do spinners help? should i get one?

Spinners do help and yes, all planes should have a spinner. The AMA requires the nose of the aircraft to have a certain radius for safety. I don't know the exact number. Here's a little info about spinners.

The AMA requires that the shaft be covered with a spinner or an acorn nut.
Purpose is to eliminate sharp edges in case something is hit.
As far as I'm concerned, beyond hold the prop on and safety, its purpose is something to put your starter on.
Both will work just fine.
KW_Counter

Depends.....
When I first started I used wood props and I hand started the engine it made no odds whatsoever.
When I got an electric starter, the props had a habit of coming loose and it was a hassle unscrewing the front of the (plastic) spinner each time to retighten the prop then get everything lined up again.
When I swtched to APC props, the situation became worse and the cutouts aren't usually big enough on most plastic props anyway, so I in turn had to switch to an alumium spinner. This combination works well when you are using a starter.
So in conclusion, if you hand start then a spinner is just for looks. If you use an electric starter then a spinner is the best surface to use, although you can reverse the rubber to use on the bare shaft.

Recently on a new engine I experimented with a 4 blade prop. The LHS didn't have a 4 blade spinner so I just used the standard nut. Later, I ordered a Harry Higley prop nut. I then found that the starter wouldn't turn the nut so I got a special Sullivan starter cone for prop nuts...

You need something on the front of the prop. Why?
1. AMA rules (ifthey affect you)
2. Spinners make electric starts easier
3. Protects the crankshaft and absorbs some impact when the plane crashes

How do i determine size needed?

i am building a spad so it is made of 2 1/2 inch gutterpipe.

what size engine for a .40 get a 2 in

a .46

Spinner size is a mix of cosmetic and practical. It has to fit, and it has to look good.

So, take your plane off to the LHS, and stick the spinner in front of the plane. Admire for a few seconds, and then say, "This looks OK". Check the price (some spinners are really expensive). Then, take your prop of choice, and make sure that the spinner fully accomodates the prop (Ask the LHS attendant to double check - open the packaging and trial fit your prop if (s)he says it does, just to make sure).

Make sure the spinner backplate is the right size for your propshaft (or that the spinner has a correct-sized collar for your propshaft). If the spinner has an "adaptor" nut in to which you bolt the spinner cone, then make sure that the adaptor nut is thread-compatible with your engine.

Almost all of the thrust generated by your propellor is generated from the outside 75% of the prop, so, a spinner that covers 25% of your prop is not a problem (a 10" prop can have a 2 1/2" spinner) without loosing any significant thrust.

gus


PS ..... I use 2 1/2 size spinner on my two .40 size planes (both with .46 engines).

Have a look at the site mentioned above (by caffeenman), and also this thread:
[link=http://www.rcuniverse.com/forum/m_123917/mpage_1/key_spinner%252Cthrust%252Cprop%252Cdrag/anchor/tm.htm#123917]Prop discussion.[/link]

For a .40, I've seen spinners ranging from 2" to 2 1/2". My two .40 size planes have 2 1/4" and 2 1/2". If your SPAD is on the skinny side (like most spads are), I suggest going with a 2" spinner.

Oh...and get one with a metal backplate so that you can really crank down the prop nut and hold that baby on (plastic backed ones will tend to spin off with a starter and can't take as much tightening).

Jim