The 'Dimple Effect"

Here are our CFD links and discussions about aerodynamics, suspension, driver safety and tyres. Please stick to F1 on this forum.
tc9604
tc9604
0
Joined: 04 Oct 2009, 01:21
Location: Bristol, UK

Re: The 'Dimple Effect"

Post

So would tyres with tread experience this effect to some extent?

Mystery Steve
Mystery Steve
3
Joined: 25 Sep 2009, 07:04
Location: Cincinnati, OH, USA

Re: The 'Dimple Effect"

Post

There might be a slight effect, but due to combination of ground effect and tire rotation it would likely be a negligible contribution (I assume, at least...). If you wanted to fill the wake behind the wheel, it would much easier to use a turning vane, or some other aerodynamic device.

Giblet
Giblet
5
Joined: 19 Mar 2007, 01:47
Location: Canada

Re: The 'Dimple Effect"

Post

When I was a young boy I fell on a fireplace hearth and cracked my cheekbone, and it left me with a permanent dimple on my face.

Luckily it landed in the exact place a dimple is supposed to be.

Am I off topic?
Before I do anything I ask myself “Would an idiot do that?” And if the answer is yes, I do not do that thing. - Dwight Schrute

Mystery Steve
Mystery Steve
3
Joined: 25 Sep 2009, 07:04
Location: Cincinnati, OH, USA

Re: The 'Dimple Effect"

Post

Do you notice any asymmetrical aerodynamic effects when you go for a bike ride then? :D

tc9604
tc9604
0
Joined: 04 Oct 2009, 01:21
Location: Bristol, UK

Re: The 'Dimple Effect"

Post

Thanks guys :)

autogyro
autogyro
53
Joined: 04 Oct 2009, 15:03

Re: The 'Dimple Effect"

Post

I know that there is a slightly greater increase in lift with the wheels still rotating than if they have been braked to stationary, when a Hawker Sea Fury takes off and as the undercarriage is retracted. There is a markedly greater sudden upward jerk.
The tyres on the main undercarriage usualy have grooves.
Work that one out aero guys.
Is it an aero effect on the rotating tyre surfaces, or perhaps the gear hitting the wheel well, increased air flow over the flaps or a pilots stick reaction to the change in control surface feed back, or something else.
Come on you guys, all that uni must be worth something!
Model it?

User avatar
mep
29
Joined: 11 Oct 2003, 15:48
Location: Germany

Re: The 'Dimple Effect"

Post

It is caused by the magnus effect.
http://en.wikipedia.org/wiki/Magnus_effect
It’s the same principle who enables you to let a football fly on a curve or to lift a spinning golf ball. In fact is is not only the dimples who let a golf ball fly far, it's more the spin you gave them.

On a tire the lower surface is covered by the road so you get even more lift.
I don't think the groves play a big role there.

On an airplane you have a lot of drag with the wheels outboard what makes it more sensitive. The magnus effect should play a marginal role there.
Your wheel suspension will suffer under high structural what you don't like on a light optimised part.

HungryHebbo
HungryHebbo
0
Joined: 04 Mar 2010, 20:21

Re: The 'Dimple Effect"

Post

But surely that would reduce the overall lift? Think of topsin and backspin in tennis. Topspin makes the ball dip...

autogyro
autogyro
53
Joined: 04 Oct 2009, 15:03

Re: The 'Dimple Effect"

Post

Dont forget, as the wheels retract on the Sea Fury, they move from vertical to horizontal and there is also the gyroscopic effect.

We can move on to light unpowered rotor autogyros with retracting gear if you like. I will be looking at one next Wednesday.
This one has teeter bearings in the rotor head rather than the helicopter collective/cyclic head. Single blade with self balancing A of I.

Question would be to model all the forces at work during take off, retract, cruise and landing.
Guess what: The CAA has not even got a definition of autogyro flight and Glasgow Uni who were paid to do a computer evaluation of the flight envelope for them got it completely wrong, so it is now an unknown technical area of aerodynamics.
The old Ministry of Aviation knew 40 years ago, just shows how backward the technology has progressed.

HungryHebbo
HungryHebbo
0
Joined: 04 Mar 2010, 20:21

Re: The 'Dimple Effect"

Post

autogyro wrote:Dont forget, as the wheels retract on the Sea Fury, they move from vertical to horizontal and there is also the gyroscopic effect.
I was thinking about that too when I posted about the topspin. If I remember correctly, both wheels move inwards, so the port undercarriage comes up anti-clockwise, starboard clockwise. Unless I'm mistaken (probably am!) the forces this would induce should cancel out - the relative input force to each gyropscope (wheel) is equal and opposite, right?
We can move on to light unpowered rotor autogyros ... just shows how backward the technology has progressed.
I have no clue whatsoever about autogyro's, so not even going to attempt to comment on them!

User avatar
mep
29
Joined: 11 Oct 2003, 15:48
Location: Germany

Re: The 'Dimple Effect"

Post

HungryHebbo wrote:But surely that would reduce the overall lift? Think of topsin and backspin in tennis. Topspin makes the ball dip...
The golf ball spins in the other direction like seen on the wikipedia picture compared to the direction the air comes from.
So the forve vektor points upwards.

HungryHebbo
HungryHebbo
0
Joined: 04 Mar 2010, 20:21

Re: The 'Dimple Effect"

Post

mep wrote:
HungryHebbo wrote:But surely that would reduce the overall lift? Think of topsin and backspin in tennis. Topspin makes the ball dip...
The golf ball spins in the other direction like seen on the wikipedia picture compared to the direction the air comes from.
So the forve vektor points upwards.
I was refering to the wheel rotation direction :)

Mystery Steve
Mystery Steve
3
Joined: 25 Sep 2009, 07:04
Location: Cincinnati, OH, USA

Re: The 'Dimple Effect"

Post

Tennis balls aren't in ground effect, though... :wink:

HungryHebbo
HungryHebbo
0
Joined: 04 Mar 2010, 20:21

Re: The 'Dimple Effect"

Post

Are the wheels of the Sea Fury in ground effect too? I have no idea. Suppose it depends how aggressive the pilot's being and popping 'em straight after rotation or what. That is the general practice I guess, get them up as soon as possible.

Auto, is it only the Fury that has this effect? I've never heard of it before to be fair (doesn't mean a lot though, turns out there's a lot of things that I haven't heard of) and it does surprise me. Wasn't it generally lobbed off of an aircraft carrier? I'd imagine something funky might happen as it clears the deck, but not an increase in lift though... hmm.

riff_raff
riff_raff
132
Joined: 24 Dec 2004, 10:18

Re: The 'Dimple Effect"

Post

The objective of putting dimples on the surface of a golf ball, or bumps on the upper surface of an aircraft wing, is to energize the flow boundary layer and hopefully keep it attached to the adjacent aero surface (and laminar in nature) for as long as possible. Thus minimizing drag losses. Small bumps tend to work better, but golf balls use dimples because bumps would get flattened out when the ball is hit.

With regards to asymmetrical lift on the Sea Fury, this condition is common to all single engine planes with a front propellor. It's mostly due to the helical wake flows from the prop blades interacting differently with the wings and fuselage from one side of the plane to the other. And not much from the spinning tires on the landing gear.

Of course, spinning cylindrical surfaces moving in a radial direction with respect to airflow (like an F1 car's tires) can produce significant lift/drag forces. And this is a big consideration with high-speed, open wheel race cars. Remember the 6 wheel Tyrrell chassis with its small diameter, shrouded front wheels?

Here's a good read on the subject:

http://www.aps.org/units/dfd/pressroom/ ... axerio.cfm

Regards,
riff_raff
"Q: How do you make a small fortune in racing?
A: Start with a large one!"