In-Line Twin Concept Plane

Advantages:

• Thrust of 2 engines- Drag of 1
• No yaw moment due to loss of power in one engine
• Ultra high reliability for total loss of power scenario (because composite reliability is the multiplication of 2 individual reliabilities)
• Wide variety of power plants (assuming 2 of the same) can be deployed because weight does not effect the position of CG.

Issue:
The basis of the argument is one of economics,

    Options:
        A) Traditional- Single 180HP certified aircraft engine retails for approximately $30k.
        B) Proposal- 2 Rotax Snowmobile (or auto) engines producing 100HP each cost $5k total

and economics has a direct effect on public saftey bacause:

• Expensive up-keep is more likely to be put off.
• Engine replacement will be put off long past when it should be.
• Used engines and components (no joke) will be used and reused in active aircraft.
• More likely that a broken or worn part will try to be repaired instead of replaced as it should be.
• Airframe manufacturers are more likely to underpower their aircraft to reduce cost of goods sold, and increase the proportion of the aircraft that they build.

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Author's Concept Plane

3 View of author's concept of inline tandem- "Cat-Dog Plane" named after the cartoon character with 2 heads.
Shown with 2 80HP Rotax Sno-Mo Engines.
Back to back seating for superior CG management and leg room.

 
 

Performance:
Conditions:  3,000 MSL, 50% Fuel, Solo, 80HP Engines
Top Speed: 195MPH
Single Engine- Full Feathering: 130MPH
Single Engine- Fixed Pitch: 90MPH

Dimensions
Length: 25'
Wingspan: 33'
Power: 2 x Inline Cylinder, 80HP each
Empty Weight: 1070 LBS
Max Weight: 1750 LBS

Brief Statistics Lesson:
If you take the loss of power in an engine in flight to be a random process of uniform distribution with probability P, then for a single engine plane the probability of being without power is simply P, but for a twin it becomes P².

So to plug in some sample numbers for the people who hate algebra, if we take the probablility of loss of power in an engine during a particular flight P to be 1/1,000 then for a twin your odds are (1/1,000)*(1/1,000)=1/1,000,000, those are the kind of odds I like.

Basically the idea is that you don't put all your eggs in one basket.

Therefore, if you have a particularly safe arrangement, you can use particularly ordinary power plants.

FAQ:
Q: With 2 engines, aren't your odds of losing an engine twice as likely?
A:  As some might note, the probability of losing AN engine in a twin is double (2P), but for a suitably (and affordably) powered twin this is not a major catastrophy.

Q: If you lose one engine in a twin, won't the other one "just carry you to the scene of the accident"?
A: Below a certain airspeed, and particularly at low altitudes, it can be dangerious, but this is exclusively a problem of the left/right traditional twin.  This design flys at 130MPH on one engine.

Q: Won't 2 engines cost twice as much?
A: No, they're approximately half size of the single, and can be less specialized single ignition types because of the robust nature of the in-line twin configuration.  Engine price is roughly proportional to rated power for a particular make.

Q: The twin engine aircraft I'm familiar with cannot climb on a single engine, the examples given of twins that have this ability are hot-rods and not representative of the typical twins mere mortals can afford to fly.
A: First off, an inline twin doen't require hard rudder (and drag) to counteract uneven thrust.  Second, get used to flying hot-rods, when engines cost $5K instead of $30k and up, you can afford to power your plane with the appropriate amount of power for safe operation.

Q: Aren't most incidents due to pilot error.
A: Yes, but why punish the safe pilots with a less safe mechanical system.  Lose of engine power rates a close second.

Q: For a particular displacement Rotax aero engine, why is the sled engine more powerful?
A: Many reasons, not the least of which being that because the penalty is so high for failure in a single engine aircraft application, they tend to derate the power (and thus your aircraft performance) in an effort ro retain some reliability.

Goals:
Raise awareness in the general aviation public of the advantages of the inline twin configuration to drive new product development with regulatory relief in the form of:
1) In regards to pilot licensing, petition FAA to count the number of thrust lines rather than the number of engines.  Would allow single engine pilots to fly in-line twins such as the Cessna 337 with just a type checkout like any other new aircraft.  This being done to reflect the particular training requied to handle the most serious issue in tradition twin engine aircraft: yaw moment induced by loss of power on one side, especially at low airspeeds.
2) Petition FAA to allow for longer (2x?) intervals between mandatory service/inspection for aircraft using in-line twin configuration due to robust operation of inline twin configuration.
3) Petition FAA to allow otherwise compliant twin aircraft with a single line of thrust (but 2 engines) to be part of the new "Sport" aircraft classification.
4) And regarding Sport classification, remove top speed limitation, the stall speed requirement is sufficient; if someone can build a wing with low speed stall characteristic and high top speed, then we'd all like to have it.