OK here's an odd question...

Just how important is the size of a spinner?

Point in fact - a local flier increased his spinner from 2.25" to 2.75" and the plane is now noticeably quieter.

I like to used aluminium prop nuts that are 'Bullet Shaped' - could I improve the efficiency of my engine by changing to a 2.5" Plastic Spinner?

Is There a 'Science' to Spinners?

OR .

Are they Just to look pretty? []

BB for sure in terms of performance efficiency spinners make a difference however I have my doubts that with our models airplanes and the speeds we operate them at among other things, I doubt most of use could tell a real differance. They may acturally affect the sound but not sure how big a change that would be.

I like you actually prefer with many of my airplanes to use spinner nuts and reccommend them to most sport flyers.

There is one factor though where spinners can make a big difference and that is cooling. On any cowled and specifically a tightly cowled installation the spinner can be an important component of the cooling system.

Here is an example of one of mine that that I beleve the spinner is an important part of the cooling system:

John

OK John, I will accept your opinion (makes sense to me)

BUT, if I may... just HOW does a spinner improve the cooling?

I think in John's case, it may have reduced the inlet area to less than the outlet area, which would cause a negative pressure in the cowling, and suck air in around the spinner.

Les

BB I beleve the spinner to provide a stabilizing effect on the air before it enters the inlets and thus allows a greater velocity.

A pressure cowl noted by Les relys on a greater sized outlet than the inlet (usually to the rear and bottom but not always) to cause an increase in the velocity of air moving through the system.

Here is an extreme example (not my airplane). Note the tiny inlet but roughly twice the outlets one on each side to the rear of the cylinder bonnet.

On A sport airplane such as a stick of some sort or one of my multi engine bashs where the engine are completely exposed all of that is moot and I love the simple spinner nuts.

John

I read an article in an RC mag a few years ago that said spinners have an effect on prop performance.....don't ask where or when I read it.
But look at it this way...noise is turbulent air.....a spinner would have the effect of causing calmer air at the hub.....I can see a spinner quieting a prop if there wasn't a spinner there in the first place.....but changing to a 1/2 in bigger spinner...would lead me to think it was more of a spinner/prop combination that lead to the quiter run

Maybe it was because of the nose shape? If the nose of the plane was wide the prop blast would produce a series of pressure waves at the middle of the prop. With a bigger spinner, the air moves more smoothly.

Many general aviation, full scale airplanes depend on the spinner to direct cooling air into the cowling,
and operation without a spinner is not recommended. I doubt that it makes much difference on our
models.
The number one reason to run a spinner on our models is to protect the engine, particularly the crank-
shaft, from crash dammage! Most cheap spinners rob more power then any benefit they might contribute.
Now that we have modern telemetry systems that can tell us airspeed, someone needs to do a spinner
study? My guess is that most models would be faster without a spinner? Keep in mind that most of our
models are so over powered, that small aerodynamic improvements will go unnoticed.
Just my opinion. Greg

"noise is turbulent air.....a spinner would have the effect of causing calmer air at the hub....."

But the majority of the noise is caused at the tip of the prop, where it can easily go super sonic.

Les

I ran a Carbon fiber spinner (5") on my Yak for one season and during hot days saw a noticable decrease in upline performance. I since have run two seasons with the spinner removed and my motor seems to be running cooler because i dont start bogging down on the uplines. That said, my Yak appears to fly a bit slower with the spinner removed. Here is a pic with it & one with out it....Gene

I understand that the blade tips make most of the noise...but the OP said that the spinner made the airplane quieter.....the prop hub still spins at engine RPM with a nut, and the place where the blade begins....now that is under the spinner and would be less turbulent...that is the only thing I can think of why it would be quieter