First projet and need your help!

[AirFlare]

Hello everyone!

I just discovered this awesome forum, and as a beginner I would need your experience and suggestions for something.

I am with a group of 5 people, and we are participating in a cargo plane competition. The goal here is to lift the maximum weight, while having a plane as light as possible. After some discussions, we decided that our target goal would be to have a 1lbs plane, able to lift a 1.8lbs weight (so overall the plane would be ~3lbs).

Now I am responsable for the engine part in our projet, and I was wondering what engines you guys would recommend for such a projet? (An engine as light as possible). I know I can do simulations on eCalc to find the best propeller type associated to my engine, but I have no idea what type of engine would be suitable for our plane, since it's my first time ever doing this.

Would you guys have any suggestions, comments or tips for me?

Thank you in advanced for all your answers, any help is greatly appreciated.

PS: Sorry if there are a few english mistakes, english is not my native language.

 

[Hai-Lee]

Unfortunately your specification is extremely vague.

For instance is the design to be a monoplane? Is it to be a single engine or a multi? Will the wing design be capable of supporting the engines with the required power/thrust? How much of the planes all up weight can be devoted to electronics Etc? Will the finished plane be required to have good low speed performance or would a very high take off and landing speed be acceptable? What propeller ground clearance has been allowed for? The list goes on!

Having said all that I do have a very cheap motor that I do like for slow flying models because of its price and thrust/current figures.

I use a few of the 2213N 800Kv from HK. It gives great thrust using large slowfly propellers and has a maximum current of around 10A.

Using 2 of this motor it would be possible to have a thrust total of over 1KG, (close to 1.5KG), using 2 12A ESCs and a single 3S battery of 1000mA to 2200mA.

My second preference would be the 2205C 1400Kv also from HK which can deliver similar thrust again using slowfly propellers and again at less than 12A, (Only just). A bit more expensive and a little lighter.

A little more info on the design might bring out better recommendations from the forum members!

If the wing mounting is not ideal then a push pull setup of the 2 motors would work well. Though based on the scant description supplied I would think a twin boom tail design like the German "Millipede" or similar is appealing with functional flaps of course.

 

[quorneng]

AirFlare
In many respects the actual choice of the motor is determined by the characteristics of the plane.
Before selecting the motor you really need to know what the general shape of the plane is going to be and its wing loading as this will largely determine the speed it will fly at and the power required to do it.
Its flying speed has a significant impact on the characteristics of the motor and propeller to give the best performance.

You don't actually say for how long or how far the plane has to carry the load.
In reality simply designing the structure of a plane that can safely carry almost twice its own weight is quite a challenge and then it also has to be aerodynamically controllable and presumably be able to take off and land.
Does any of the team have RC plane experience?
I would suggest holding off selecting a motor until at least the air frame design is finalised and what it is going to be made of.
We may then be able to advise on a suitable motor/battery/speed controller combination.

 

[AirFlare]

Thank you guys for your fast detailed answers and support!

You are absolutely right, I completely forgot to include basic information on our plane. My coworkers (which are experienced) are currently working on the body design of the plane, and they asked me to do a preliminary check on the engine types that could suit us. So let me give you all the available details I have.

We must build a plane that can fit in a cylinder box (70 inches length and 6 inches diameter max). Of course you would need to assemble/diassemble it. The smaller it is, more bonus points we can get. This is the only restriction on the size of the plane itself. In addition, we have a restriction on the battery: It must be a Lithium Polymer battery, with max 3 cell 2200mAh. Finally, the total score of the competition is given by the Total weight/Empty plane weight. Now that's pretty much it for the details of the competition.

With that in mind, so far we decided that we would only have 1 engine (unless you guys recommend 2?) and if possible to have the less electronic weight as possible. Additionally, we estimated that our plane would fly at a speed of 11m/s. Finally, there is no ground take off, we will throw the plane from hand, and it will do a "belly" landing in the grass.

Hope this will be enough to give you guys more concrete idea of what can help me. Thank you again for your help and support, I'm definitely considering making a projet of my own at some point when I see how helpful and nice you guys can be.

Just a last note, the price of the engine(s) should not be too important, as we have some good sponsors and funding from our school.

 

[quorneng]

My first comment is that is an interesting challenge. Does the competition set what a 'flight' has to consist of? Normally it is at least a return to a point no more than a set distance of the starting point.

Next hand launching a plane that flies at 11m/s (24 mph) will not be that easy unless it has sufficient 'spare' thrust to accelerate it to its required flying speed before it sinks to the ground and spare power means extra weight.

Lastly as your points are based on the ratio of empty to loaded weight not the size of the actual figures you should consider the 'size scale factor'. The smaller anything is the relatively stronger it becomes. A matchstick can support a great many times its own weight. A 20M wood beam of the same proportions struggles just to support its own weight.

If you look at the micro RC type plane that only weighs, say 1 oz 30g complete, structurally it can easily carrying that weight again. The down side is that aerodynamic efficiency falls at small sizes, but then I am not entering the competition!
There is no technical benefit I can think of to use two rather than one motor.

 

[Hai-Lee]

We must build a plane that can fit in a cylinder box (70 inches length and 6 inches diameter max). Of course you would need to assemble/diassemble it. The smaller it is, more bonus points we can get. This is the only restriction on the size of the plane itself. In addition, we have a restriction on the battery: It must be a Lithium Polymer battery, with max 3 cell 2200mAh. Finally, the total score of the competition is given by the Total weight/Empty plane weight. Now that's pretty much it for the details of the competition.

If you are really looking to fit everything into a 6" diameter cylinder then the wing is going to have a maximum cord of 6" and the maximum span possible to lower the wing loading! Interesting problem! Whilst I am aware that you are not tasked with the planes design I would say that a multi-wing layout, (Fully cantilevered Biplane), would ease the weight Vs structural strength problem considerably. With the aim of locating the load between the wings, (on the CG).

If you are required to make the plane a monoplane then consider designing the wing to split spanwise and have the 2 halves slide into each other along the main spar. A cord of 9" could then be fitted into the 6" cylinder.

With the loaded weight being so high I would also recommend a broad span aerofoil profile horizontal tail to help broaden the CG range a little. With a such a massive load any slight shift in the load away from the required CG could be catastrophic.

You do not mention the size and shape of the load. or even whether the load must be carried internally in the aircraft or if it can be carried externally!

As for saving weight and space a Pod and Boom design would be a method of saving weight.

Finally Just a recommendation to keep the weight to an absolute minimum, the materials I would select are Ply, Carbon Fibre and Balsa With plastic film covering of the wings.

 

[Hai-Lee]

There is no technical benefit I can think of to use two rather than one motor.

Technical benefits abound.

In the case of a heavy load carrying aircraft the fuselage tends to be large in cross section and therefore can impede airflow from a single fuselage mounted tractor style engine installation. A wing mounted multi engine installation's thrust is not impeded by the fuselage regardless of the fuselage'e cross-sectional area.

A large single propeller driven source of thrust will have a large "P" or torque factor which if the aircraft is large and to be hand launched can provide strong roll forces at a time when the wings might not give sufficient roll control to cancel out the effect. A wing mounted multi-engine design can use motors that rotate in opposite directions and the "P" factor or torque effects are almost zero.

Increasing the motor size and the prop diameter is more impinging on ground clearance than the wing mounted motors of a twin engine design. With the wing on a heavy load carrying design normally being mounted centrally to high on the fuselage the higher thrust line can allow for greater propeller diameters.

A single fuselage mounted motor normally does not avail itself for such high features as "Blow" Flaps or ailerons. A wing mounted multi-engine design can provide a strong thrust over flaps and ailerons at and below take off speed and can allow for functional control surfaces at speeds even below stall speeds.

During an unfortunate loss of control and resultant impact a heavy plane can damage or destroy a large, expensive, (relatively), motor installation as well as the usual fuselage and wing damage. A wing mounted Multi-engine design will NOT stop out of control incidents and ground impacts but the motors will not be the "Ground contact point" and will suffer less damage. The fuselage and wings are the low cost parts in the equation.

There are technical advantages in all designs and some designs are better suited to certain applications than others. If that was not the case then all planes would be similar in design!

:black_eyed:

 

[quorneng]

Hai-Lee
Whilst I don't disagree with your list there are disadvantages as well.
A single big prop is considerably more efficient than a smaller one, particularly at model speeds, and up to a certain size so is the motor itself resulting in a overall lighter power plant for a given level of thrust.
If you are trying to achieve the maximum empty to loaded ratio, as the OP is, then this is very important attribute.

 

[kdobson83]

Just a thought? Does it have to be a plane YOU designed? Lol Flitetest's Mini Guinea weighs in at a pound with battery, and I could only imagine it's payload would be much more than u need. Plus, if u used the emax motors reccomended I see no problem with speed. Don't know if it fits ur size parameters though...

As far as motors go, the 24 gram Hextronic motors are nice. On a 8045 prop on 3s they do like 450 grams of thrust on a 8amp ESC. The Blue Wonder would be the name brand of this one.

Anyway, goodluck.

 

[Hai-Lee]

Hai-Lee
Whilst I don't disagree with your list there are disadvantages as well.
A single big prop is considerably more efficient than a smaller one, particularly at model speeds, and up to a certain size so is the motor itself resulting in a overall lighter power plant for a given level of thrust.
If you are trying to achieve the maximum empty to loaded ratio, as the OP is, then this is very important attribute.

Yes, I agree that a large prop is more efficient especially at low speeds hence we have a great range of "Slow Fly" props to suit various motors from the smallest to some really large and powerful motors.

As the original specification was a "First Project" and had severe size restrictions and a weight limit I was more interested in a heavy lift platform which had as many aerodynamic advantages as possible as well as a "beginner" level of controllability.

If I was designing a racer or combat design I would definitely power the aircraft with the largest prop motor combination I possibly could and streamline the fuselage as best I could. When designing a heavy lift aircraft I would want a maximum lift design with flaps and even prop driven airflow across the wings and flaps to provide extra lift especially for low speed takeoffs and landings.

Too many Newbies suffer from not FULLY understanding all of the forces at work in their design and often get discouraged as a result of advice given when they have not fully explained their design parameters.

The entire internet is littered with the wreckage of newbies trying to fly their new creations either at below stall speed or with severe "P" factor/torque roll characteristics. I was one of them for many years until I listened to advice in an advisory role only and worked through the problems myself balancing what were sometimes contradicting requirements.

I will repeat that your statement is correct when seen in isolation but if you are able to throw a 3KG plane made of foamboard, over your head at close to 30 mph to ensure the model has adequate aerodynamic control to counteract "Torque Roll" then you should see about a career as a major league pitcher because I know I definitely could not do it.

 

[quorneng]

Is there a danger that a critical bit of this competition as outlined in AirFlare's post (#4) is being over looked?

Finally, the total score of the competition is given by the Total weight/Empty plane weight.

If this is indeed the only equation used in scoring then the actual weight of the plane is not a factor.

In this sort of competition it pays to not only understand the limits but also the factors that determine the scoring.

 

[Hai-Lee]

The only equation apart from the physical size restraints and payload.

When designing any flying model the structure must support the load including the motor, battery, etc. This support is directly responsible for the majority of the airframe weight. A higher stress load on any part normally requires that part to be stronger and therefore heavier.

With a model in which the majority of the weight is in the fuselage and the wing provides the majority of the lift then the wing must have the structural strength to support the weight of the fuselage structure, (Including cantilevered motor), through the wing to fuselage joint or connection. With the majority of the weight being in the fuselage then the wing affixing apparatus must be strong enough to support the loads multiplied by the maximum "G" force anticipated during maneuvering.

Additionally the fuselage must be also designed to cope with the stress load of the heavy cantilevered motor mounting against the same "G" force expectations. As I tried to explain earlier removing weight from the fuselage and moving it to the wings themselves makes the fuselage structural strength requirements far less as well as the required strength and hence weight of the fuselage to wing jointing mechanism.

A wing on a conventional aircraft design has to centres of lift span-wise which calculated by the addition of all lift across the wing span and finding the centre of the lift. normally this is approximately 40% of the span from the wing/fuselage union. This effective centre of lift can be used to calculate the forces applied by lift which are effectively trying to fold the wings together. The small this force then the lighter the wing structure can be.

With wing mounted motors the weight of the motor installations are directly subtracted from the lifting forces generated by the wing and allow for a lighter wing structure at the wing joint/s. Further more as the wing is most likely a cantilever design the effective internal structure of the wing, (Spars), can also be lighter.

Fore and after pressures on the wing structure are far less than that required of a single motor installation of the same power and the shorter moment from the spar to the motor keeps the weight down and brings the CG closer to the centre of lift (Cordwise).

One other advantage is that designed properly there is no real need for a bulky fuselage at all assuming that the load could be attached to the underside of the wing centrally at the appropriate, (CG balance), point, though without a fuselage and depending on the tail weight and moment the motors could be extended forward, (with structure), to balance the aircraft.

Why have a fuselage if it is not needed? A fuselage is dead weight if the load does not require it unless you need it and its inherent weight to support a large motor and larger control surfaces on the tail. As for the old CHERRY that the long nose on a fuselage with motor nose mounted helps with CG! Well so does sweeping the wing forward or having a TE that sweeps forward. If we are designing based upon weight of structure then surely we should engineer our way around the problem by removing weight wherever and however possible and in this exercise maximise lift which in itself means less structure required because of the resultant reduction in required wingspan.

Without a bulky fuselage in the packing container we have far more room for additional lifting surfaces if required!

 

[Hai-Lee]

Before the debate takes another turn I will add that an external load could be supported using a couple of velcro loops, or straps attached internally to the wing structure. And to further distribute the load to a lightweight built wing is the use of a few struts or in the above example some lengths of fishing line attached to the wing spar and connected to the load supporting velcro with a couple of velcro tongues could be used.

This would allow the load to be more evenly distributed across the wing spar and therefore allow for a far lighter, more uniform and simpler build. When not carrying the load the "Struts" and the load support could be taped flat against the underside of the wing.