# Lack of twin engine E/ABs

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#### nerobro

##### Well-Known Member
I know that the original 2 seater was a very attractive aircraft. However, the fuselage extension done on this aircraft is also a very attractive modification. You need to see it in person, it looks like a bullet.

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That plane is really, really pretty.

Your product is really, really, impressive too. I believe I met you at oshkosh. I like what you're doing.

#### SpruceForest

##### Well-Known Member
Wilson Global Explorer. I recall that project...fly to remote places and explore... lots of internal volume...performance was more stately progress than fast, efficient, but you have to admit it has an interesting look. Two built... both crashed due to pilot error.

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#### PMD

##### Well-Known Member
I know that the original 2 seater was a very attractive aircraft. However, the fuselage extension done on this aircraft is also a very attractive modification. You need to see it in person, it looks like a bullet.
There are two things that bother my eye: the fuselage extension changes from constantly changing rate of change to a "straight taper" in the plug. The second is strangely the paint job: those three "chevrons" behind the cabin I find a bit offesnsive. The latter easily corrected.

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#### PMD

##### Well-Known Member
You are talking about 2 different families of airplanes.

John Thorp designed his T-18 homebuilt plus the Fletcher/Cresco/PAC 750XL series that is still built in New Zealand. John Thorp also designed the first Derringer before production was taken over by George Wing.

The Grumman-American series of light singles is based upon the BD-1 light 2-seater designed by Jim Bede. Under GAA management, they developed some 4-seater variants. I have a few hours flying a GAA Cheetah and rather enjoyed it.
This light twin is based upon a GAA single.
Why this example (N134GB) has the word "Derringer" on its engine nacelle is a mystery to me.
Being one of the founding year members of the AYA, I promise you I DO remember who designed the BD-1 (but in all fairness, NOT the AA-1). To say that butchering one of the greatest little airplanes of all time is offensive to me is an understatement, but I was pointing out who's engines were included in this heinous offense on one's senses.

#### SpruceForest

##### Well-Known Member
It's certainly possible to get something with decent range, payload, handling, ergonomics, and - yes - looks in a small 2-4 place twin, as the Diamond DA-62 shows, but the cost to tool up for what would be a pretty niche market would have to be done by a kit manufacturer with a couple other successful aircraft or someone with fairly deep pockets and no expectation of moving out of the red for a decade or two. Liability would be an interesting issue given the additional opportunity for legal mayhem, and I have to wonder where liability law will go if we continue to move towards penalizing industries that we don't care for or don't contribute sufficiently to our preferred sorts of villains.

#### pfarber

##### Well-Known Member
Read this and this and this very carefully. I'm a big fan of LSs for aircraft use but a pilot should give some very sober consideration to all of the potential failure modes and ask himself if he'd like to double those odds of something going wrong. This is not a level of complexity that many homebuilders want to mess with. Three times the power of an O-360 (540-600hp) is decidedly in forced-induction racing territory (i.e. higher than Sweet Dreams' normally-aspirated SBC) unless one is willing to sacrifice reliability for higher mean piston speed.
EPI is IMHO not authorative. They have all the answers yet no marketable designs.

A turbo car engine is not a mystery. And even if you have a TBO of 500 to 1000 hours so what. Every five to ten years you spend $10k on two new engines. Dual IO-360s or bigger might last longer but will be 4-8x more expensive. You still win economically. #### pfarber ##### Well-Known Member The number one thing experimental builders complain about are engine prices. Now we are going to have to purchase 2? Hard pass. The cost of a car motor is maybe 25% of a certificated one. The cost is not a significant factor. When I dug into this a few years ago, there is a "moving" liability insurance requirement in almost every state; especially if the airport gets any state funds. All the aviation insurance web sites state that insurance is required,yet none have shown a FAR or state law. I did find one state that requsted a copy of the insurance policy to determine AC value for tax purposes, but upon further reading it was only for commercial operators. I asked at my old home airport and they did not require it, and while one Aviation insurance company said that most FBOs require insurance to use their facilities, i have never been asked to show any documents, ever, at an FBO for parking or fuel. #### Pops ##### Well-Known Member Supporting Member Wing Derringers biggest problem is they are slow. They should be a rocket but they aren’t. Cool for sure. Cockpit is small. Aircams are a blast. They fly like any floaty plane of its type. Cost is pretty rough. Most cool kids would probably go for a Backcountry Cub in that price bracket. Aircam’s biggest problem is it’s not certified. It’s a plane that needs a job. 80% helicopter in an airplane. Photographs, aerial surveys, wildlife counting. I could put one to work right away. Surprising what you can do with a lighten straight tail C-172 with one notch of flaps at 60 mph. #### Aesquire ##### Well-Known Member A LS powered semi-scale P-38 would be a great sport plane. Since you wouldn't have 4 heavy machine guns and a cannon in the nose, there's room for a pilot's legs without ruining the look of the pilot pod. Visibility would be an issue, but not that much worse than the original. The twin boom configuration offers room for fuel and maybe turbos, for the race version. #### BJC ##### Well-Known Member Supporting Member A LS powered semi-scale P-38 would be a great sport plane. Jim O’hara built a very nice scaled P-38. BJC #### Dan Thomas ##### Well-Known Member The cost of a car motor is maybe 25% of a certificated one. The cost is not a significant factor. Really. All converted for aircraft use and everything? So why don't we see way more of them flying? I'll tell you. It's because they're nowhere near ready for flight. One reason is because the car's engine generates its HP at RPMs totally unsuitable for propellers. So we need a redrive, one that will convert the engine's high RPM/low torque to the propeller's low RPM/high torque needs, and we need it to do that without coming apart or putting excessive radial loads on the crankshaft or weighing too much or costing as much as an aircraft engine. This stuff has been beat to death on HBA, since so many seem to think it's easy, while those of us who have done it, and watched others do it, know it's not easy or cheap at all. Torsional vibration is a real thing and it kills drive systems. Our own wsimpso1 was an automotive engineer that designed transmissions and other stuff and spent lots of years learning the physics of it all in university. He has filled us in on what causes those destructive forces and how they must be handled, but still a lot of folks don't believe him because it's just not convenient to their daydreams. It's sad. Well, there's one aspect. Another involves reliability. With auto engines we're dealing with single ignition, and we need to make sure that setup gets the power it needs. Sure, the EI systems are pretty good today, but if the alternator and its charging systems fails, no amount of reliable electronics is gong to make a spark. Alternators on cars last a long time because they're turning at a quarter of their rated RPM most of the time. In airplanes, they wear out way faster because they're running near their redlines all the time. They're geared or belted to the engine so that they will reach redline at engine redline, and car engines cruise at a quarter or a third of redline, while the aircraft engine cruises at 90% or better of redline. As an aircraft mechanic who also does all his own vehicle maintenance, and has done so for over 50 years, I can tell you that your auto conversion's alternator will not last all that long. And if you don't believe that, you will have an inflight failure. All the same factors apply to the EFI system, too. No power? Instantly dead engine. These systems, between the two, can use 20 amps, so a battery can't keep them alive for more than a few minutes. Once the voltage drops to 11 or so, they quit. And that's the reason why cars now typically have much more powerful alternators than they used to. They're working harder. Cooling is a hassle. That system has to be capable of shedding waste heat at full power for extended periods. The exhaust system also is severely stressed. Aircraft exhaust systems are running red-hot at full power. You need good steels or nickel alloys to deal with that, and the system has to be cooled, and be located so it doesn't set other stuff afire. Then there's the mounting of auto conversions. They don't come from the GM factory with convenient lugs on the back or bottom of the engine. That stuff has to be designed and attached to robust points, then a tubular mount designed to transmit weight, thrust and torque forces to the airframe. And that mount has to hold the engine at a place where it balances the airframe, to get the empty CG where it needs to be. You do a lot of math and geometry. This all adds up mightily. I know. I did it, installing an already-converted Subaru into a Glastar. Had to design the mounting system, the mount itself, the cooling system, the fuel and electrical systems, and get them all to work properly. Working the bugs out took months. Just dealing with the TV issues at 1400 and 2800 RPM was a problem, fooling with cog-belt tensions. I made seven different mufflers for the thing, trying to quieten it without adding bulk and weight. In the end, a Lycoming would have been flying in a week instead of over a year, at the same cost, with better reliability AND performance, and the resale value would have been double. Hamilton-Stoddard had all the stuff: the mount, the cowling, the baffling, the exhaust. Everything. Thankfully, I didn't have to design the redrive to take a constant-speed prop. That adds a whole other level of headache to the project, delivering oil from the prop governor through the redrive to the prop, and finding and manufacturing a way to drive the governor. So saying that an auto engine costs 25% of an aircraft engine is completely meaningless. You're missing the bigger picture entirely. Last edited: #### Dan Thomas ##### Well-Known Member Twins also typically have constant speed props. This means it can be really tricky to determine if there's a problem. It also means that the cost just went way up. CS props cost WAY more than fixed-pitch props. #### Riggerrob ##### Well-Known Member For series built planes: Rutan Defiant Velocity V Twin Cri-Cri Lazair For one offs: Twin Engine Quickie 1 RV6a Built as a Twin Zenith Gemini Rutan Voyager To directly answer your question, you don't see a lot of twins for a whole slew of reasons. Most practical, is due to common design practices, you end up with lots of interference drag, and despite having some large multiple of the a single engine's horsepower, you typically don't see a whole lot more performance. One big engine generally works out better. Your suggesting that they aren't overly complicated is .. well lets talk about that. Sure, you can build a powerplant, and supporting systems, and then do it twice. Good! That's the easy side. Once one engine goes out, a whole lot of things can go very wrong. This is compounded with the idea that most twins are setup as planes designed to be fast, so there's a lot of compromises to the flying qualities of the plane, so you get the higher performance. When things go wrong in a twin, they go "a lot more wrong". Lets say you manage to get it into a spin. Your plane has engines way out at the ends of the airframe, giving you a large polar moment of inertia. If your spin develops at all, you're not fighting a pair barbell with 300lbs on each end, that's 20' long. While if you're flying a cherokee or something, nearly all your mass is near the center. High performance twins also tend to have small control surfaces so they behave well in their designed operating speeds.... Which makes their surfaces pretty marginal in an emergency. It's possible to design a twin that has no VMC, but you quickly lose performance. There are some planes that are like that. They're not fast... Twins also typically have constant speed props. This means it can be really tricky to determine if there's a problem. There's a whole lot of other factors that make figuring out a POH for your experimental that makes it a very complex issue. It's also likely you don't have good enough test pilot to get that entirely sorted out. And if they DO find a problem, are you a good enough engineer to sort it out? Single engines just don't have any of those issues. I've been thinking about this sort of thing for a year or two now. Sadly those industrial v twin's don't have great power to weight. Why stop at two though? With two, when one dies, you have 1/2 your horsepower gone, and some significant drag. If you have four... an engine out would only be 25%. I also pondered some sprag clutches and driving the same prop... Completely nonsense, but it was fun to consider. Think about how the larger single-rotor helicopters (EH-101 Cormorant and Sikorsky CH-53 Stallion) have three jet engines connected to single transmission and the connection is through Sprage clutches, so if any single engine quits, they only lose 33 percent power and the quiet engine does not drag on the transmission. #### dog ##### Well-Known Member automotive conversions,done so that they approach aviation standards are cheaper per hp,and will be more fuel efficient,but installed cost per installation are still going to be in$10k chunks,variable pitch prop,chunk,turbos and
air conditioning,chunk chunk,dry sump,chunk
titanium exhaust,chunk

#### SpruceForest

##### Well-Known Member
Think about how the larger single-rotor helicopters (EH-101 Cormorant and Sikorsky CH-53 Stallion) have three jet engines connected to single transmission and the connection is through Sprage clutches, so if any single engine quits, they only lose 33 percent power and the quiet engine does not drag on the transmission.
Seems like a good way to disconnect an engine when failed. One-way freewheeling clutches are found on single or multi-engine helos... location for single engine usually on the trans; for multi-engine, varies with whether there is an intermediate trans like an engine nose-box or c-box. On the 212/412, clutch integrated into the combining gearbox of the PT-6 TwinPac. Biggest issue with clutches is hard engagement from over-running...that can be a noisy and expensive problem. Def one of the checks we did on second engine start was to make sure the clutch engaged as N2 came up, and then disengaged with spool-off of other engine.

#### Riggerrob

##### Well-Known Member
It's certainly possible to get something with decent range, payload, handling, ergonomics, and - yes - looks in a small 2-4 place twin, as the Diamond DA-62 shows, but the cost to tool up for what would be a pretty niche market would have to be done by a kit manufacturer with a couple other successful aircraft or someone with fairly deep pockets and no expectation of moving out of the red for a decade or two. Liability would be an interesting issue given the additional opportunity for legal mayhem, and I have to wonder where liability law will go if we continue to move towards penalizing industries that we don't care for or don't contribute sufficiently to our preferred sorts of villains.
Airplane crashes are expensive and painful, but nothing compared to the agony personal injury lawyers can inflict on anyone vaguely related to the crash.
Tort lawyers WILL drive you insane.

Hint: I suffered multiple injuries when a Beechcraft King Air crashed in 2008. The pilot compounded the problem by shutting off the engine that was still generating power.

So the simple solution is to build your own light twin for your own amusement, but refuse to sell it to anyone else. D
When you have satisfied your curiosity, dismantle it and sell off the parts separately.

#### Dan Thomas

##### Well-Known Member
automotive conversions,done so that they approach aviation standards are cheaper per
hp,
Once you add it all up at the end of the project, they're not even that.

#### dog

##### Well-Known Member
Once you add it all up at the end of the project, they're not even that.
thinking of some v8 conversions,robinson sticks out,litteraly,as they have three different extension
lengths availible on there psru,so that cg can be
put where you want it,shortest is about as stubby as a gear reduction can get,and mid length,and a long one looks to be just about 20",an option just not availible off the shelf in a certified unit,