# Twin engine powered sailplane.. Cinema 2

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#### danmoser

##### Well-Known Member
Found a link to info. on a WWII-era powered two-seat sailplane..
Frankfort Cinema 2
A unique twin engine arrangement: mounting on the struts!
This avoids a lot of problems with other installations: landing gear mods, CG shift, non-ideal thrust line, prop clearance issues, turbulence/interference with other surfaces..
The struts turned out to be more than strong enough to handle thrust loads in this old design.
It even had counter-rotating props to cancel torque.. probably not even necessary with such small engines.
Perhaps this concept is one worth reviving with modern construction & power options? :cheeky:

#### Topaz

##### Super Moderator
Staff member
Log Member
It might make a nice self-launch option for an SGS 2-33. 'Course, you'd either have to get an STC or take the individual airplane experimental.

#### jedi

##### Well-Known Member
There was a similar configuration with a tandom wood and fabric (Cherokee II, I think) glider at The EAA fly in many years ago with two single cylinder engines mounted to a strut (looked like a 2x4) extending through the fuselage and the recoil starters routed inside. Looked like it flew well and was fun to fly with one person aboard. Don't know if it flew with two up?

#### Matt G.

##### Well-Known Member
There was a similar configuration with a tandom wood and fabric (Cherokee II, I think) glider at The EAA fly in many years ago with two single cylinder engines mounted to a strut (looked like a 2x4) extending through the fuselage and the recoil starters routed inside. Looked like it flew well and was fun to fly with one person aboard. Don't know if it flew with two up?
Cherokee II is a single-seater, so it definitely couldn't have flown with two people in it.

##### Well-Known Member
HBA Supporter
Jedi had the right plane, it was a Cherokee II. Ken Flaglor (who later developed the Flaglor Scooter) did the conversion, with two go-cart motors on a beam extending through the fuselage under the wing. It had tiny little direct drive props. I have no way of knowing, but I imagine it sounded like a swarm of angry bees.

#### danmoser

##### Well-Known Member
Leave obsolete designs to the history!
You cant make it simplier as that:

Front Electric Selflaunch/Sustainer - Future of gliding
Simple, yes.. and beautiful, but at around $30,000, there will be few buyers. And the sellers of FES insist on approving every installation.. you can't buy one without disclosing your detailed design to them and getting their OK. "Old" is not synonymous with "obsolete.".. good ideas can last a very long time. #### ultralajt ##### Well-Known Member #### autoreply ##### Well-Known Member Simple, yes.. and beautiful, but at around$30,000, there will be few buyers.
There will be a huge amount of buyers. Remember that 3/4th of the worldwide fleet doesn't have an engine and can't be retrofitted with a conventional (pylon) one (too much non-lifting weight). That leaves either a jet or a FES.
The price difference between conventionally powered, but otherwise similar sailplanes is far more as 30K US$, so there's a huge market. A PIK20E for example is 4 times as expensive as a PIK20D. Same for the DG200/17 vs DG400 and so on. And the sellers of FES insist on approving every installation.. you can't buy one without disclosing your detailed design to them and getting their OK. Hardly a surprise, given that most of what they've done is putting existing components through certification. You can buy their components off-the-shelve and buy one yourself. The batteries alone are 7500 US$ (saft V5LM prices), motor another 3000 US\$. The components for a jet (sustainer) are about that expensive, for a SL-capable engine count on a 3-fold.

#### bmcj

##### Well-Known Member
HBA Supporter
I don't know... I'd be wary of anything that might compromise the integrity of the lift struts.

#### ultralajt

##### Well-Known Member
Yes henryk, the spinner gap and prop ruins the laminar flow around fuselage at its very tip of the nose, what is not so good for performances, but this is what you must trade for simplicity of this system.

The majority of motorgliders and sustainers crashes, was caused by a fact that each pilot wait to the last minute to start engine. But in case of retractable engine, the time to erect it to the full power is very long (around aminute) and rised drag of idle powerpod worsen the gliding of the glider significantly, and ground came faster that one expect...

At FES system from decision to run the engine to full power pilot needs just a pushing to the button. A matter of two seconds...

#### henryk

##### Well-Known Member
At FES system from decision to run the engine to full power pilot needs just a pushing to the button. A matter of two seconds...

=FES+opened/closeed containers \like folding wheel\?=clean surface for laminar flow...

##### Well-Known Member
=FES+opened/closeed containers \like folding wheel\?=clean surface for laminar flow...
The gap, any gap will ruin laminar flow, with a large drag penalty. About 4% afaik on the Lak17 FES at high speeds.

#### henryk

##### Well-Known Member
The gap, any gap will ruin laminar flow, with a large drag penalty. About 4% afaik on the Lak17 FES at high speeds.
-I have in minde gapless construction!

#### danmoser

##### Well-Known Member
This was not intended to degenerate into an electric vs. gasoline or laminar vs. turbulent argument..
I just thought there might be some who are interested in an alternative engine mounting arrangement.. it is worth exploring if you happen to be planning to power an aircraft with wing struts..
Certainly, the strut-mounted twin engine concept is quite adaptable to electric propulsion as well as conventional fueled engines...
BTW, aircraft designs with wing struts have been finding some advantages over cantilevered wing structures, as recently "re-discovered" in some in modern aircraft trade studies .. struts allow you to reduce structural weight and utilize thinner root airfoil sections.
An example: Truss-Braced Wing | AOE| Virginia Tech

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#### danmoser

##### Well-Known Member
I don't know... I'd be wary of anything that might compromise the integrity of the lift struts.
According to the article on it, the struts were load tested to 600 pounds in the thrust direction. . and the small engines were only putting out 37 pounds of thrust.. a safety factor of 16.. that's static.. of course, combined loading of lift & thrust as well as vibration needs to be looked into, but I think this would be the least of your worries, unless you go overboard and try to mount two O-200 engines on the struts. .. wouldn't be posted in "the light stuff area" then.

In any case, you should load test your own struts to verify... don't rely on analysis or "that looks strong enough" feelings.. nuthin' better than a load test! :gig: