Freewhwwling impellers/props

[Scrooge Howard]

I read recently that using low throttle inputs on E.D.F`s, such as when landing increases the drag, (& could in fact be employed as a method of "Air braking" ), above that which would be experienced, with the throttle closed & the impeller merely freewheeling. Similar statements are often made regarding "Prop` driven" aircraft , but in my "Embryo" days of aeromodelling, (1940s, Ajax, Achilles etc),a "Freewheeling Prop" was considered to be an aid to reducing drag on the glide not increasing it.
There must be more to all this than meets the eye methinks!.

[Mike Rolls]

I raised that point on another forum a while back without getting a definitive response. Thinking it over - we used to use freewheelers aas they gave less drag than a fixed prop. However, by 1952 or so the folders were really taking over and the featherers were also known, and in order of decreasing drag it seemed to be:
Fixed
Freewheel
Feather
Fold.
There is also the point that on rubber models the prop is far larger, in proportion to the airframe, than fro power models so propeller drag is a more significant element of total drag.

If you think of multi-engined full size with an engine out - the dead engine (hopefully!) has its blades feathered; it isn't left to freewheel.

Mike

[Chris Dickens]

There is a difference between a prop/impellor driven at low power and a freewheeling one.
If at high speed you reduce power the prop will slow down and present a disc to the airflow which increases the total drag of the model until the point where the airspeed is steady, i.e the speed at which the thrust is equal to the drag of the airframe.
A freewhelling prop not providing thrust and is always at revs consumate with the steady speed of the model. i.e if you dive and increase the airspeed the prop will speed up.
I know this is a bit simplistic but a driven prop will provide more drag than a freewheeler.
Chris

[Mike Rolls]

As an aside - I was once flying a control line stunter on a windy day and decided to change the prop. I took of the old one, put the new one on the shaft and before I could tighten it up it started whizzing round in the breeze. Out of idle curiousity I put the tacho on it - over 1,000 RPM. Now if I could have found a strong enough wind to get it up to the 8,000 I flew at....:-)

Mike

[Alistair Taylor]

Scrooge

There is more to this than meets the eye, and there's HEAPS of confusion about this, mostly centering on the use of the brake function of ESCs.

I was going to take issue with Mike's list of prop arrangements, and suggest that a freewheeling prop is draggier than a fixed prop, but it seems that a completely unhindered freewheeling prop (as is the case with free-flight rubber-powered planes) does seem to produce less drag than a fixed prop at constant speed (spot on Mike).

The "completely unhindered" and "constant speed" bits are important. If there is any form of brake on the prop from the speed controller, or friction from motor bearings, then the windmilling prop will create lots of drag, probably more than a fixed prop. Also, if you want to change the speed of the model, e.g. accelerate by diving, or slow by climbing, a windmilling prop will take time to accelerate/decelerate, and so add to the inertia of the model.

So - with the EDF plane you mentioned, the freewheeling impeller is probably not producing much drag, and is spinning at a fair lick. So how does applying low throttle affect it? It would probably slow the impeller down, as it "catches up" with the speed that the motor wants to turn at.

Why? Well the electric motor will spin at the speed you tell it and will actively RESIST any attempt to make it spin faster.

AlistairT

[Scrooge Howard]

What prompted me to display my ignorance in public ?, Nigel Hawes article , Pg71 , RCM&E, July 07. Glad I did , Thanks guys for some very interesting contributions to the discussion . Hi Mike Rolls , so I`m not the only old time "Balsa basher" brought up on "Freewheelers ,Featherers& folders " , & I certainly wouldn`t entertain a stationary prop on any of the rubber driven F/Fs Ifly today.
Alistairs point about the impeller speed , IF , higher when freewheeling , than that of the commanded (Low revs) , of the motor , dragging down the impeller speed to match, it is something I will bear in mind in future when trying to tickle my E.D.F. over the "Threshold" on the glide by adding a "Smidgin" of Throttle (now it will be all or nuthin) , just joking .

[Chris Dickens]

Scrooge
As they say 'many a true word spoken in jest'if you are undershooting with an EDF then you will need a burst of at least half power to extend the glide, anything less will have no effect (unless it's grossly overpowered!)
Chris

[Alistair Taylor]

There's plenty of us balsa bashers out there.

Now I've got a shed I've at last made some progress on the West Wings Hawker Typhoon and BF(ME)109 that have been languishing part-completed on a shelf for too long.

Stringers....ahhhh those were the days.

My brother in law gave me a WW Sea Fury for christmas that will no doubt benefit from the rekindling of building skills by the Tiffie and Schmitt.

AlistairT

[Mark Lubbock]

I think the main reason for freewheelers on rubber models was not to reduce drag, but to avoid upsetting the trim on the glide. Those props were half the wingspan in some cases & the twist would act as a very warped flying surface. My KeilKraft Gypsy spun in when the freewheel stuck!

[Mike Rolls]

Mark
That was also true - but the free-wheeler did quite definitely reduce drag compared to fixed. It also had a dampening effect on glide stability - although there was a school of thought that claimed that the 'dampening effect' was actually a euphemism for an under-elevated glide!

Mike

[Scrooge Howard]

Mark.
Excellent example . The prop was obviously transmitting energy to the airframe when it stuck ,& to prevent the "Spin" , energy would have to be generated, (By the flying surfaces), to oppose this & by so doing create yet more drag , so freewheel it!. In
these days of heavy , powerful , fast models using,by comparison very small props the effect must be hugely reduced , but is it totally eliminated ?. Ummm .