Minimal twin

In the mean time, on the back of my head, I have also been thinking the twin concept. What is the minimum power feasible for the twin for being safe in single engine situation, and what can be the maximum weight and maximum wing loading of a plane which is equipped with two HKS700E engines (only 60 hp each).

Known thing is that Diamond DA42 climbs still at 22 lbs/sqft wing loading and 24 lbs/hp power loading on single engine. However, there is quite a bit more excess power on 135 hp Thielert than on a 60 hp engine. I am feeling that I am getting too optimistic results from the sizing equations with either Raymer or Anderson method.

I have estimated that the plane should not weight more than 700 kg (according to the equations) to still be able to take off and climb with single engine. This may be too optimistic figure, I have been thinking that the limit might be rather near 650 kg or maybe even a bit less.

Thinking pessimistic: the plane can have positive climb rate with 60 hp single engine mode if the gross weight is 600 kg. That gives:

600 – 55 kg – 10 kg – 55 kg – 10 kg = 470 kg for the airframe + useful load excluding engines.

For useful load, minimally needed is:
– Two big adults, 95 kg including heavy clothes per each
– 5 kg baggage per each
– 120 liters of gasoline = 85 kg

This becomes:
95 kg * 2 + 10 kg + 85 kg = 285 kg.

For the plane to be minimally useful, it must be able to carry 285 kg in addition to its own weight. There are two engines and to have useful endurance the amount of fuel has to be double the size of a single engine plane.

The airframe + systems maximum weight excluding engines then becomes:

470 kg -285 kg = 185 kg

This means that the airframe + systems excluding engine can only weight 185 kg. This is a very hard goal to achieve.

The aircraft empty weight then becomes:

185 kg + 65 kg + 65 kg = 315 kg

The empty weight to gross weight ratio becomes:
315 kg / 600 kg = 0.52

This ratio is very challenging to achieve for a twin where the airframe must be carrying in addition to the occupants instead of one engine, two engines, and their fuel.

If we could still take off at 650 kg, then this becomes:
Airframe weight can be increased with 50 kg: 185 kg + 50 kg = 235 kg

235 kg + 65 kg + 65 kg = 365 kg

Looks like now we are talkin. This looks like a figure which might be theoretically possible, even though this is still very hard goal. As seen on ultralight planes, achieving empty weight under 300 kg is very hard. Adding extra engine on top that requires aircraft that is as lightweight than best ultralights equipped, plus can still take the additional engine.

But this is just theoretical thinking and whether or not it may be feasible, the discussion can continue:

The empty weight to gross weight ratio then becomes:

365 kg / 650 kg = 0.56

Historical data shows that at least on a bit larger aircraft, the 0.56 value is pretty well achievable.

Lets consider now the performance for the 650 kg case:

Single engine produces only 60 hp power. Only the excess power can be used for climb. This means that in a side slip of asymmetric thrust and climb angle of attack, the total drag (drag due to lift + fuselage drag) must be less than the thrust of 60 hp at best climb speed with a propeller that has efficiency of 0.7 (for pessimistic evaluation, I prefer to not use 0.85) by a large margin, and then the climb rate pretty much becomes from the weight to be lifted and how much excess power is still left.

The power loading for single case would be: 23.8 lbs/hp. This would be about the same as Diamond DA42. The drag must be low in order to ensure that the power needed for level flight is small, and there is excess power for climb, even with very low power.
Then comes the disaster of increasing wing area, this increases drag, but on the other hand, increases also lift. However, to get good cruise performance on the low power, wing size should be as small as possible. So some compromise is needed here. Increase in wing loading has to be accounted with increase in aspect ratio to keep the induced drag the same. Increase of aspect ratio may increase weight, but does not necessarily always do so. For example the earlier mentioned LH10 has very light wings, despite of aspect ratio of 14. So it worths researching on this area. A good design is a synergetic design which combines couple of good things into one good compromise.

I maybe need to redo the calculation yet another time again.

Why I am thinking this?
– For a plane that I would design for myself, I could choose Rotax 912ULS, and get two used engines with about half the price of a new Rotax 912ULS. This would be roughly the cost of a pair of new HKS700E.
– However, if we think a kit-builder who wants to have a twin with shoestring budget. Many aviators are limited with budget (aviators are always rich simply does not seem to be true, and if they originally were, they no longer are after starting spending to flying). So we have been thinking of a concept of a light plane with two engines with good performance. Any twin out there, even used ones, cost many many times more than it would cost to build a plastic one with two little HKS700E engines.
– I think that twin engine aircraft are not so popular, not because they require the additional license, but because people do not opt for the additional license, because the cost of the twin is prohibitive. There is absolutely no twin out there where one could log twin engine time and which would not cost a fortune of a millionaire to own or cost a fortune of of a normal people to maintain and operate.
– It is often explained that twins are more dangerous than singles. However, the context seems to be forgotten. Single engine limits the use of the plane and with two engines, people may often go to more dangerous situations.
– And it is not only a bad thing, consider this: You live in Finland and want to visit for example Greenland. What do you do if you want to fly there by yourself and not to sit as a passenger on an Airbus? You go and start your C172 and head towards Greenland. If the one old-fashioned engine that is almost approaching car engines in reliability, that is there, quits, then you are in biiig trouble. Wouldn’t it be great if there was a second engine and you could still fly even if the one failed. Even if the climb rate with single engine is poor, you could still maybe get out of there alive. Your speed would get slow, but also your fuel consumption becomes half because only one engine is drinking the fuel. You actually might make it and your relatives don’t need to arrange funerals.

Any comments on this?

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    • Matthew
    • January 22nd, 2009

    This is an interesting puzzle, that you have a twin engine plane which in theory could fly with on 1 engine of 60hp, but trimming forces might dangerously hamper its performance.

    maybe the best solutions might be a twin boom pusher/puller aircraft.

    Should one engine fail, there should be no trimming forces needed and therefore no extra drag.

    Also, as you mention about the cost of engines, this layout means that they engines do not have to be identical. This might make it easier to source cheaper engines.

    • Matthew
    • January 22nd, 2009

    This is an interesting puzzle, that you have a twin engine plane which in theory could fly with on 1 engine of 60hp, but trimming forces might dangerously hamper its performance.

    maybe the best solutions might be a twin boom pusher/puller aircraft.

    Should one engine fail, there should be no trimming forces needed and therefore no extra drag.

    Also, as you mention about the cost of engines, this layout means that they engines do not have to be identical. This might make it easier to source cheaper engines.

    • Matthew
    • January 22nd, 2009

    Another thing I forgot to mention…

    The airflow around the fusilage of most aircraft is disturbed by prop wash, cowl vents, cockpit protrutions, and this is difficult to solve. Having a pusher/puller twin (such as the adams A500 or cessna 336) alows the wings to be relitivly clean compared to a conventional twin (no engine pods, etc)

    I really dont know much about aerodynamics, but maybe you can consider these options and offer an opinion?

    • Matthew
    • January 22nd, 2009

    Another thing I forgot to mention…

    The airflow around the fusilage of most aircraft is disturbed by prop wash, cowl vents, cockpit protrutions, and this is difficult to solve. Having a pusher/puller twin (such as the adams A500 or cessna 336) alows the wings to be relitivly clean compared to a conventional twin (no engine pods, etc)

    I really dont know much about aerodynamics, but maybe you can consider these options and offer an opinion?

    • Karoliina Salminen
    • January 22nd, 2009

    I have thought push-pull also.

    The problem is that the rear prop operates in the prop wash of the front prop.

    In my idea, the engines mounted on the wing are in pusher configuration and instead of disturbing the air over the wing, they actually suck the boundary layer for some extent which could be beneficial near the trailing edge where the boundary layer is turbulent for sure.

    The front fuselage cleanliness has challenges and there are some compromises to be made in a small aircraft. In a larger aircraft it would be possible to locate a door behind the laminar turbulent boundary layer transition and not put any protrusions or discontinuity to the front. However, a small plane may require a canopy, and it can be as well sealed as on a sailplane in the best case but not much better than that, and that is not perfect.

    One easy way to not do things wrong is that the nose is clean from pitot tubes etc. One can place the pitot tube e.g. under a wing. Why not spoil the laminar boundary layer by putting it to the nose like on a jet fighter, it can not do much good there if laminar flow is the goal. If turbulent flow is assumed, then of course the nose can be used for pitot tube and vents. Cabin air vents have no requirement to be front of the cockpit. The cooling air can be ducted behind the cockpit to the cockpit quite easily. In fact, in our previous plane, the TL-96 Star, the rear fuselage was leaking air into the cockpit and there was blow from rear towards front to even uncomfortable extent.

    • Karoliina Salminen
    • January 22nd, 2009

    I have thought push-pull also.

    The problem is that the rear prop operates in the prop wash of the front prop.

    In my idea, the engines mounted on the wing are in pusher configuration and instead of disturbing the air over the wing, they actually suck the boundary layer for some extent which could be beneficial near the trailing edge where the boundary layer is turbulent for sure.

    The front fuselage cleanliness has challenges and there are some compromises to be made in a small aircraft. In a larger aircraft it would be possible to locate a door behind the laminar turbulent boundary layer transition and not put any protrusions or discontinuity to the front. However, a small plane may require a canopy, and it can be as well sealed as on a sailplane in the best case but not much better than that, and that is not perfect.

    One easy way to not do things wrong is that the nose is clean from pitot tubes etc. One can place the pitot tube e.g. under a wing. Why not spoil the laminar boundary layer by putting it to the nose like on a jet fighter, it can not do much good there if laminar flow is the goal. If turbulent flow is assumed, then of course the nose can be used for pitot tube and vents. Cabin air vents have no requirement to be front of the cockpit. The cooling air can be ducted behind the cockpit to the cockpit quite easily. In fact, in our previous plane, the TL-96 Star, the rear fuselage was leaking air into the cockpit and there was blow from rear towards front to even uncomfortable extent.

    • dodlithr
    • January 22nd, 2009

    Good thinking. But let’s add some constructive critisism here.

    Why do you have to be all the time pushing the limits? Why not relax and make a decent design?

    The twin engine concept will not save you in all situations. There are lot of cases where the fatal accident happened regardless of the number of engines: fuel starvation, mid-air collision, bad weather, etc. etc.

    If you want to fly to Greenland and only maximize your safty, then it is best to take the Airbus. You can never beat airliners in safty.

    Maybe spend some time reading typical multiengine fatal accidents and you notice that twin enigine is not any “Holy Grail”. More often it might be to take enough fuel before take off or really just fly VFR for example ūüôā (or some similar very sad human factor).

    • dodlithr
    • January 22nd, 2009

    Good thinking. But let’s add some constructive critisism here.

    Why do you have to be all the time pushing the limits? Why not relax and make a decent design?

    The twin engine concept will not save you in all situations. There are lot of cases where the fatal accident happened regardless of the number of engines: fuel starvation, mid-air collision, bad weather, etc. etc.

    If you want to fly to Greenland and only maximize your safty, then it is best to take the Airbus. You can never beat airliners in safty.

    Maybe spend some time reading typical multiengine fatal accidents and you notice that twin enigine is not any “Holy Grail”. More often it might be to take enough fuel before take off or really just fly VFR for example ūüôā (or some similar very sad human factor).

    • dodlithr
    • January 22nd, 2009

    cri-cri might be a very minimal twin, check the link ->

    • dodlithr
    • January 22nd, 2009

    cri-cri might be a very minimal twin, check the link ->

    • Karoliina Salminen
    • January 23rd, 2009

    dodlithr: I am interested in a serious IFR travel machine in the twin concept. I have separate track for single concept which has different criteria.

    You are not actually considering it all, I mean not thinking it from pilot’s perspective: Pilots need to accumulate certain number of hours per year per rating to keep their licenses valid. Therefore if I am going to finish my multiengine license, I need to fly certain amount of time with a multiengine plane or my license will become void. But wait, I don’t have a multiengine plane, or can afford one realisticly. Diamond DA42 costs over 500000 used. Especially with current economic situation, it is not a feasible sum to invest. So here is the catch, I can potentially build one.

    What licenses do you have and how much time you have logged? Do you have experience of these things, not from pure transportation point of view, but requirements point of view and about the things pilots must do in order to continue flying? I am going to renew my SEP soon and there is always the requirement of having enough hours during the last 12 months (for the SEP) and later for the MEP, to keep it valid, otherwise the license is void. Fortunately my FAA hours are counted for the SEP renewal, otherwise I would not make it.

    • Karoliina Salminen
    • January 23rd, 2009

    dodlithr: I am interested in a serious IFR travel machine in the twin concept. I have separate track for single concept which has different criteria.

    You are not actually considering it all, I mean not thinking it from pilot’s perspective: Pilots need to accumulate certain number of hours per year per rating to keep their licenses valid. Therefore if I am going to finish my multiengine license, I need to fly certain amount of time with a multiengine plane or my license will become void. But wait, I don’t have a multiengine plane, or can afford one realisticly. Diamond DA42 costs over 500000 used. Especially with current economic situation, it is not a feasible sum to invest. So here is the catch, I can potentially build one.

    What licenses do you have and how much time you have logged? Do you have experience of these things, not from pure transportation point of view, but requirements point of view and about the things pilots must do in order to continue flying? I am going to renew my SEP soon and there is always the requirement of having enough hours during the last 12 months (for the SEP) and later for the MEP, to keep it valid, otherwise the license is void. Fortunately my FAA hours are counted for the SEP renewal, otherwise I would not make it.

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