First attempt at design for electric delta pusher

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Sockmonkey

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Design the plane for the desired flight characteristics without the pilot then find where the center of mass is. Put the pilot at that location! Problem solved. Pilot weight will then not affect the static margin at all!
That's pretty much what I'm saying.
Batteries in front, but with an empty place ahead of the pilot's feet for a landing viewport.
 

RCBinChicken

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Apologies to all for lack of replies to input, I'm an engineering student and the end-of-term assignments and exams have swallowed up all time for the last few weeks... Now it's quietened down a little, I can finally go through and respond properly to everyone! Thanks to all of you for chipping in so many suggestions, it's given me a lot more of an idea of where to focus/what to fix.


A 6m span delta is going to have quite a lot of induced drag. Have you run any drag numbers yet?
The important thing about thrustline is its height from cg. With the prop well behind the cg, you can angle it to get things right.
Calculating usable drag estimates is not within my current skillset, I'll have to work my way up to that one :) but yes, I did assume that induced drag would be the single largest issue with my chosen layout. What pushed me into the "okay this is maybe worth trying anyway" zone was reading an interview with Barnaby Wainfan in T.W.I.T.T, where he was asked a fair few questions about drag on the Facetmobile, which has (I think!) a lower AR than my design and still achieved impressive performance on a smallish powerplant. I would tend (possibly naively/incorrectly) to attribute that to the favourable wetted aspect ratio, like the oft-cited (by starry-eyed delta fangirls like myself) comparison between the B-47 Stratojet and the Avro Vulcan, which had widely different AR as one would expect, but similar WAR and thus similar endurance (when adjusted for other factors.) The Bloop-esque super-low wing loading should also help keep induced drag lower than it might otherwise be.

And yes, RE: thrustline, that's a big part of why I wanted to stick the motor so far back - glad to know I wasn't too far off-base with that.

I'm no engineer or anything, but 70KG sounds extremely light for a man-carrying aircraft. Particularly one that will be fully-skinned. Curious to see where this goes, though.
You're absolutely right to be skeptical, and I may well fall short of that goal. 70kg is ~154lb, and I'm using the later Bloops (~204lb) as my initial feasibility benchmark, my bet being that I should be able to save some weight on a simpler, or at least more compact overall structure. If my final weight (batteries but no pilot) falls anywhere in the 160-210lb range I'll be well satisfied.

Landing gear creates problems for many delta wing designers. John Dyke and Mr. Verhees took the simplest approach by fixing landing gear length at the ideal angle of attack for tak-off and landing.
...
Dean’s other problem was that when he chopped power (to abort the take-off) turbulent backwash from the propeller cloaked elevons reducing their effectiveness.
This is why several deltas (e.g. Rutan Vari-Viggen) have vertical fences or vertical stabilizers immediately outboard of their pusher propellers. Those fences/fins limit spanwise travel of turbulence, smoothing airflow over elevons.
100% with you here. I'll go with gear fixed at ideal AoA as suggested. Sounds like it will solve a lot of headaches.

Good point also on the turbulence cloaking issue. I'm hoping that the very-rearward prop will avoid this too.

The model aircraft design I've been playing with variations-of for most of my time in the hobby, coincidentally, is essentially a VariViggen without the canard. I only found the VV a few months back and was quite tickled to find my silly-looking model had a bona-fide real-life counterpart! :p

If you hope to sell plans for your Mark II...
Oh heck no! :) BEST case scenario, if this thing works, it might provide inspiration or a useful case study for people wanting to build similar designs.

But your point on pilot weight is entirely valid, and I'm thinking maybe this is another area where I should look to the Facetmobile, i.e. just scoot the pilot back as suggested and solve the high-AoA visibility issue with some see-through floors.

I would extend vertical fins so that they overlap the wings’ trailing edges.
That sounds good. Did you mean extend rearwards horizontally behind TE, or downwards making them more like wingtip plates? Or both?

A bit of philosophy from an "arm chair tailless designer".
The problems with the Dean Delta were mentioned. This should be taken as NOT implying that the design won't work, but as an indication of potential problems. Take these in mind and don't repeat other's mistakes. Get a copy of "Landing Gear Design for Light Aircraft, Vol. 1" by Ladislao Pazmany (Vol 2 was never published).

Get a copy of Nickel's "Tailless Aircraft in Theory and Practice".

Learn about flat wrap and Conical Lofting unless, of course, you are building a Facet Mobile like aircraft or doing wet lay-up fiberglass over foam.

Consider putting some fixed canard surfaces near the nose, like on the Saab Viggen, but smaller. I don't have it in front of me, but have a paper around here that says it can do nice things to a delta wing design, if placed properly.
I'll look out for those books. Ta for the recommends!

I'm going to basically learn about every construction method I can, as I'm by no means sure which will suit the design best. I am heavily drawing inspiration from the Facetmobile in a lot of respects, but not yet sure if its construction method will prove ideal in my case.

I considered canards initially, but after reading everything I could get my hands on, I suspect they wouldn't offer much benefit for my target flight envelope - I won't have the raw engine power for flying around at the ultra-high-AoA regimes where their interactions with a delta planform really shine.

As it's an electric, I'd stick the batteries in the nose and/or wing leading edge and scoot the pilot back slightly so variable pilot weight won't change the CG much.
As above, increasingly leaning that way, and Riggerrob and Aeroworx seem to agree with you. Also if worst comes to worst, that way I won't have half my own weight in batteries trying to fly forward THROUGH me if I get in a crash. :)
 
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RCBinChicken

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Aaaaannnd having just seen in another thread that the Facetmobile's designer is a member of this forum, I really hope my own references to it are not embarrassingly factually incorrect. My sincere apologies to Mr. Wainfan should this prove to be the case! :eek:
 

pictsidhe

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Here's the wikipedia page on lift induced drag, now you can ballpark it! I usually calculate induced power
Pi= Di*V/prop_efficiency
That tells me how much engine power is sucked up by the induced drag.

For parasitic drag, the appendix in far103.7 should put you in the ballpark.

Ballpark being the operative word.

With some fairly simple calculations, you can get an idea of whether your magnificient idea is going to soar like an eagle, or fly like a turkey.

Apologies to Barnaby if I'm not entirely correct: The Facetmobile is competive on total drag for a few reasons. The big one is that the lower AR enables many structural components to be reduced in size. It's root thickness of over 3' needs way less spar material than a conventional wing that is under a foot thick. The ~1" tubes used to create the shape double as the spars. Being a geodetic structure, is noticeably lacking in heavy lumps of metal. It's also a BWB (blended wing body), so the fuselage weight and drag are gone. Almost all the surfaces are lifting. The exceptions are the prop and fins. Getting rid off those is tricky, though not impossible.
There's a report out there on the facetmobile.

I did investigate building a Fauxetmobile-103, but decided that the AR was too low to work well within the constraints of 103, and I don't think I'm smart enough to stretch it out enough sideways.
 
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Doggzilla

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This is completely feasible but has some minor issues.

This is no more impossible than a powered harness. It’s simply thickening the wing and placing the pilot inside.

https://en.m.wikipedia.org/wiki/Swedish_Aerosport_Mosquito

To keep it simple the pilot and battery should hang from a harness within the aircraft so that they are always at the proper CG, just like a weight shifter. A two person harness will do just fine. Controls would only need to be a tube to push yourself within the aircraft as weight shift. Turning would be superior to a hang glider due to the superior proximity to the centerline.

As for the flight time, 100lbs of standard lithium batteries is about 6-7kwh (16lbs per kWh from Chevy Volt lithium batteries). At 30% this would be an hour, but it takes nearly no thrust to keep a hang glider airborne. If you pushed a twin harness to it’s limit you could get at least 2 hours. Very likely more.
 

Doggzilla

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Also, there is the issue of getting in and out of such aircraft. They are not difficult to pick up, so using a bottom door that you pull over you like a backpack is completely feasible.

And yes, I have flown hang gliders before.
 

Speedboat100

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Probably some regulatory reason not to. As well as avoiding other unwanted negative attention.
NEGATIVE ?

Isn't any attention a good attention ? You get the thrust line lower if you have more motors closer to the trailing edge.
 

Doggzilla

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NEGATIVE ?

Isn't any attention a good attention ? You get the thrust line lower if you have more motors closer to the trailing edge.
Yes, skirting the rules so obviously attracts nit pickers who then go on crusades to find petty reasons to file complaints. Whether or not they actually exist or not.
 

Speedboat100

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Yes, skirting the rules so obviously attracts nit pickers who then go on crusades to find petty reasons to file complaints. Whether or not they actually exist or not.
See this air foil..that is easily made into a pusher..just add pilot and pusher engine ( pun intented ).
 

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RCBinChicken

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This is completely feasible but has some minor issues.

This is no more impossible than a powered harness. It’s simply thickening the wing and placing the pilot inside.

https://en.m.wikipedia.org/wiki/Swedish_Aerosport_Mosquito

To keep it simple the pilot and battery should hang from a harness within the aircraft so that they are always at the proper CG, just like a weight shifter. A two person harness will do just fine. Controls would only need to be a tube to push yourself within the aircraft as weight shift. Turning would be superior to a hang glider due to the superior proximity to the centerline.

As for the flight time, 100lbs of standard lithium batteries is about 6-7kwh (16lbs per kWh from Chevy Volt lithium batteries). At 30% this would be an hour, but it takes nearly no thrust to keep a hang glider airborne. If you pushed a twin harness to it’s limit you could get at least 2 hours. Very likely more.
Thanks for that link - I had no particular knowledge of "powered hang gliders" as a category, and this is giving me lots more feasibility data points! I like the Mosquito's simple design. That ballpark estimate of flight time is also on the "encouragingly hopeful" side, though 100lbs is approx. 45kg so I may not carry quite that many cells.


How about having at least 4 x 5 KW engines and smaller dia props in the tail ?
My reasoning (and I may well be off-base on any of these points!) was as follows:
- One large-diameter blade, if your airframe can accommodate it, is always more efficient than an output-equivalent quantity of little blades. I'm going to be carrying very minimal e-fuel as it is, so I'll need to make the most of it.
- Even if blade diameter/ground clearance were not an issue, I'd want the centers of the props well above the trailing edge anyway, so they ingest as little of the wing wake as possible (far less noise and hopefully less stress on the blades.) The thrust line height I've designed in should have less impact than, say, a high-engine-mounted floatplane, though I'll need to verify that with a lot of math (and some RC scale models) before putting my neck where my mouth is, so to speak. :p
- Additional wiring, multiple speed controllers etc. presents cost, complexity and weight issues, even allowing for the likelihood that the individual multiple parts would require lower current rating. (That said, if multiple lower-spec components turn out to be significantly/disproportionately cheaper than the power system I had in mind, I might revisit this, even at the cost of total flight time as per point 1.)
 

litespeed

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Nothing will beat a single electric engine efficiency and weight wise.

Extra motors will just add weight and have low thrust compared to a single with bigger prop.

Why not design for the part 103 limit of 254 lbs, seems a lot more attainable and less prone to fragility. If you get the design correct the weight should not be a performance killer.

You can find much better large capacity controllers and engines quality wise than the little cheap ones.
 

Doggzilla

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You could even do 100kg if you wanted. I was 100+ kg (over 2m) and hang gliding with an instructor and the glider was still delightfully responsive vertically. The only issue is turning is like Jello because the controls are at the end of a long bar and so rolling is very restricted.

If you did some hang gliding with the extra weight of an instructor you would see how well it would fly without power.

And you don’t have to limit yourself to such low power levels. The great thing about electric is that you can do nice bursts of 30kw on takeoff and then pull it back to 2-3kw and just float around for a while

I highly recommend trying out tandem hang gliding if you have not already. It will give you a very good idea how your aircraft will handle in real life.
 

Speedboat100

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Nothing will beat a single electric engine efficiency and weight wise.

Extra motors will just add weight and have low thrust compared to a single with bigger prop.

Why not design for the part 103 limit of 254 lbs, seems a lot more attainable and less prone to fragility. If you get the design correct the weight should not be a performance killer.

You can find much better large capacity controllers and engines quality wise than the little cheap ones.

I agree with efficiency...but overall weight and cost might be factors too.
 
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This is a (scruffy) 3D model of my ideas so far..









What a joy to see, somebody else also working with a pusher, that is like in the middle between a deltawing, a flying wing, and a blended wing body :) I had the same idea, just a 4-seater, in stead of a single searter. I made some very early and preliminary 3D-drawings. (They still need to a lot of details, like air intakes, control surfaces, doors, etc. But already in this unfinished state, I think it looks promising. (and gorgeous)
 

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Doggzilla

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I wonder what it would take to get the effect of a double delta.

Double Deltas have vastly better handling characteristics.

Saab double Deltas are so maneuverable that they can outperform Russian SU-27s.

There is video of them doing the “cobra” maneuver at high speed and extremely nimbly, greatly exceeding the flanker. Makes the Flanker look like a B-52 in comparison.
 
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