Single Seat De-regulated (SSDR) Microlight twin - Conceptual Design

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Floydr92

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Some initial sizing with the twin wing mounted pusher config. Not sure if I can get away with slightly reducing both horizontal and vertical tails based on the additional prop wash, or if I should be sticking the horizontal up on the vertical to form a T-tail. This reduces my required horizontal tail area by 0.05m^2, which isn't much but then the tail being in the prop wash might reduce cruise speed? Advice here is appreciated.

I estimated the CLmax as 2.5 with slotted fowlers, which I think is achievable given the dyn-aero at up to 3.0

I understand my writing is crap but if anyone could have a browse over these it would be great. Thanks...image.jpg
 

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addaon

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Is that an 0.03 vertical tail volume coefficient? Seems a bit light; what's the rudder deflection OEI?

With a chord below 0.6 m, your Reynolds number is below a million... CLmax of 2.5 seems optimistic, even with high-complexity flaps.
 

Floydr92

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Is that an 0.03 vertical tail volume coefficient? Seems a bit light; what's the rudder deflection OEI?

With a chord below 0.6 m, your Reynolds number is below a million... CLmax of 2.5 seems optimistic, even with high-complexity flaps.
yes, 0.03 is the coefficient as taken from tables in raymers books. Sailplane was at 0.02 while homebuilt at 0.04, so i assumed mine would sit somewhere between the two.

Sorry, i don't know what you mean by OEI? Rudder as guesstimated is 40% chord, -20 deg to 20 deg

What i like about the Dyn-aero double slotted fixed fowlers, or 'complicated flaps', is that they're very simple.

It might be worth 1: building the wings last once i have the empty weight of the fuselage actually measured, and 2: building them a bit longer just incase, and trimming down to suit real world conditions? what CLmax would you say is reasonably optimistic, between reasonable and optimistic? ;)

If your heart is set on twin pushers, maybe a configuration like the Beachcraft Starship?
I was discussing a canard design with a friend this afternoon and i'd love to do a canard, but i don't think i'd ever get the CLmax high enough for 35kts stall and a nice fast 160kts cruise.
 

Matt G.

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Your vertical stabilizer sizing is going to depend on how much rudder control power you need to maintain controlled flight with one engine inoperative. The values in the table in Raymer are "typical values" and not "point design values". The vertical tail volume coefficient for a twin engine GA airplane would be a more appropriate choice, but it is still only a starting point. Your stability and control analysis will dictate the sizing, and the values of the tail volume coefficients will end up where they may.

I will again suggest that you do some more research before you dive into designing and building.

Edit: A CLmax of 1.8-2 would probably be a bit closer to reality for a combination of leading edge and trailing edge devices, and based on my preliminary design class from my AE degree. If I remember right, the prof for that class said 1.5 is a realistic number for wings with trailing edge flaps only.
 

Floydr92

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I will again suggest that you do some more research before you dive into designing and building.
It's a conceptual design, got to learn somewhere! (and conceptual design IS research)

I intend to build it, but i'll be waiting till i have a solid design completed and through all the stages, and i have completed my AE degree. Just a few years behind you mate, down off the high horse. I'll start building when i see fit to do so, till then, design. end of discussion.

Thanks for the encouragement :gig:
 

addaon

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Yep, OEI = one engine inoperative. You'll need more vertical control authority than a single engine plane for this case; potentially substantially more, depending on how far laterally offset your engines are. More offset is structurally good, but ups the tail size drastically.

Slotted flaps have critical dimensions for slat geometry to actually achieve published CLmax. As your wing gets smaller but tolerances stay similar, it becomes harder and harder to do this. You're looking at a wing thickness under three inches… how accurately do you think you can put your flap hinge line, especially on a high aspect ratio (flexible) wing?

Reynolds number also comes into this… The DC-9 wing apparently achieved (3D numbers) a CLmax of 1.65 unflapped, 2.15 heavily decorated (50° flaps, vortilons, fences, /and/ tip extensions) at Re=10M… but down at Re=1M, these numbers were 1.3 and 1.7, respectively. (High Lift Systems: Predicting CLmax)

You're at an even lower Re than 1M, and you're probably not going to have that level of complexity, even if you do end up with Fowlers… targeting a 3D CLmax of 1.5 is probably a somewhat but not unreasonably conservative starting point.
 

Matt G.

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It's a conceptual design, got to learn somewhere! (and conceptual design IS research)

I intend to build it, but i'll be waiting till i have a solid design completed and through all the stages, and i have completed my AE degree. Just a few years behind you mate, down off the high horse. I'll start building when i see fit to do so, till then, design. end of discussion.

Thanks for the encouragement :gig:
Chill, dude...no high horse here. Speaking from experience. You asked for an opinion and you got one, and I figured it'd be a bit more value-added to cite my source instead of pulling something out of my ear.

You will probably learn more in the last year or so of your degree than the first 3 combined, and then still probably find you want to learn more before starting construction. Two days ago you were ready to start building a fuselage.
 

Floydr92

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Chill, dude...no high horse here. Speaking from experience. You asked for an opinion and you got one, and I figured it'd be a bit more value-added to cite my source instead of pulling something out of my ear.

You will probably learn more in the last year or so of your degree than the first 3 combined, and then still probably find you want to learn more before starting construction. Two days ago you were ready to start building a fuselage.
sorry Matt, I was up late lastnight, got a bit cranky. Two days ago I was going to start building a fuselage mock up to check sizes/anthropometrics/ergonomics, not a flight worthy fuselage.

Thanks.
 

gtae07

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Chill, dude...no high horse here. Speaking from experience. You asked for an opinion and you got one, and I figured it'd be a bit more value-added to cite my source instead of pulling something out of my ear.

You will probably learn more in the last year or so of your degree than the first 3 combined, and then still probably find you want to learn more before starting construction. Two days ago you were ready to start building a fuselage.
Spend a couple years out in industry, and you'll learn a whole lot more--especially when it comes to detail-level parts and construction vs. conceptual and top-level design. Same with some construction/maintenance experience on a real aircraft.

I'm not trying to discount an AE degree--I have one myself--but it doesn't really do much for you on its own when it comes to actually making flying hardware.
 

Grelly

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Hi Floyd,

I'm late to the thread, but I read it all with interest.

You surprised me a bit, 'cos I have assumed up to now that somewhere in the rules that cover SSDR it would say "single engine". However, I checked and can find no such rule.

However, I think I'm right in saying that that you need a twin rating if you intend to fly a twin. If I'm wrong, wow that opens up some exciting possibilities...

*Really* love Duncan's in-line design. Somebody make it happen - please!

Grelly
 

Floydr92

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Spend a couple years out in industry, and you'll learn a whole lot more--especially when it comes to detail-level parts and construction vs. conceptual and top-level design. Same with some construction/maintenance experience on a real aircraft.

I'm not trying to discount an AE degree--I have one myself--but it doesn't really do much for you on its own when it comes to actually making flying hardware.
I agree, and I've had a few years experience in another industry totally non related (left school and started working before deciding to go to Uni) but I still know more about that subject than I do about aero engineering.

Fortunately, the Uni I attend also has a subsidiary company which is a part 145 approved training organisation, an alongside my degree I can do all the part 66 modules. Also due to this we have a fair bit of practical and hands on work with actual flying aircraft in the hanger, along with some non flying parts aircraft and a bunch of engines, all of which we can tear down and re-assemble. So it's not all theory, which is why I chose here over Glasgow strathclyde which ranked higher worldwide but everything is theory, no hands on work. (Apart from maybe riveting a little sample in a workshop).

Grelly, I've read fairly extensively into the twin engine SSDR, and no twin engine rating is required. Regarding opening up the possibilities...that's what I thought! (And they dropped the wing loading rule so it's only limited by stall speed with as much flaps and high lift devices as you can manage).

I'm not sure how to calculate the vertical tail volume coefficient with so many unknowns? I don't know the volume, so can't work out the coefficient from the raymer equations. I presume it's related to the moment required about the vertical axis at stall in an engine out, so this will include the thrust line span wise position , direction and magnitude, - I just can't find a suitable equation. Would it also include the side area of the rear of the fuselage minus the front, and their respective moments given a yaw angle.? Any suggestions?

if the later is the case, it might be worth doing some more work on the fuselage to get it a little more refined before properly sizing the tail and wings?

thanks.
 

addaon

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If you want a really rough approximation, choose a Vmc speed (Vs is a good target); at that speed, assume 100% thrust on one engine, windmilling drag on the other (at most the drag of a disk the size of the prop disk). Calculate the yaw moment from this (which is where engine offset comes in), then figure out how much rudder deflection you need to counter that yaw (treating the rudder as a plain flap like any other control surface). If you're over 20°, or if your rudder control forces are over max (150 lbs? don't recall), make the tail bigger or increase the tail arm.

Obvious approximations here: (1) The center of thrust of the engine isn't at the crankshaft, hence why one engine is critical; (2) a windmilling prop's drag depends on prop speed, etc; (3) you can actually design to a stopped prop for the steady state if you have feathering or a prop brake; (4) you don't need to achieve zero yaw OEI, and allowing some yaw means the entire vertical tail, not just the rudder, will be working for you; (5) even if you can stabilize yaw, the drag of doing so (along with reduced thrust) is very likely to put you at negative climb.

Still, it's a start and will give you a sense of the magnitude of the forces involved.
 

autoreply

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With a chord below 0.6 m, your Reynolds number is below a million... CLmax of 2.5 seems optimistic, even with high-complexity flaps.
The Dyn'Aero does really get close (but not to) 3, with it's tiny chord.
Edit: A CLmax of 1.8-2 would probably be a bit closer to reality for a combination of leading edge and trailing edge devices, and based on my preliminary design class from my AE degree. If I remember right, the prof for that class said 1.5 is a realistic number for wings with trailing edge flaps only.
I heard those numbers too. One of the issues with academia without a cross-breeding with industry. Too much book-knowledge.

Your typical sailplane (tiny chord, low Re) get between 1.5 and 1.7 with flaperons real numbers, certified (CAS, not IAS). With more plain flap and reversed aileron numbers are similar. With drooping ailerons and 50/30/20 degrees of droop (inboard flap, outboard flap, flaperon), you'll be just shy of 2.

Real world effects have a lot to do with it. A high-AR wing by definition have higher Clmax. Winglets help a lot too.

On the Dyn'Aero MCR01 (with double-slotted, laminar wings) I came to a measured Clmax of over 2.75 (2-way GPS tracks with a known true wind direction)

*Really* love Duncan's in-line design. Somebody make it happen - please!
Me too. If I ever get to the point where my design is flying and I can afford it, a push-pull 4-seater is next. Or a balls-to-the-wall unlimited sailplane. So little time, so many designs to make ;)
 

Matt G.

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Spend a couple years out in industry, and you'll learn a whole lot more--especially when it comes to detail-level parts and construction vs. conceptual and top-level design. Same with some construction/maintenance experience on a real aircraft.

I'm not trying to discount an AE degree--I have one myself--but it doesn't really do much for you on its own when it comes to actually making flying hardware.
Having a few years of experience, I agree; however, there is still value to the background knowledge, i.e. knowing where to look for things and what questions to ask. Big-picture stuff. Industry is great for filling in all of the details.

I've worked with a couple of people who must not have paid a lot of attention in school- they probably couldn't pass a statics final exam at this point, yet somehow have some sort of engineering degree. They will probably have fairly limited industry careers because they have no foundation on which to build the experience.

autoreply said:
I heard those numbers too. One of the issues with academia without a cross-breeding with industry. Too much book-knowledge.

Your typical sailplane (tiny chord, low Re) get between 1.5 and 1.7 with flaperons real numbers, certified (CAS, not IAS). With more plain flap and reversed aileron numbers are similar. With drooping ailerons and 50/30/20 degrees of droop (inboard flap, outboard flap, flaperon), you'll be just shy of 2.

Real world effects have a lot to do with it. A high-AR wing by definition have higher Clmax. Winglets help a lot too.
Agreed, and I'm pretty sure the un-mentioned asterisk on that data point was "for 'conventional' relatively low-aspect ratio GA aircraft".
 

BJC

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One of the issues with academia without a cross-breeding with industry. Too much book-knowledge.
Some years ago, while on a recruiting trip to a major university, I met a Phd Mechanical Engineer / college professor who expressed a desire to get some "real world" experience. He took the summer off, and I hired him to work with some of our engineers in a large power plant. It was a win-win; he got real world experience, and our engineers got answers to all of their questions about how theory fit in with issues that they were working on. He came back the next summer, then left the university to start his own company with some very specialized thremodynamic performance software that he had developed with us.

He was a really nice guy, too.


BJC
 

autoreply

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Sounds familiar. One of the points where TU Delft truly excells is that industry and university work closely together, both in aerospace and many other fields and they literally walk into eachother on a daily basis. Many big high-tech companies are within walking distance of campus.

Many of the fresh promising (engineering) employees are sent to production first. First qualify as a certified welder or CNC-machinist, then start work as a structural engineer.

Not to mention a lot of ground-breaking projects where students take a year (or more) off to do stuff like Nuna, Wasub, etc.
 
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