lift devices are required considerations from the very beginning because it affects the needed wing size radically (through wing loading). Stall speed shall not exceed 55 kts and high lift devices can not be bolted on but are part of the consideration with optimization of wing profile. At no circumstances stall speed can exceed 61 kts, if that happens, it requires complete redesign for the wings and its high lift devices.

    6) I don’t see the point of using two propellers for single engine. I would see the point of using two engines and one propeller. Two propellers for single engine don’t add redundancy, they add complexity and more parts which can fail and makes the reliability worse than with single propeller. The reduncancy of twin engine is achieved when the two engines have completely individual systems and one failing does not affect the other engine in any way.

    7) Cool if you have FEA programs. They are very useful in designing structures. The use of NISA FEA program is described in Martin Hollman’s books, but it is quite apparent that these programs are very complicated and hard to use and they require long learning curve.

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