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Apr 05, 2018, 05:36 AM
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Self designed twin boom pusher rc plane for endurance

Hi. I'm new to this forum. I need some help in improving my design. My team is going to compete in a national rc plane competition. We are rookies and this is only our second plane. We have designed this plane completely on our own. The aim is to make a design which can perform basic aerobatics (Roll, loop etc) in one mission and then fly for the longest duration in another mission. The constraint is that we can only use 3S lipo with 2200 mah capacity at max. Our wing design is half rectangular with some taper from mid-span towards tips. Other design details are:

1. Aspect Ratio: 8
2. Wing loading: 16-18 oz/sqft (estimated)
3. Weight 2.0 kg-2.3 kg (estimated)
4. Wingspan: 75 inches
5. Cruise Speed: 25 mph
6. Chord: 11 inches (rectangular part), 7 inches (tips)
7. Airfoil: N10 (wing), NACA 0012 (Stabilizers)

We are planning to use Ntm prop drive 3542 1250 KVA brushless out-runner with 12 x 6 propeller. We would be happy with 1.6 kg-1.8 kg thrust. Are we going in right direction in the propulsion department? What better can be done in this regard, considering the fact that we need long endurance and maneuverability?
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Apr 05, 2018, 05:59 AM
Wake up, feel pulse, be happy!
Piece's Avatar
Well you're on the right track for sure. That wing looks nice.

Are you allowed to switch props between the various events? The battery size (i.e. total available energy) is rather small for the size of the plane. This isn't a problem for either specific task, but the ideal props for each mission are somewhat different. You need relatively little thrust for endurance flight, which leads to a smaller prop with coarser pitch to achieve a usable pitchspeed at a lower overall power draw. For aerobatics, it's the opposite - you want a bunch of thrust to pull through a loop, and it's no big deal if you chew up all your available electrons in the process.

For the endurance side, I would be inclined to use something like an 11x10, whereas for the aerobatics, a 13x6 (or whatever pleases thee - I'm just extrapolating from your 12x6).

If you can't change props, then I would err on the side of endurance by running relatively high pitch (e.g. 12x10) and paying attention to throttle usage. Here the goal would be a setup that probably pulls "too much" current at full throttle, but flies happily at 1/2 throttle. You can push a motor pretty darn hard for the 10 seconds it takes to run up to a loop. The ESC should be the strong link in your system.

And don't crash it!
Latest blog entry: Jeti ESC resto-mod
Apr 05, 2018, 06:06 AM
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Extreme Sports's Avatar
Very nice plane, but is there any particular reason why you chose the twin boom configuration? Its not wrong, but I can't see what advantage you get from it for your particular goals.

But the bit that really jumps out is your expected AUW. A 2200 3S lipo is not going to get you much duration on a 2.0-2.3kg plane. At a guess, not much beyond 5 mins before you start to damage the battery. Your target should be under 1kg, probably closer to 700g.

Duration usually suggests a glider like airframe. Long, thin wings, minimalist fuselage. Designed carefully, this will do all basic aerobatics...just don't expect too much from the rolls. If you go this route, you could drop the motor size dramatically - from the ~150g of your suggestion to under 50g, maybe even below 30g.

I used to have a Drifter II glider that I (over-) powered with a cheap 39g motor and a 2200 3S lipo - AUW under 700g. It flew away one day due to a bug in INav. On that flight I only had a 1300mAh in it, but I calculated that it would have remained airborne for at least an hour before the battery died. It could do snap rolls, and in theory, by removing the polyhedral and adding ailerons I could have performed axial rolls. I'd argue that such a configuration would run rings around your twin boom in the competition you refer to .
Apr 05, 2018, 06:20 AM
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Extreme Sports's Avatar
For comparison, I have a very similar twin boom, with an almost identical motor (KEDA 3542 1000KV). It is slightly larger (2.3m span and 2.5kg AUW) and has to carry a pixhawk, other UAV paraphernalia and twice the battery weight as yours, but best stable cruise I can get is at about 5-6A. This seems on par or better than the Skyhunter, which is closer in size to yours. At 6A cruise, higher current for take off, and 80% use of capacity, your Lipo will struggle to get you a 10 min flight. That's not really 'endurance'.

EDIT: I guess what I am saying is that the twin boom pusher is a strange choice for your task. It is neither particularly aerobatic or efficient. Unless of course the rules allow you to put multiple 2200 Lipo's in parallel...then the twin boom is a great configuration as the big box fuse can be packed with batteries. That is what the Skyhunter folks do - up to 20,000mAH IIRC. I've calculated I could get 2h, >100km out of mine this way
Last edited by Extreme Sports; Apr 05, 2018 at 06:38 AM.
Apr 05, 2018, 06:38 AM
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Thread OP
First of all, thanks for all the responses. I forgot to mention earlier. We cannot change props between missions. Another constraint is that the plane should be a pusher type. So, we decided to make a twin boom as a single boom under the propeller could have tail-strike issues during take-off roll or even landing. Also, it should get airborne within 60 ft and the weight should be no more than 3 kg. The aircraft will carry a 300 grams payload internally during all missions. And the weight that we have estimated is after including the payload. We are trying to keep weight as minimum as possible. Well, maybe we can reduce weight in the wing because I think we have over strengthened it a bit?
Last edited by Mohid94; Apr 05, 2018 at 06:43 AM.
Apr 05, 2018, 06:58 AM
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Extreme Sports's Avatar
OK, so the pusher is a design constraint you have to live with.

Here are two other data points:

1) Swept wing twin boom pusher - 1.65m span, 2200mAh Lipo and this motor AUW about 700g

2) Polyhedral twin boom pusher - 1.6m span with a 2200mAh Lipo and this motor: AUW was about 700g as well - can't quite remember as this was a once off experiment.

Bear in mind that all of my planes are built from correx. You should be able to build considerably lighter with foam or balsa and film if longevity is not your primary criteria.

But the main point is that these two use MUCH smaller motors than you propose. Together this suggests that you can save considerable weight on the airframe, the motor and probably the electronics as well. Less weight translates directly into longer endurance.

Set yourself a design target of 600g
Apr 05, 2018, 07:13 AM
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Thread OP
Thanks for your guidance. Yup, as u said, we should aim to reduce weight as much as we can.
Apr 05, 2018, 09:04 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Originally Posted by Mohid94
... We are rookies and this is only our second plane. ...
Some well-structured reading and handy e-tools for rainy/windy days.
Will save you, and us a lot of questions. Notably the 'what went wrong?' kind of questions
Will also prevent you from burning up several controllers and/or motors and/or battery:
E-flight primer and tools

And please, do your RC equipment, wallet, ego, battery, controller, motor, house/garage/car a big favour ... get a watt-meter. It will more than pay for itself, will save you at least one fried motor and one fried controller. Will also help you finding the best setup.
And keep watt-/multi-meter wires short
too long wires batteryside will kill ESC over time: precautions, solutions & workarounds

Vriendelijke groeten Ron
Apr 05, 2018, 10:44 AM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
Originally Posted by Mohid94
We are rookies and this is only our second plane.
Was your first plane successful ?

Do you have an experienced RC pilot who will be flying your plane ?

For maximum endurance , a pusher design such as this ↓ one would probably be best .
Last edited by balsa or carbon; Apr 05, 2018 at 10:52 AM.
Apr 05, 2018, 12:08 PM
Registered User
Extreme Sports's Avatar
Yup, that is the sort of 'pusher' glider I had in mind

At the other extreme of the spectrum, take a look at this thread - a 845mm span store bought foamie, with a distance record of 100km!!. All on flying weights around 400g! Granted, they are using a different battery technology (18650 Li-Ion cells), but I bet you'd easily get over an hour's duration with a standard 3S 2200mAh Lipo - much the same weight as these guys fly:

These are all just suggestions, intended to be helpful. But I think it is clear that your initial design choice of a large twin boom pusher is not going to be very competitive, no matter what motor and prop you go with. Do you still have time to revisit that decision?
Last edited by Extreme Sports; Apr 05, 2018 at 12:14 PM.
Apr 05, 2018, 11:27 PM
B for Bruce
BMatthews's Avatar
In the framework picture of the first post what I don't see is a secondary spar out to the booms. You need TWO spars to support the booms well through the center section. The main can be one of them but you want something more than the trailing edge or the sheeting (if it is fully sheeted) to support the spar some distance behind the main spar. A bit of a rough landing and the torque on the booms will bust away the portion behind the main spar pretty easily if there's no rear spar to hold the booms in place.
Today, 05:53 PM
Registered User
The power required to fly is almost directly proportion to weight and speed so for endurance you need light weight and slow speed.

You are stuck with the weight of your 2200mAh 3s LiPo at about 188 g so the solution is to make the rest of the plane as big and as light as possible.
With a really efficient structure the battery should constitute about 33% of the plane's ready to fly weight.
If you use various materials to their best advantage in other words everything is designed to be just strong enough for its duty and with a span of say 1200 mm it should end up at around 500g.
If it is aerodynamically efficient and has a well matched prop and motor it should then be able to fly for over 2 hours on a 2200 mAh 3s battery however it is likely to require much testing and redesigns before you will achieve such targets.
It took me the best part of a year testing and several rebuilds but this finally did 2 hours (my limit not the plane!) on a 2s and with capacity to spare.
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Yes it can loop and roll!

What sort of duration target had you in mind?

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