Whatif I design a plane

[Caveman]

I have just packed all of my modelling stuff for moving house so I wasn't going to compete. But This book escaped the pack process so far. I guess this will be a picture with a backstory. I've had this book for a while and have read through it cover to cover a couple of times. It's not particularly long and this is a very abbreviated design process hopefully I'll be able to produce something within the time remaining.



Moderators, do you think this would count?

[perttime]

I'm not a moderator ... but I don't see anything in The Rules against it. On the contrary.

[Old Wombat]

Nor am I a Moderator but I would think it'd fit into the "Pictures & Pixels/2D/CGI" grouping.


EDIT:

Specifically this rule;

3)  You may build, draw or write as many entries as you like (good luck!).  The posting of in-progress pictures is encouraged as always.  Back stories - however long or short - win extra points.  Well OK they don't but they're always nice to see.

[kitbasher]

You crack on with the idea, mate.  It in scope.

[Old Wombat]

And that IS the Moderator talking writing!

[Caveman]

Cool thanks guys.

[Caveman]

Not a typical "build". I hope that this will ultimately end up with a 3 view drawing of an aircraft along with the reasoning (backstory) for its design. I'll follow the book pretty much exactly but will only look at structural "considerations" and mass distribution "assumptions" rather than full calculations. Hopefully this will be interesting for people to read as well as for me to play.

My inspirations for this are the Fournier RF4 and the Schleicher ASK 14 motorgliders.





I think they are both very aesthetic aircraft and I want one of my own. My challenge and the bit I'm bringing to the design (instead of just buying one of the above) is to make the aircraft compliant with the UK's Single Seat DeRegulated (SSDR) rules (see below if you're interested).

https://www.bmaa.org/files/til_045_ssdr_handbook.pdf

Summary: must be less than 300kg loaded and have a stall speed < 35 KCAS ~40mph.

Just to make it more interesting I want to make it electric too. To my mind motor gliders are the most logical application for electric propulsion with current battery technology. The motor provides instant and reliable start performance - giving the pilot confidence to push the gliding aspect (think low altitude restarts). And the glider aspect allows the pilot to eek out more flight duration for the same battery mass.

So that's the premise. Next step is napkin sketch and requirements. This will drive the initial sizing.

[PR19_Kit]

LOVE the Fournier! 

I've flown the two-seat RF5 owned by one of the club members at my gliding club, a cracking aeroplane. 

[Caveman]

LOVE the Fournier! 

I've flown the two-seat RF5 owned by one of the club members at my gliding club, a cracking aeroplane. 

Very jealous Kit! I've only been able to read about pilot experiences and glean what I can from videos.

So here is my rough digital napkin sketch - hopefully it looks a bit like a motorglider.



Its fairly conventional in layout, very similar to the RF4 and ASK14. Tractor, low wing, monowheel, taildragger. Some specific design cues from the ASK14 - the bubble canopy and relatively skinny tail section. Nose profile (which Im NOT totally enamoured with on this sketch) is more RF4 than ASK14, an electric motor for this sized aircraft will be very small I, so I will have more freedom to play with profiling the nose. Wing geometry - I have shown approximately the geometries of the RF 4 (red) and the ASK14 (blue). I would like to try to have the highest aspect ratio possible for the available weight (Kit). Both wings are slightly forward swept at the 1/4 chord line, the ASK more than the RF this will be for stability reasons and Ill allow any sweep required to be determined similarly. Wing position is slightly further back relative to the pilots head than for either aircraft. I'm hoping that with a light motor forward I can push the pilot slightly further ahead of the leading edge to get better sideways and down visibility. Empenage is configured closer to the RF4 than the ASK14 with the H-Stab fully infront of the rudder hinge line. This should give better spin recovery/resistance as it will limit the hstab from blanking the rudder. I've gone for straight tapered surfaces rather than the RFs slightly swept tail and backward leaning hinge though.

What would I use such a plane for: Hours building and some gliding. Must be able to achieve some pattern work, say 5 circuits on a single charge. Or take-off and climb to find some thermals and complete several reclimbs as conditions (and skill) dictates. (Ill refine what this means in terms of stored power later)

"Mission" Requirements for sizing (these are guestimated based on the performance of my two reference aircraft):

Rate of climb: goal 600 fpm, threshold 400 fpm
Endurance: goal 1hr, threshold 45 minutes
Stall speed: 35KCAS
Cruise speed: goal 90KCAS, threshold 60KCAS (powered cruise speed isn't that important for this)

Payload: goal - 95kg (me plus some baggage), threshold - 85kg (me plus some water and a sandwich)
With 300kg max weight this leaves me with 205kg (215kg) for the aircraft structure, equipment, motor, batteries etc.
The RF4 is 270kg empty (no fuel or pilot) and the ASK14 is 245kg

[Tophe]

So here is my rough digital napkin sketch - hopefully it looks a bit like a motorglider.

And it looks a bit like a Mustang P-51 or FTB also. Good!

[Tophe]

it looks a bit like a Mustang P-51 or FTB

I have found in archives the missing links:

[Caveman]

 

[Caveman]

Ok, so today has mainly been about utilising some "reasonable" assumptions and relationships between geometric parameters to define the size of the wing, work out how much power I am likely to need and provide an initial SWAG at "cruise" L/D.. If people are interested I can go through the steps but the upshot is that:

I get to cheat a bit because I know how heavy the aircraft will be. I want the battery pack/wing to be as big as I can fit within the weight limit so there is no advantage for me to have a lighter aircraft.

The wing area looks to be about 10.7m2. I may round this up to 11 for some conservatism. This size was derived in order to have a reasonable figure for coefficient of lift at minimum flying speed (stall), without the requirement for flaps.

I have split a couple of trade studies as I go in order to make an informed decision further on. The first is the wing aspect ratio. As discussed above, I'm not sure if I want to use the high aspect ratio of the RF-4, or the very high aspect ratio of the ASK14. Ill use the ASK14 if I have the weight budget for it. So using those two aircraft as a guide I get a wingspan of 11m for the "short" wing and 13.2m for the long wing. I have also worked out the root and top chords of the wing. So I have a reasonable idea on the wing geometry. Except for what aerofoils Ill use. and the wing sweep. and the wing twist.

Power loading (kg/kW) was an assumption based on the RF4 and ASK14. The RF 4 exceeds the climb rate that I am after so I reasoned its power loading figure would provide reasonable performance for my aircraft (I shall determine is this really is the case later). It comes up with a figure of 22kWs for the motor size. Coincidentally (really I didn't plan it), this is exactly the size of the Front Electric Sustainer (FES) system that is becoming a popular way of self launching high performance gliders. Additionally the motor is tiny. 8kg, 10cm long by 20cm diameter. That seems like an ideal candidate donkey to me.

With such few parts of the aircraft sketched and calculated, coming up with a prediction for overall drag coefficient should be taken with a significant chunk of salt. But its a chicken and egg situation and this is an iterative process. So we take a SWAG using previous experience and come out with a coefficient of drag around 0.01. Which seems ok(?). Wikipedia tells me this is ~1/20th the value for a Tesla Model 3 (hows that for an apples to pineapples comparison?).

But having this figure lets me work out an approximation for L/D at cruise speeds and therefore will let me approximate cruise drag, which will then allow me to try to work out how much battery I will need to meet my endurance requirements (phew).

That wasn't a very visually appealing update I am afraid. Hopefully I can rustle up something for the eyes for the next one.

Edit: Here's a quick picture of the wing plans sketched up in fusion 360 just to give an idea of the different sizes. long with has zero sweep along the 25% chord line. Shorter wing has zero sweep along the 50% chord line. Diagonal lines are a graphical way of determining the mean aerodynamic chord (as a check of the algebraic method)

[Caveman]

Gah! Real life has somewhat interrupted this.

Anyway, I have made some further progress. I have worked out a first approximation for cruise drag and from this calculated how much power I will need and therefore the mass of batteries required for a 1 hour cruise. I was surprised at the masses required. I've used figures from the FES battery packs for this calculation but simply scaled them linearly. They come out at 131 Wh/kg.

The masses for my aircraft are as follows:
11m span 60 KTAS cruise = 51kg
11m span 90 KTAS cruise = 94kg
13.25m span 60KTAS cruise = 42kg
13.25m span 90KTAS cruise = 88kg

Given that I have ~ 200kg mass budget for the entire structure of my aircraft I don't think a 90KTAS cruise speed is going to be feasible. It will be interesting to see how much of the ~ 10kg weight saving on batteries is eaten up by the extra wing span and higher aspect ratio, if I go for the long wing (actual wing area is the same between the 11m and 13.25m wings). Even so, battery mass is going to account for almost 1/4 of the aircraft. One potential advantage of having such a dense object in the aircraft is its positioning may allow me to tailor the cg position somewhat - and hopefully allow me to achieve a more forward pilot position relative to the wings.

[Caveman]

Ok, so this is probably as far as I will get with this in the GB unfortunately. Not quite as far as I had hoped to get. But, I have completed the calcs for the initial drafted sketch, just not been able to complete the sketch.





Getting a completed first sketch is what I was hoping for in this week GB but I am probably going to keep pressing on with this anyway. The follow on steps would look something like:

Once I completed this drawing, I would be to go back to the assumptions based on the napkin sketch (stuff like wetted area) and rework them. At that point it would be worth my while investing some time in looking into aerofoils, mass estimation for wings and fuselage, mass distribution and then stability calcs. At that point, Id redraw it again.