# Grenet G.47 very light twin from 1949

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by cluttonfred, Mar 27, 2019.

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1. Mar 27, 2019

### cluttonfred

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There very few multiengine homebuilts out there and for pretty good reason. Engines represent an enormous part of the cost of an aircraft project and the cost per horsepower tends to go down with increasing size. Add in the cost of accessories and propellers and it's easy to see why two 50 hp engine installations are going to cost more than one 100 hp installation. Here is the Grenet G.47, an amateur-built light twin from 1949 powered by two 40-45 hp Salmson radials, performance is modest but it's still an interesting concept. What do you think?

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2. Mar 28, 2019

### Vigilant1

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1840 lbs MTOW and one 45HP engine out there on each wing. Does the text mention anything about two-engine climb rate, single engine rate of descent, or minimum controllable airspeed?
It's not ugly to look at IMO. The gear is quite "beefy."

3. Mar 28, 2019

### TFF

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I know someone who has one of those radials, but the plane was so underpowered he got a field approval to put a Continental 75 on it. Engine is for pretty on a stand. It would barely fly. I bet that twin is flyable as a single seater.

4. Mar 28, 2019

### TFF

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How much Jodel is underneath?

5. Mar 28, 2019

### cluttonfred

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No Jodel influences as far as a know, this appears to be an all-metal design with fabric covering.

There was clearly a focus on low-speed handling rather than maximu-speed performance. The text mentions a minimum level speed of 65 kph and landing at 60 kph, microlight speeds, thanks to the nearly full-span fixed leading edge slats. No single-engine climb speed is given but a level speed of 90 kph on one engine is mentioned, which suggests some ability to climb if enough rudder authority is available. It manages this with just 90 hp total and a whopping 835 kg/1841 lb gross weight.

With modern techniques, materials, components, and instruments it ought to be possible to reduce that weight substantially. At something closer to LSA gross weight of 600 kg/1320 lb (LSA gross weight, though this wouldn't qualify as an LSA) then it seems workable. Twins are often more expensive per hp than singles but with a pair of industrial V-twin engine and redrives this starts to look like an attractive concept.

Here are some detailed pics and the specs table from the article.

Last edited: Mar 28, 2019
6. Mar 28, 2019

### TFF

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I bet there is not a lot of meat on the bone to remove. Turning those big high pitched propellers for the engine size is what flew it at all. Like most prototypes, genius Built it. No ham fisted homebuilder makes weight go down.

7. Mar 28, 2019

### Vigilant1

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I don't think it's realistic to believe it climbs at all on one engine. Imagine a C-152 at max gross trying to take off on 40% power. Now add another 170 lbs of weight (a stowaway pony in the area behind the seats?). Now add the drag from slats. Now add the drag from that large prop that is stopped. Now add the drag from the frontal area of both of those radial engines. Now add the drag from the rudder and aileron you've added to keep the plane flying straight. No way. I think they just skipped that part in the story.

If we want a two-seater that can still climb on one engine, they'll have to be some very big industrial engines (60 HP+) and the airplane will still have to be clean and light. When we get to industrial engines that big, an 1835cc VW looks like a better alternative for both cost and (esp) weight. Significantly, the pilot will need a multiengine rating (no cost if you aready have one, bring a few thousand  if you have a SEL rating and want to get a multi rating from your local FBO). There might be ways to reduce this cost significantly (e.g. a CFI with a Beetlemaster and a LODA could take students up in his plane and train them to fly their own Beetlemasters. The flying hour cost of 2 VW engines would be a lot less than an Apache.)

If we want a single seat, it looks like we can achieve single engine climb with a clean airframe and two approx 28-30 HP engines (based on my limited figuring with the Micromaster idea). From the legal side, the FAA allows a single-seat E-AB airplane to be flown without a multiengine rating if the DAR who inspects the plane writes the Operating Limitations to allow it.

From the EAA web site:
I think I've read that at least some CriCri's operating in the US are flown by pilots with SEL ratings, but the close-in spacing of the engines and the way the airflow interacts with the canop apparently makes the plane fairly docile with one engine out, which might encourage a DAR to leave off the Multiengine class. I think a centerline twin design might also be more likely to receive this treatment than a twin with engines farther off the centerline and the resultant single-engine handling issues.

Last edited: Mar 28, 2019