Does the size of spinner effect prop performance ?

[LHR Dave]

This is possibly a daft question !

If you have a large spinner on a prop on an IC engine will it affect performance of the prop.? ie with a smallish spinner for example there is 4 inches of prop showing either side and with a larger spinner there is only say 3 inches of prop either side, these sizes are only hyperthetical.

[Chris Walby]

IMHO In theory yes, in practice no

Why, the air is very turbulent near the spinner and cowl + the props angular velocity is low compared to the tip.

Others more expert than me will correct me, but I think its the outer 2/3 of the prop that does the work so the size of the spinner (within reason of course) will no make that much difference.

Hope that helps

[Former Member]

[This posting has been removed]

[fly boy3]

I agree with Chris. Many many years ago I asked a similar question after building a Magnatilla. The cowl was so large I was concerned re power of the prop, Only about half the prop wash was providing air over the control surfaces. I was told all was well, as the outer tip portion provided most blow. Cheers

[Martin Harris - Moderator]

Little or no sign of performance issues during 1/12th scale combat bouts with this - however odd the prop looked. No the tips are not concealed by the grass!

7"x5" prop to comply with combat rules for the .15 engine and a 2 3/4" spinner to conform to the scale outline of the fairly little known Me309 which I "designed" when looking for something a little more individual than a Spitfire to use for competition...

[Jon H]

the effect is usually very small but if you like you can test it.

Run the engine with your chosen prop but no spinner and check the revs. Fit the spinner and run again. If it revs faster, and it should, its likely the rpm rise will offset the lost blade area. If the rpm rise is significant, you might get away with fitting the next prop up. Say 16x8 up from 15x8 or whatever. 

But, dont loose sleep over it. Its more of a 'for science' test than anything else.

Edited By Jon - Laser Engines on 31/03/2020 23:00:31

[Tim Flyer]

Frontal area of an aircraft is important for drag . A large spinner that matches the cowl and blends with the frontal profile will reduce drag and help the plane reach higher speeds and reduce propeller slip. In the trade off between loosing a bit of inner prop area, and reducing drag, reducing drag normally wins especially for higher speed aircraft.

[Jon H]

Posted by Tim Flyer on 01/04/2020 10:44:06:

Frontal area of an aircraft is important for drag . A large spinner that matches the cowl and blends with the frontal profile will reduce drag and help the plane reach higher speeds and reduce propeller slip. In the trade off between loosing a bit of inner prop area, and reducing drag, reducing drag normally wins especially for higher speed aircraft.

Yep, but clearly it isnt that important as the canadians flew all their Hurricanes like this

[Tim Flyer]

I’m sure wartime creates many more issues other than simply top speed of aircraft 😉

Edited By Tim Flyer on 01/04/2020 20:27:09

[Paul Marsh]

My me109 doesn't have much prop, the spinner is nearly half the diameter of the prop, but has no thrust loss at all.

[Frank Skilbeck]

If you look at it from the prop swept area then a 4" dia spinner on a 12" prop is around 10% of the theoretical swept area.

[Robert Welford]

Introducing some thread creep. What effect do large radial cowls have on the thrust generated?

I have a Flair Hannibal with no much of the prop disc area outside the cowl footprint, but it flies fine on a Laser 120 using a 16" dia prop.

[Guvnor]

I worked on some prop designs in a previous life. I'd need to get my notes out, but I seem to remember that the inner 15% of the radius did nothing at all, the critical area was something like 60% to 90% radius.