# Twin engine powered sailplane.. Cinema 2

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##### 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..
I wouldn't call it "degenerate", those are the very reasons we pick some configurations over others.
Certainly, the strut-mounted twin engine concept is quite adaptable to electric propulsion as well as conventional fueled engines...
I doubt it. Some ballpark numbers show that you need a rather large strut to cope with the torsion from precession. Engine weight being significant, you'd have to beef it up a lot to take the bending load, and you'll probably end up with what is essentially an underwing-mount. Check out some of the engine mounts for an idea of the loads.

The Do28 (or was it 27) is a good example; by the time they ended up with significant mounts, it's structurally almost a wing ;-)
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
On big planes... yes, but not on small ones. Scaling not being linear means spar weight is insignificant in anything with a practical aspect ratio. Skin is the majority of the structural weight.

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#### danmoser

##### Well-Known Member
I wouldn't call it "degenerate", those are the very reasons we pick some configurations over others.

I doubt it. Some ballpark numbers show that you need a rather large strut to cope with the torsion from precession. Engine weight being significant, you'd have to beef it up a lot to take the bending load, and you'll probably end up with what is essentially an underwing-mount. Check out some of the engine mounts for an idea of the loads.

The Do28 (or was it 27) is a good example; by the time they ended up with significant mounts, it's structurally almost a wing ;-)

On big planes... yes, but not on small ones. Scaling not being linear means spar weight is insignificant in anything with a practical aspect ratio. Skin is the majority of the structural weight.
Well, the Cinema 2 example had "jury struts" at its midpoint, so that is an essential detail in determining off-axis loads.
Jury strut - Wikipedia, the free encyclopedia

As to the gyroscopic precession loads, they are no worse with an electric engine than with an IC engine, so I don't see your point as to its validitity in the electric vs. IC engine argument. In fact, a rational case can be made that dynamic IC engine loads are far worse than electric motor loads, considering that dynamic vibrations in all 6 axes (up-down, left-right, back-forth, yaw, pitch and roll directions) are more intense with IC (Internal Combustion) engines than they are with electric motors.

The Cinema 2 powered glider example presented in this thread is for two low-power engines mounted on wing struts.. I think it is a worthy design option for some (but not all) glider motorization schemes. Criticize it, if you must, but it IS a viable design option .. subject to proper engineering rigor, of course, as are all other options.

As to your assertion that thickness of airfoils being only applicable to larger, faster aircraft (i.e. higher Reynolds number).. that is incorrect.
Lower Reynolds number airfoils (as in ultralight aircraft) tend to be thinner sections to achieve best L/D efficiency.
Ultralight airfoils tend to be 15% of chord or thinner.. cantilever structures sometimes require 20% or greater thickness at the root.

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#### henryk

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

Front Electric Selflaunch/Sustainer - Future of gliding
"2. Nose section should be round as possible. It could be also slightly elliptic or eggy shape,

but not too much.

For most types of sailplanes we could find a solution with special spinner of if this would be not possible we could adjust front part of fuselage so that shape would become more appropriate."

"but not too much."=???

="mooving" nose \STEMME-like\ have not this probleme !

##### Well-Known Member
Well, the Cinema 2 example had "jury struts" at its midpoint, so that is an essential detail in determining off-axis loads.
Jury strut - Wikipedia, the free encyclopedia

As to the gyroscopic precession loads, they are no worse with an electric engine than with an IC engine, so I don't see your point as to its validitity in the electric vs. IC engine argument. In fact, a rational case can be made that dynamic IC engine loads are far worse than electric motor loads, considering that dynamic vibrations in all 6 axes (up-down, left-right, back-forth, yaw, pitch and roll directions) are more intense with IC (Internal Combustion) engines than they are with electric motors.

The Cinema 2 powered glider example presented in this thread is for two low-power engines mounted on wing struts.. I think it is a worthy design option for some (but not all) glider motorization schemes. Criticize it, if you must, but it IS a viable design option .. subject to proper engineering rigor, of course, as are all other options.
Not sure whether in the end it's worth it, but it's certainly a viable possibility.

The Eurosport aircraft (sic?) also has an interesting configuration. Fold two props sideways in the fuselage.
As to your assertion that thickness of airfoils being only applicable to larger, faster aircraft (i.e. higher Reynolds number).. that is incorrect.
Lower Reynolds number airfoils (as in ultralight aircraft) tend to be thinner sections to achieve best L/D efficiency.
Ultralight airfoils tend to be 15% of chord or thinner.. cantilever structures sometimes require 20% or greater thickness at the root.
Huh?
I spoke about struts, not chord thickness. Virtually all modern sailplanes have airfoils that are already at the aerodynamic optimum (11-12.5% for lifting surfaces, much thinner for non-permanent lifting surfaces) and still the spars are light as a feather
Point is that spar weight (potential weight savings with a strut) is simply too small a weight fraction to get any significant structural savings. That's different in airliners, where the non-linear scaling means you can make the wing much lighter (stressed skin), relatively speaking and reducing the bending moment (struts) simply means you can cut down skin thickness.
In short; struts don't make sense for structural reasons on "small" airplanes.

#### danmoser

##### Well-Known Member
Yes, it all comes down to the details.. struts are neither better or worse than a cantilevered wing (except to the structural analyst who thinks "easiest to analyze" is the best design .. )
A strut-braced design can be lighter than a cantilevered design, but only if the engineer is rigorous in his or her work.
Streamlined struts are essentially airfoils, so an aircraft using such struts is essentially becomes a "joined wing" design... struts become contributing lift members, not just drag producers.

As to the claim than cantilevered composite sailplane wings are "light as a feather": .. I can only smile at such an obviously incorrect assertion... this coming from a composites guy..

##### Well-Known Member
As to the claim than cantilevered composite sailplane wings are "light as a feather": .. I can only smile at such an obviously incorrect assertion... this coming from a composites guy..
Correct. Your assertion is obviously incorrect, since I didn't write anything like that.

And yes, I know what I'm talking about. Neither my design work, nor the scales lied so far. Throwing a 20' spar almost through the ceiling is great fun

#### danmoser

##### Well-Known Member
Virtually all modern sailplanes have airfoils that are already at the aerodynamic optimum (11-12.5% for lifting surfaces, much thinner for non-permanent lifting surfaces) and still the spars are light as a feather
So.. yeah, you DID say it, autoreply.. I didn't make that up.. but I know you probably meant that as tongue-in-cheek, but still, you DID say it -- deny it all you want -- the record is there! :speechles

My main point is: as wonderful as modern composites are in terms of strength-to-weight ratios, struts and wires can indeed still reduce structural weight - but whether or not it is worth it overall (drag vs. weight trade-off) .. you have to get into the nitty-gritty details.

Now, can we finally end this pointless bickering and discuss something relevant on the topic?

#### akwrencher

##### Well-Known Member
HBA Supporter
I believe Autoreply said "spars", not wings, were light as a feather. small, but infinitely valid detail......

##### Well-Known Member
So.. yeah, you DID say it, autoreply.. I didn't make that up.. but I know you probably meant that as tongue-in-cheek, but still, you DID say it -- deny it all you want -- the record is there! :speechles

Now, can we finally end this pointless bickering and discuss something relevant on the topic?
That's the third time in this topic you're accusing me of something, while I said something completely different.
A little bit less condescending and a lot more careful reading what has actually been said, before you reply the next time, please.

My main point is: as wonderful as modern composites are in terms of strength-to-weight ratios, struts and wires can indeed still reduce structural weight - but whether or not it is worth it overall (drag vs. weight trade-off) .. you have to get into the nitty-gritty details.
I doubt that, see earlier posts, save double wires and torsion-supported wings obviously. Struts and their structure are easily heavier as the complete spar of a cantilever wing. Real numbers of real aircraft...

#### LimaZulu

##### New Member
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. Unfortunately it is not possible to build FES at price of$15,000, as only price of material is much higher. My idea is to sell FES at low prices, but at small series this does not work. We do not isist on approwing of every installation, but there are many tricks we know about, and so some initial check is required in order to find out if FES is suitable for a project as we would like to have a happy customers.

Regards,

Luka

#### LimaZulu

##### New Member
We put a lot of effort into development, so why we should show all the details so that others would copy them? When something is done it looks easy, but it is not at all.

Regards,

Luka

#### Aircar

##### Banned
The very small prop diameter possible with a nose mounted propeller (on a typical modern 'low profile' fuselage and 'body of rotation' shape makes the take off run and excess thrust unsuitable for autonomous operation -- the Stemme is a bit better from sheer size and the nose high attitude it has (massive undercarriage) but again it is fearsomely expensive . I designed a 'retrofit' nose engine and prop for my old Sagitta in the 80s and balanced the extra weight up front with a small canard so that the aircraft became three surface --older gliders have deeper fuselages and more volume in the nose to bury a two stroke (210cc Solo in that case or Fuji Robin 250cc ) things conspired to prevent carrying it out but I believe it would be practicable for lots of older gliders and cheap .

For the HP 18 I toyed with a nose end prop and 'spinner'/nose cone that was articulated to give a much larger prop diameter possible --described on the HP gliders forum way back (so preventing patent..) and since improved upon -- if the task is redefined to include a take off run constraint or static thrust minimum (for safety reasons mostly) then simple or simplistic solutions are not adequate. The scope for real invention is wide open in these sort of things but still the same handful of methods reappear despite the costs and drawbacks (eg the 'pop up' motor and or prop cutting out big chunks of rear fuselage structure and being airbrake like during use and very expensive still . The first such modification was done on a Nimbus at Schempp Hirth in '73 but used a privately modified Scheibe Sf 27 as the 'prototype' --the Finnish Pik 20 self launcher also drew upon an amateur conversion -- homebuilders found a way first in both cases. Of all the possible permutations for placement of props and engines,folding etc etc the majority have never been built (there is some mutual cross pollination possible here between roadable aircraft and gliders in the area of folding/stowing of various parts and configurations. )

#### Aircar

##### Banned
I used to get a laugh out of asking LAME's (licenced Aircraft Maintenance Engineers) used to toting light aircraft wings to just 'quickly lend a hand' when rigging a glider in my old workshop next to a GA business --and making sure that they had the heavy end of a wing (eg a 19 metre Kestrel ) --expecting only a light thing because it was just a single seater, looked so small and had no motor etc they would start to lift but be perplexed when it didn't move --and kept lifting till the back breaking weight was obvious (!) -- they had a new respect for how heavy gliders really were . Among non cantilever gliders the standout from the early 70s is probably the Aviafiber 2CFl -- 90 lbs for 44 ft span and 30;1 l/d -- and rigid enough for rough handling due to it's configuration --another case for 'strut' mounted engines and props in this fashion.

#### oriol

##### Well-Known Member
What a strange aircraft the Aviatik!

I never heard of it, It´s claimed gliding coefficient is of 35/1, much better than the SWIFT (20/1).