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?