How far forward should the prop go?

[e-geezer]

I now understand how to calculate neutral point position (having read Ollie's answer to the GWS Spitfire thread) and how that relates to wing and stabilizer areas and the centre of gravity.

Now I need to know this: What, apart from its effect on the centre of gravity, determines how far forward the engine / prop should go?

One author gives as a rule of thumb that the prop should be (1.5 to 2 times the wing chord) ahead of the wing's aerodynamic centre.

Must this be true? And why?

[Sparky Paul]

There are no immutable "shoulds" in aerodynamics.
The prop "should" be placed where it does the job intended for the airframe.
Here's two at the extreme ends of the spectrum.. poorly placed, both of them
The upper plane's nose moment was so short it required an enormous amount of ballast to achieve a flyable c.g.
The lower plane's motor was so far aft it also required a large amount of ballast to get a flyable c.g.
For a front-mounted prop, 1.5 x isn't a bad location.
For a pusher, as close to the trailing edge as possible minimizes the ballast situation.
For instance, for a photo plane, you want a slimer- motor behind the camera.
Therefore you go pusher.
It's all relative to the task...

[Bill Glover]

Originally posted by Sparky Paul
For instance, for a photo plane, you want a slimer- motor behind the camera.
Therefore you go pusher.

Not me! :)

[e-geezer]

But - aerodynamically - is getting the right centre of gravity the ONLY consideration in positioning a prop and motor?

[e-geezer]

... assuming a tractor prop placed somewhere near the centre-line of the fuselage, can I place the prop as close as I like to the leading edge of the wing - provided the centre of gravity of the plane ends up in the right place - or should I stick it out further forward for some other reason?

[e-geezer]

OK, I want the wing to be far enough back so it doesn't foul the folding prop...

[e-geezer]

... but are there any other factors on how far forward the prop needs to go?

[Sail 'n Soar]

In terms of for and aft, getting the CG right is the first priority. Personally, I think the 1.5 - 2.0 rule of thumb is overrated and missleading. The 1.5 - 2 times the cord length rule will generally result in a configuration that looks about right and is relatively easy to balance - for models with wings in the typical 5 - 7 aspect ratio range. It results in low aspect ratio winged models with noses that are too long and in high aspect ratio winged modes with noses that are too short.

The second priority is the thrust line. A high mounted engine, e.g., pylon mounted, will produce a pitch down moment and a low mounted engine will produce a pitch-up moment. Both of these thrust line configuration will result in model trim changes with throttle changes. The trim change with throttle change tendency can be at least partially offset with up or down thrust. For example, a high wing model will require more down-thrust than a lower wing model to minimize the throttle induced trim changes.

[Bill Glover]

Practically, 99% of the time the position of the prop is determined purely by where you need components like the motor, pack, (fuel tank for IC) etc. to go in order to achieve the correct CG position. Then sometimes considerations like ground clearance etc.

Aerodynamically, the prop does actually act like a vertical surface (side area) ... so gives a stabilising force if it's right at the back and a de-stabilising force if it's far forward. But the size/shape/position of the fin, shape of the fus., nose and tail moments, etc. are all part of the same equation ... so I wouldn't worry about it. There are loads of planes (model and fullsize) with the prop miles ahead of the CG, it just means they need a fractionally larger fin than they would have done otherwise.

[e-geezer]

From these replies, it looks as if I can have the nose as short as I like, provided the c/g is OK. There's no MINIMUM distance from the l.e. to the prop?

[vintage1]

Ok...there are a FEW issues, to do with airflow over the wings etc. that make a difference. Its not a BIG diffrence tho. In general, its all about making the ruddy thing balance.

Pushers are more 'ideal' since the prop wash doesn't hit the plane - thats better efficiency, but they got a bad reputation in the early days because the engines used to kill the pilots in nose overs.

3D flyers make use of the propwash over the wings and tail to get control authority at zero airspeed. a 3D pusher simply wouldn't work I suspect..So for 3D you want (as well as for other reasons) a biggish prop not too far in front of the ailerons.

But mainly its the CG.

[Bill Glover]

Originally posted by vintage1
Pushers are more 'ideal' since the prop wash doesn't hit the plane - thats better efficiency

Actually ... as a general rule, I believe pushers are supposed to be less efficient. The fact that the prop is operating in 'dirty' air (the wake from the airframe) normally gives higher losses than a tractor installation where the propwash hits the airframe. But I'm sure it depends on lots of factors.

[KillerWatt]

Until it's no longer attached to the fuselage, and then back-up about an inch or so.......... kw

[BMatthews]

E-geezer, you totally confused yet?...... :D

Out of all this I gather your main concern at the moment is getting enough clearance for a folding prop. Within the limits of "conventional" design just use any nose length that puts the motor in a good spot to balance the model. As long as you avoid extremes you'll be good to go.

If your prop is a very large one then there's a few gearbox units that use an extension for placing the prop further forward than the motor location. For example I've got an older Leasure gear drive that uses a 3 inch extension built into the gear box. I don't see too many of these around nowadays and I'm not parting with mine for the same reasons as you're finding now.... :D Some of the planetary drives are a little longer due to their design and need to have reasonable bearing spacing. Perhaps those would allow you to keep the motor back far enough for good balancing and yet keep the prop hub forward enough for prop fold clearance.

The only rule is that there IS NO rule..... usually.... :D

[e-geezer]

Thanks all. No minimum distance it is then. So this would be alright:

http://www.databasedesign.co.uk/modelplanes/sketch01.jpg

?

[BMatthews]

Technically that is fine. Practically it won't work for most outdoor RC models for the reasons we've all posted above.

But I've done just what you show there on an indoor rubber free flight model where the CG is back around the trailing edge of the wing. It was actually STILL too stable but I couldn't move the wing any further forward without hitting the prop.

[Bill Glover]

Of course the IFO design has the prop virtually on the LE, and that seems to fly ok! ;)

[e-geezer]

So why does Andy Lennon in his 'R/c Model Aircraft Design' (p125) give this '1.5 to 2 x chord' rule?

[Sail 'n Soar]

Originally posted by e-geezer
So why does Andy Lennon in his 'R/c Model Aircraft Design' (p125) give this '1.5 to 2 x chord' rule?

I have his book. You will find the "1.5 to 2 x chord" rule given in other model aircraft design books as well. That said, if you look at the drawing on your page 125 reference, you well see that Andy uses a quarter cord CG as the reference point, not the leading edge.

There is nothing wrong with using that rule. But there is nothing magic about it either. If you check out Andy Lennon's "Basic flying boat proportions" on page 126 you will notice that the prop is within ~1/4 chord of the leading edge. As mentioned consistently in this thread, the bigger driver is achieving a reasonable CG. If you follow Andy's design proportions (vs. rules), you will have a greater chance of building a model, which will be relatively easy to balance correctly, will "look about right" and will most likely fly reasonably well.

Bottom Line: Following Andy's rules is something like baking a cake from a recipe. If you follow the recipe and don't substitute ingredients you are likely going to be successful even without knowing the chemistry of baking soda.

Are you a cook or a chef?

[e-geezer]

Thanks for the common-sense and analogies.

[HELModels]

e-geezer,
Your diagrams look alot like those little rubber powered models you buy for a $1.00. As a kid, I used to play around with moving the wing way forward or way Aft(aft looked kind of like Gee-Bee to me). I would then use modeling clay to make it flyable and watch the changes in flight characteristics.

This might still be a useful exercise and only costs a $1.00 or so.

[e-geezer]

ElectroStorch,
Yes I did (and do) that too, going back to boyhood in the 60's. But I can never decide if I have taken absolutely everything into account. Hence the appeal for other peoples' experiences.