I'm not sure whether or not this is best place for this post so if one of the moderators wants to move it feel free to do so!
Over the last few months I have been conferring with other modellers on the subject of model / engine / prop combinations, the discussions really spurred on by my experience, or lack of it, with the SE5a; I thought a précis of the conversations might be of interest.
I should point out right at the beginning that none of us have any qualifications in aerodynamics, but we do have between us many years experience of model flying, and a modicum of common sense! If any aerodynamicists read this and would care to comment, their input would be more than welcome.
Firstly the blindingly obvious; for anything to fly it has to have enough lift to overcome gravity, therefore:
The (Scale) Model:
We cannot do a lot with the wing design, the aerofoil section could be changed somewhat if we’re not too bothered about scale fidelity, but that’s about it.
Speed is also important, basically anything will fly if it goes fast enough, but we really want the model to fly as near to a “scale” speed as possible.
That only leaves the weight; so the idea is to build as light as possible commensurate with sufficient strength. This is a bit of a juggling act, in my opinion it’s no good having a model that flies beautifully if it ends up in a bin bag after the first not perfect landing! A model a bit on the heavy side will still fly, although it will have to fly faster to provide the extra lift required to overcome the extra weight.
I’ve seen a few “3D” models virtually explode in incidents that would only require a few hours work and some cyano to repair for the average sports / scale model.
The Engine:
Without sufficient power the model will never attain flying speed, whether this be a “scale” speed or faster for a “heavy” model, but if too much power is available it can always be controlled by using less throttle. This is why I think it’s a good idea to use the biggest engine that will fit in the model; in fact I actually design my models to be as small as possible whilst still allowing a fully enclosed Laser 70.
So far everything is pretty straightforward; build light and fit a big engine! But now things begin to get a bit more complicated.
The Propeller:
In order to move the model through the air the engine power must be converted into thrust by the propeller. For any given power there are 2 extreme ways to produce the thrust, either a large area of air moving slowly or a small area of air moving quickly; and of course all points in-between. The model can never move forwards faster than the air is being pushed backwards by the propeller, therefore for speed a small area of air moving quickly will be best, but at the expense of acceleration and pulling power, a large area of air moving slowly produces better initial acceleration and pulling power, but at the expense of final top speed. We’ve all seen the tests were a car out accelerates a jet plane, for the first few hundred yards at least!
There are 2 important measurements, diameter and pitch, which affect the amount and type of thrust a propeller develops. Firstly the diameter, obviously as it increases the area of air moved by the propeller also increases; the pitch is a measurement of how far the propeller would move forwards during one complete revolution if it was 100% efficient, which of course it isn’t! So the higher the pitch the quicker the air is pushed backwards at any given revs.
The type of thrust needed depends on the type of model; a “slippery” streamlined model can use a small diameter high pitch prop and it will eventually attain a high speed, but a “draggy” un-streamlined model would never get up to speed. In fact the drag is what limits speed far more than weight and once again there is little we can do to reduce it on a scale model.
So for a “draggy” model like the SE5a a large diameter propeller seems to be the way to go, the trouble with large diameter propellers is that more of the engine power is “wasted” in just turning the weight of the propeller, thus reducing the revs and so the models possible speed. Reducing the pitch will help to re-gain some of the lost revs and enable the engine to operate nearer to its optimum revs but at the same time it will reduce the speed for any given revs so may not in fact be helpful.
Although we know the general principles for choosing a prop: small diameter / high pitch for speed, large diameter / low pitch for pulling power, the correlation between diameter and pitch remains pretty much a mystery. As with many aspects of modelling it is a case of starting with an educated guess and progressing via trial and error and just hoping you’ve still got a model at the end of it!
Over the last few months I have been conferring with other modellers on the subject of model / engine / prop combinations, the discussions really spurred on by my experience, or lack of it, with the SE5a; I thought a précis of the conversations might be of interest.
I should point out right at the beginning that none of us have any qualifications in aerodynamics, but we do have between us many years experience of model flying, and a modicum of common sense! If any aerodynamicists read this and would care to comment, their input would be more than welcome.
Firstly the blindingly obvious; for anything to fly it has to have enough lift to overcome gravity, therefore:
Lift > Mass
As we aren’t talking about airships, hot air balloons or spacecraft we can expand the equation somewhat. Lift is generated by the wing and is dependant upon the design of the wing and its speed through the air; also mass can be replaced by weight, therefore:Wing Design ≈ Speed > Weight
Now that we have identified the variables we can look in detail at what we can and cannot do to increase the likelihood of our models actually leaving the ground.The (Scale) Model:
We cannot do a lot with the wing design, the aerofoil section could be changed somewhat if we’re not too bothered about scale fidelity, but that’s about it.
Speed is also important, basically anything will fly if it goes fast enough, but we really want the model to fly as near to a “scale” speed as possible.
That only leaves the weight; so the idea is to build as light as possible commensurate with sufficient strength. This is a bit of a juggling act, in my opinion it’s no good having a model that flies beautifully if it ends up in a bin bag after the first not perfect landing! A model a bit on the heavy side will still fly, although it will have to fly faster to provide the extra lift required to overcome the extra weight.
I’ve seen a few “3D” models virtually explode in incidents that would only require a few hours work and some cyano to repair for the average sports / scale model.
The Engine:
Without sufficient power the model will never attain flying speed, whether this be a “scale” speed or faster for a “heavy” model, but if too much power is available it can always be controlled by using less throttle. This is why I think it’s a good idea to use the biggest engine that will fit in the model; in fact I actually design my models to be as small as possible whilst still allowing a fully enclosed Laser 70.
So far everything is pretty straightforward; build light and fit a big engine! But now things begin to get a bit more complicated.
The Propeller:
In order to move the model through the air the engine power must be converted into thrust by the propeller. For any given power there are 2 extreme ways to produce the thrust, either a large area of air moving slowly or a small area of air moving quickly; and of course all points in-between. The model can never move forwards faster than the air is being pushed backwards by the propeller, therefore for speed a small area of air moving quickly will be best, but at the expense of acceleration and pulling power, a large area of air moving slowly produces better initial acceleration and pulling power, but at the expense of final top speed. We’ve all seen the tests were a car out accelerates a jet plane, for the first few hundred yards at least!
There are 2 important measurements, diameter and pitch, which affect the amount and type of thrust a propeller develops. Firstly the diameter, obviously as it increases the area of air moved by the propeller also increases; the pitch is a measurement of how far the propeller would move forwards during one complete revolution if it was 100% efficient, which of course it isn’t! So the higher the pitch the quicker the air is pushed backwards at any given revs.
The type of thrust needed depends on the type of model; a “slippery” streamlined model can use a small diameter high pitch prop and it will eventually attain a high speed, but a “draggy” un-streamlined model would never get up to speed. In fact the drag is what limits speed far more than weight and once again there is little we can do to reduce it on a scale model.
So for a “draggy” model like the SE5a a large diameter propeller seems to be the way to go, the trouble with large diameter propellers is that more of the engine power is “wasted” in just turning the weight of the propeller, thus reducing the revs and so the models possible speed. Reducing the pitch will help to re-gain some of the lost revs and enable the engine to operate nearer to its optimum revs but at the same time it will reduce the speed for any given revs so may not in fact be helpful.
Although we know the general principles for choosing a prop: small diameter / high pitch for speed, large diameter / low pitch for pulling power, the correlation between diameter and pitch remains pretty much a mystery. As with many aspects of modelling it is a case of starting with an educated guess and progressing via trial and error and just hoping you’ve still got a model at the end of it!
Comment