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Need a little advice on motors and props in general, and specifically for my custom design.

Vimana89

Well-known member
#1
I've been flying for the better part of a year now, a lot of the planes I fly are custom home builds that are a bit crude by some standards. I've been building mostly planes in the mighty mini size range, small park flyer sized, and have used only 1806 radials, 2205 radials, and Graupner 2806 Ultra 1900 KV, which is similar in performance to the 2205. I have experimented with these motors on 2 and 3s with different prop combos, generally knowing what was within the safe and effective range, but I never got too deep into the specifics of selecting the right motor for a build or building the right plane around a motor. I'm really taking educated guesses and going off recommended or compatible with lists on the products I'm using. I'm going to have to do a little research into this and ask around.

What prompted me to really want to polish up my knowledge at this point was building a larger version of my V Sliver RET slender delta design. The original park flyer size model with the 2205 Radial or 2806 has tons of power to spare with a 3 bladed 5x5.5 Hulky prop on 3s. It can tail stand and float around and even maneuver and turn while maintaining a ridiculous high alpha, it can stay very steady in slow flight, and really pick up some rocket speed on full throttle. It had about a 16-17" wing span by about 30" chord. I have attempted a larger version twice, and both came out about the same as each other, but not as well as the small version.

My most recent one flew decent, but suffered from some structural integrity issues and random bad luck. It was not optimal though. It had a very low aspect ratio of 18" span by 40" chord with a custom style KFM wing, which is actually perfectly functional with all the vortex lift and wing area, and it flew nicely, but the 2205 and 2806 are my biggest motors, and on the larger model they are underpowered. I get a very turbulent wing rock if I fly it excessively slow or try high alpha stuff like the smaller model. The only time the smaller model got such violent wing rock was when I rode the battery too long and it lost power in the air. That's telling me the motor just isn't making enough extra thrust to keep this thing up and steady at low speeds.

I can post pics of both the larger and smaller slender deltas if necessary, but what motor and prop combo would be best for a 42-45" long slender delta, with a stipulation that it uses a high mounted pusher and the best clearance is for a 5" prop, what motor, prop, and battery combo would be best to give me some stability on high alpha and at low speed, and why? What makes it the appropriate choice? I think generally I want higher KV for a smaller diameter prop if I'm not mistaken, that's all I know.
 

Brett_N

Well-known member
#2
Weight is the bigger question. 5 inch blades are only going to do so much, and, from your previous experiences, seems like you already tapped out the thrust to lift ratio with the motors you already have.

Time to figure out how to turn a bigger prop.

My bigger flying deltas (usually 30"x20" ish) fly 2218's with 8" pusher props on 3S.

I'd aim for figuring out how to get something like that setup. 3S is fine, 30 to 40A ESC's. I dunno - put it on a pylon and notch the fuze?

Stability at high alpha goes out the window with a slender delta style wing, especially a pusher. It just doesn't work well. Look up the Kolibri High-Alpha for ideas. Wide wing at the rear, with a pusher prop, but the prop is way up at the front so it acts more like a tractor. High alpha stability is all about maximum airflow (from the prop) over your control surfaces.
 

Vimana89

Well-known member
#3
Weight is the bigger question. 5 inch blades are only going to do so much, and, from your previous experiences, seems like you already tapped out the thrust to lift ratio with the motors you already have.

Time to figure out how to turn a bigger prop.

My bigger flying deltas (usually 30"x20" ish) fly 2218's with 8" pusher props on 3S.

I'd aim for figuring out how to get something like that setup. 3S is fine, 30 to 40A ESC's. I dunno - put it on a pylon and notch the fuze?

Stability at high alpha goes out the window with a slender delta style wing, especially a pusher. It just doesn't work well. Look up the Kolibri High-Alpha for ideas. Wide wing at the rear, with a pusher prop, but the prop is way up at the front so it acts more like a tractor. High alpha stability is all about maximum airflow (from the prop) over your control surfaces.
Thanks for the advice! I am familiar with the Kolibri, it came out of a lot of cool discussions I had with @Dr. Looping Looie , and it is his special take on low aspect deltas. It is a good design and I already borrow somewhat from it with the high mounted pusher, and my smaller slender delta does excellent high alpha, it's just translating that to a bigger build.

You are right about the 5" props not being enough. Today I just retested this larger airframe with a new nose set up to run 6" props, and noticed a lot better slow speed performance. I'd say it needs like a 6x4 three bladed to really get that spare trust, but the 6x3x2 is all I got till I order some, and it's a bit underpowered, but working for now much better than the 5" props. Super big props aren't usually practical on very slender deltas. I think 6" will end up being great on this build , but would try an EDF for anything bigger, for one to keep the diameter down, two to maybe position the EDF and thrust tube to blow over my elevator more. When I get a chance I'll snap a pic of the big delta with the new nose, and share a vid of my smaller one doing the ridiculous high alpha.

I was thinking it may also be worth adding some small canards to the front of this one for better pitch stability too. I didn't see a need with the 30ish inch one, but the big version might gain a noticeable improvement.
 
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Vimana89

Well-known member
#4
Here's the video of the smaller V Sliver slender delta. High alpha stuff starts just past 1:10 and gets pretty extreme. This plane is a lot of fun, and at 17" span by 30" chord, with a light weight and big 2205 or 2806 and some aggressive 5" props, this one has a lot of spare power for slow flight and high alpha.

This is the large one, which is heavier and has a lower aspect with only an 18" span by over 40" chord. It is really drastic, to see how far I could sweep it and still be a practical flyer. This video is with the same 5x5.5x3 props I use on the smaller model, and doesn't perform well with this setup. The attached picture is after a couple crashes and repairs with the new nose and 6x3x2 prop.
I need to get s new camera then I can get some footage testing different 6" props.
 

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Hai-Lee

Old and Bold RC PILOT
#5
Your problems actually stem from the fact that you require a Specific Excess Thrust to maintain flight and stability. Aerodynamically the planes you are flying are very poor and without the effects of the high speed airflow caused by the prop they cannot withstand the non-uniform nature of free space/air unless flying at very high speed. You require powerful control surfaces to maintain control, (due to their short coupled nature), but without adequate high speed airflow the control surfaces are ineffectual! Sadly these issues are well known in such designs!

As you scale up the current design the amount of the control surfaces that will fall outside of the thrust cone will increase and this will cause a lessening of adequate control unless you also scale up the prop diameter accordingly. There are good aerodynamic reasons for the fact that "Slender Deltas" are rare among real aircraft and those examples that existed not only needed to fly and land quite fast but also had a poor reputation in operation in relation to their stability, (often causing a high crash rate than other types of aircraft).

Good luck with your ongoing experiments!

Have fun!
 

Vimana89

Well-known member
#6
Your problems actually stem from the fact that you require a Specific Excess Thrust to maintain flight and stability. Aerodynamically the planes you are flying are very poor and without the effects of the high speed airflow caused by the prop they cannot withstand the non-uniform nature of free space/air unless flying at very high speed. You require powerful control surfaces to maintain control, (due to their short coupled nature), but without adequate high speed airflow the control surfaces are ineffectual! Sadly these issues are well known in such designs!

As you scale up the current design the amount of the control surfaces that will fall outside of the thrust cone will increase and this will cause a lessening of adequate control unless you also scale up the prop diameter accordingly. There are good aerodynamic reasons for the fact that "Slender Deltas" are rare among real aircraft and those examples that existed not only needed to fly and land quite fast but also had a poor reputation in operation in relation to their stability, (often causing a high crash rate than other types of aircraft).

Good luck with your ongoing experiments!

Have fun!
This makes sense.Thanks for your response and advice👍. I am aware of most of these setbacks, but they can be worked around to make a fun and agreeable flying plane for sure. My main inspiration is the HP-115. It is the most slender delta ever flown, yet did not exceed I believe 200 something miles per hour, and could fly safely as slow as 80 mph. Every pilot that flew it said it was fairly easy and docile. Part of what it had going was a big long wing and not much fuselage(like my builds)which is a lot easier on loading than slender wings on a more bulky fuselage. A 6x3x2 is already a big improvement over any five inch prop,but underpowered.I think 6x4x3 should work well. I doubt it will have the high alpha to the extent of the small model, but should be very capable. I'm not expecting the slender delta planform to be some super aerobatic miracle plane that does everything, but I know I can make a solid flyer out of it. Even as is, it isn't terrible and can fly ok, but it does need that spare thrust to fly particularly well.
 

Hai-Lee

Old and Bold RC PILOT
#7
Your quoting the HP115 is expected but you missed a few details of the design. It had a biconvex airfoil to improve aerodynamic lift at low speeds and was relatively light with a very short difference between empty and AUW, it also had a very short endurance!

It was built for a specific purpose, (research into the landing characteristics of highly swept delta aircraft), and it is noted that the results of its test flying were incorporated in the Concorde which actually did not continue with the same aspect ratio or wing planform.

My original statements on the aerodynamics surrounding slender deltas remain and you will need to allow for the fact that air does not scale in your experiments/calculations. I am sure that if your models flew at the near the same speeds as the HP115 they would fly very well but until then you need to assist the control surfaces with prop wash to give adequate control.

Even with conventional model aircraft the wing loading MUST be lighter than the original or chances are it would only fly at relatively high speeds! Please continue with your experiments as you may have a unique aircraft or series in the end!

Have fun!
 

Vimana89

Well-known member
#8
Your quoting the HP115 is expected but you missed a few details of the design. It had a biconvex airfoil to improve aerodynamic lift at low speeds and was relatively light with a very short difference between empty and AUW, it also had a very short endurance!

It was built for a specific purpose, (research into the landing characteristics of highly swept delta aircraft), and it is noted that the results of its test flying were incorporated in the Concorde which actually did not continue with the same aspect ratio or wing planform.

My original statements on the aerodynamics surrounding slender deltas remain and you will need to allow for the fact that air does not scale in your experiments/calculations. I am sure that if your models flew at the near the same speeds as the HP115 they would fly very well but until then you need to assist the control surfaces with prop wash to give adequate control.

Even with conventional model aircraft the wing loading MUST be lighter than the original or chances are it would only fly at relatively high speeds! Please continue with your experiments as you may have a unique aircraft or series in the end!

Have fun!
I hear ya, what you say is definitely true enough and well reasoned. The wing loading on the smaller model is very good, the coefficient is less than 6, which puts it in the "trainers, park flyers, and 3d" category, which is surprisingly light. That, the thrust to weight ratio, and the closer proximity of the elevator to the thrust stream, as you explained, are the things that help this one perform pretty well and do high alpha. I have not run the math on the larger model :rolleyes:. I can imagine it's not quite as stellar.

I'll try running some more powerful varieties of 6" props on the big model, but I might also want to take a crack at something on the smaller end, like a 30" or so model with 5" prop that has a slightly reduced aspect to mirror the very extreme sweep of my larger model. I could go very light with an 1806 motor, 650 3s or perhaps even 2s. The general rule of thumb I hear with most planes is that increasing the scale makes them a bit easier and better performing, better handling, and more forgiving, but for the slender delta, the small scale might be easier in terms of delivering the oomph it needs at a very low weight and wing loading.

Thanks for your very thorough, thoughtful, and informative replies and constructive criticism! I am enjoying experimenting with this idea. I know it's not magic, and don't have unrealistic expectations for it. I like it for what it is, "warts and all", and working with some of its unique quirks and challenges and strengths and weaknesses is part of what makes it enjoyable and a good learning experience. The first 30" model that I already have as a resource is a good start, but I think I can do even better if I experiment at different scales and perhaps different control layouts.