81 replies to this topic

[MattPete]

Formula Libre is an open, 'bring anything' sort of a formula - a catch all sort of a class. Light weight is very important and Malcolm's car is considered very light weight.

Unconventional cars do occure e.g. Bill Norman's GEK

 

 

Is that Honda's aerokit for the DW12?

Edited by MattPete, 23 July 2016 - 02:54.

[NeilR]

I'm curious what his weight distribution is? You'd expect a higher percentage on the rear for an open-wheeler. The numbers sound legit, though.

 

It's important to quote the speed at which you're measuring the downforce, but don't worry much about 'low-speed' downforce vs. 'high-speed' downforce. A lot of people seem to worry that measuring DF at one speed is invalid for a different one. It's a V^2 relationship. Ride heights will matter, but those can be measured on their own.

 

Having said this, I will say that big-ish gurney's seem to keep the wings more DF stable in nasty (i.e. bouncing, side-flow, dirty air) environments. They're draggy, but they work.

 

I think he said his weight distribution was 45% front and 55% rear - the underfloor starts under the drivers thighs, so perhaps that adds to the rear DF bias? As the monash tunnel is not a moving floor it was felt that it did not contribute much in the testing

[Fat Boy]

How big is "big-ish"?

 

4-5% of wing chord. Smallish would be 1%. Middle-ish would be 2-3%. They all have an effect, but as they get bigger there are diminishing returns. (I'm hoping my wife sees things the same way)

[Fat Boy]

I think he said his weight distribution was 45% front and 55% rear - the underfloor starts under the drivers thighs, so perhaps that adds to the rear DF bias? As the monash tunnel is not a moving floor it was felt that it did not contribute much in the testing

 

The A2 tunnel in North Carolina is also fixed floor, but you can get real numbers from it, regardless. Making a couple ride height adjustments would tell you quickly how much influence the underbody contributes.

 

Generally speaking, if a car is 45% front weight, you'd want several percent fewer than that in aero contribution, maybe 42%. It will be driver dependent, but most guys don't like a car that's super responsive at high speed.

[NeilR]

Brett is from a motoX and rallycross-type background prior to hillclimb. His big advantage has always been that he continues to drive hard when the car gets loose in the rear, whereas my reaction has always to back off, so perhaps it is to his preference? There have been some circuit racers try to hill climb and not do well at all, a couple of others I have spoken to have said that learning to hillclimb had given them an advantage over some circuit-racing peers in that they had been able to drive much harder on the opening laps as they were mentally prepared for the cold tyres and reactions needed. Having said that we are talking about club racers and not professional drivers - we had Steven Richards, a then V8 Supercar driver enter one champs in his BDA powered RS1600 Escort and he was stunningly fast up and down the hill (I don't recall if he won his class though) - and was a really nice gut to boot.

[Kelpiecross]

I have just been reading about flap or wing "blowing" in aeroplanes by using part of the jet exhaust - apparently this can increase the lift by three times or so. How about in a hillclimb car diverting the engine exhaust into the rear wing and out through slots in the rear wing's under surface so as to prevent flow separation at high angles of attack? Two to three times the amount of downforce for the same wing area could be useful. In aircraft it also apparently makes the exhaust note very quiet.

[Fat Boy]

I have just been reading about flap or wing "blowing" in aeroplanes by using part of the jet exhaust - apparently this can increase the lift by three times or so. How about in a hillclimb car diverting the engine exhaust into the rear wing and out through slots in the rear wing's under surface so as to prevent flow separation at high angles of attack?

 

It's cool, but _way_ beyond the scope of this project.

[Fat Boy]

Brett is from a motoX and rallycross-type background prior to hillclimb. His big advantage has always been that he continues to drive hard when the car gets loose in the rear, whereas my reaction has always to back off, so perhaps it is to his preference? There have been some circuit racers try to hill climb and not do well at all, a couple of others I have spoken to have said that learning to hillclimb had given them an advantage over some circuit-racing peers in that they had been able to drive much harder on the opening laps as they were mentally prepared for the cold tyres and reactions needed. Having said that we are talking about club racers and not professional drivers - we had Steven Richards, a then V8 Supercar driver enter one champs in his BDA powered RS1600 Escort and he was stunningly fast up and down the hill (I don't recall if he won his class though) - and was a really nice gut to boot.

 

Preference...kinda. If MotoX and/or Rallycross teaches you anything, it's aggression and how to deal with something sliding. He's just more comfortable with a car that's moving around underneath him. Visually, you'll be able to pick out the times where the rear is moving around, but I bet he deals with excessive U/S at times also. You're driving on cold tires...every imbalance will be magnified.

 

Having said this, the cold tire thing may be a good argument for running a high front aero percentage. You can warm the rear tires easily by spinning them. The fronts have to be loaded laterally to put energy into them. Because of this, they're likely to lag behind the rears in terms of grip. Running excess front downforce may allow a cold front to kinda-sorta balance out a warmer rear tire. By the time you get to the end of the run you'll likely be aero-loose, for sure you would be if running a full practice session where everything got up to a working temperature. If you're prepared for the end-of-run oversteer, though, which is only for the last couple corners, then it was a good compromise.

[gruntguru]

 

4-5% of wing chord. Smallish would be 1%. Middle-ish would be 2-3%. They all have an effect, but as they get bigger there are diminishing returns. (I'm hoping my wife sees things the same way)

Thanks Fat Boy.

[gruntguru]

 

I have just been reading about flap or wing "blowing" in aeroplanes by using part of the jet exhaust - apparently this can increase the lift by three times or so. How about in a hillclimb car diverting the engine exhaust into the rear wing and out through slots in the rear wing's under surface so as to prevent flow separation at high angles of attack? Two to three times the amount of downforce for the same wing area could be useful. In aircraft it also apparently makes the exhaust note very quiet.           

I would like to see a full blown underbody aero system (tunnels and Lotus 88 type skirts) with exhaust blown tunnels. (No wings required)

[Fat Boy]

I would like to see a full blown underbody aero system (tunnels and Lotus 88 type skirts) with exhaust blown tunnels. (No wings required)

 

Underbody aero is a great thing. You can get a huge bang for the buck. Exhaust in the tunnels is good in terms of numbers, but you've just made the car total downforce and balance sensitive w.r.t. throttle position. That can get interesting in a hurry.

 

Even if you had all the other stuff, for this type of a car, you'd probably still want the wings. The penalty for carrying extra drag just isn't there. What percentage of the time is at maximum throttle/speed? Very little.

[Fat Boy]

4-5% of wing chord. Smallish would be 1%. Middle-ish would be 2-3%. They all have an effect, but as they get bigger there are diminishing returns. (I'm hoping my wife sees things the same way)

 

I throw comedy gold at you guys and don't even get a sniff of a laugh. C'mon, _all_ of you can't be this nerdy!

[Canuck]

The post like I left is my polite, apologetic Canadian laughter.

[gruntguru]

 

I throw comedy gold at you guys and don't even get a sniff of a laugh. C'mon, _all_ of you can't be this nerdy!

I was feeling too embarrassed for you to comment.

[desmo]

Assuming all undercar aero DF, how do you tune the f/r aero balance? Or total DF for the track? Even if you don't need the additional DF, don't you need at least vestigal wings?

[gruntguru]

Ride height, rake, "trim tabs"?

 

Can't imagine ever needing to reduce DF. Normally only done to reduce drag.

[NeilR]

As in the CAMS manual the aero regulations are:

 

Any specific part of the automobile which has

an aerodynamic influence on the stability of the

automobile must be mounted on the entirely sprung

part of the automobile and shall be firmly fixed whilst

the automobile is in motion.

Neither the safety cage structure nor any of the

units associated with the functioning of the engine

or transmission shall have an aerodynamic effect by

creating vertical thrust.

The leading edge of any aerofoil fixed to the front of the

automobile shall not be sharp.

 

No element of coachwork may exceed in height a

horizontal plane situated at 900mm above the ground.

Neither the safety cage structure nor any of the units

associated with the functioning of the engine shall be

included. Measurements are to be taken with the driver on board

[NeilR]

I've been working on my vacuum setup for moulding composites. I will shortly have a variable vacuum setup that switches on/off when needed - a great saving in my attention and time.

[Canuck]

Why would you switch it off? I seem to recall, perhaps erroneously, curing ovens with vac lines, and certainly in autoclaves.

[NeilR]

An oil-bath vacuum pump such as I have (usually used for AC repairs etc) creates a plume of oil if left running at full vacuum which is not ideal, but they are cheap and reliable high-ish volume, single phase pumps. So the solution is to use a reservoir and vacuum switching arrangement - the pump evacuates the bag and the reservoir to say 20" H2O and switches off. It will then restart if there is a leak and the pressure in the system climbs to say 10" H2O. It will monitor and keep switching on/off as required, so vacuum can be had overnight - though I would not leave it in my workshop.

I must add that this is not an original idea - I bought a kit to do it from here: http://www.joewoodwo...EVS/concept.htm

Edited by NeilR, 28 July 2016 - 05:04.

[gruntguru]

That would be 20" Hg.

 

Must admit, vacuum would be one of those physical properties with the widest array of confusing units.

Edited by gruntguru, 28 July 2016 - 06:36.

[Kelpiecross]

As in the CAMS manual the aero regulations are:
 
Any specific part of the automobile which has
an aerodynamic influence on the stability of the
automobile must be mounted on the entirely sprung
part of the automobile and shall be firmly fixed whilst
the automobile is in motion.
Neither the safety cage structure nor any of the
units associated with the functioning of the engine
or transmission shall have an aerodynamic effect by
creating vertical thrust.
The leading edge of any aerofoil fixed to the front of the
automobile shall not be sharp.
 
No element of coachwork may exceed in height a
horizontal plane situated at 900mm above the ground.
Neither the safety cage structure nor any of the units
associated with the functioning of the engine shall be
included. Measurements are to be taken with the driver on board

I am not quite sure what most of this means - does it mean that any type of exhaust blowing is banned? "Safety cage structure" not allowed to produce vertical thrust? - meaning?

[gruntguru]

Safety cage not to be made of airfoil section tube, oriented to produce DF?

 

Shame if these rules apply to Formula Libre. I had always imagined un-sprung aero on one of those cars.

[Kelpiecross]

Have to agree - Formula Libre should really be "libre".

[NeilR]

Probably right about 20"Hg, I have not looked at the pump specs for a while and probably thought of flow benches or something.

[gruntguru]

Absolute vacuum is about 30" Hg below atm.

 

20" below is only a moderate depression - you can see that sort of vacuum in the intake manifold of a throttled IC engine.

[Canuck]

An oil-bath vacuum pump such as I have (usually used for AC repairs etc) creates a plume of oil if left running at full vacuum which is not ideal, but they are cheap and reliable high-ish volume, single phase pumps. So the solution is to use a reservoir and vacuum switching arrangement - the pump evacuates the bag and the reservoir to say 20" H2O and switches off. It will then restart if there is a leak and the pressure in the system climbs to say 10" H2O. It will monitor and keep switching on/off as required, so vacuum can be had overnight - though I would not leave it in my workshop.

I must add that this is not an original idea - I bought a kit to do it from here: http://www.joewoodwo...EVS/concept.htm

Thanks for the link - that's an interesting idea.  I managed to pick up a dual-stage Welch pump a couple of years ago and it has the inverse issue - the more time it spends at or near atmospheric, the more oil mist it will discharge. They recommend an exhaust oil recycler for operation above 1 torr (just to add to the confusion of units).

[NeilR]

Safety cage not to be made of airfoil section tube, oriented to produce DF?

 

Shame if these rules apply to Formula Libre. I had always imagined un-sprung aero on one of those cars.

The rules are somewhat archaic in that they are a hang-over from previous years via the FAI

Canuck you are welcome re the link. Price was the deciding factor in choosing the pump I have and since I rarely use it the oil-plume issue was not that much of a problem. However I hardly want a thin film of oil settling over everything so the easiest solution is to put it outside. I am finding that all of the issues I think I may have are already solved by people flying RC gliders and the like. They make thin 4m long wings in CF and have been doing so for many years. Since this is a hobby my 13YO son is getting into as a father/son thing, I have a great resource in the local flyers and online groups. The flap sections of the wing will pose the greatest challenge - the glider techniques may not apply completely to such a heavily cambered airfoil

Edited by NeilR, 30 July 2016 - 02:22.

[Fat Boy]

Assuming all undercar aero DF, how do you tune the f/r aero balance? Or total DF for the track? Even if you don't need the additional DF, don't you need at least vestigal wings?

 

Well, I do think you're getting a little ahead of yourself, but here goes.

 

Aero balance of the underwing is tuned with chassis rake and small flow devices. Turning vanes, exit gurneys, extensions, infills, etc. all have an effect. In a car like this, you'd be max DF all the time.

 

Total DF is similar. Generally speaking, as you lift the rear you get more front % and more total. As you reduce rake it goes the other way. When you see Indycars on big ovals with the rear very low and the front all jacked up, that's to reduce drag.

 

Regardless of the underwing, I'd put the biggest wings on it I could package. You can tune the aero balance with the wings and leave rake for a mechanical balance tool.

 

For your application, I think this guy gets it about right. Yes, it looks a little absurd, but I bet it's a rocket on a tight, low grip track.

 

Edited by Fat Boy, 02 August 2016 - 19:56.

[NeilR]

I've watched video's of the A-mod cars. I cannot follow that path as the rear wing cannot be more than 900mm high, but I think a double or triple element front and rear will be the go. I've been advised to get the handling spot on, as in car losing grip equally front/rear in a corner,  before adding wings

[Fat Boy]

I've watched video's of the A-mod cars. I cannot follow that path as the rear wing cannot be more than 900mm high, but I think a double or triple element front and rear will be the go. I've been advised to get the handling spot on, as in car losing grip equally front/rear in a corner,  before adding wings

 

Maybe you can't follow the exact path, but this explains the sentiment.

 

As far as chassis tuning then adding wings, I have 2 thoughts.

 

A. If the wings are the last thing made and the car can be running for a couple months prior to the wings being built, then, by all means, run the car sans wings. If anything, you'll sort some driver fitment, reliability and engine tuning issues.

 

B. Assuming the entire car is finished about the same time, then run the car as a whole, wings and all. If you tune the chassis without the wings, you'll end up with a completely different setup than tuning it with wings. Springs, bars, rides, damping, etc. will all be different with the wings. If you get the chassis sorted without wings and then just plunk them on, you'll find you're completely lost again.

 

This feeds into ones philosophical approach to a race car. It's not a collection of individual systems. It's one thing. You can't tune any of it in a vacuum. There are no 'perfect' springs, there are springs that provide more or less advantageous compromises based on how they interact with the everything else, but none are perfect. Same with any other component on the car. If you have the car handling 'perfect' and then change the driver, you'll often find a whole new set of problems. Let's say you keep this 'perfect' car and driver together, but go to a different track...it won't be perfect any longer. Hell, change the weather and it'll probably all go to ****.

[NeilR]

I've been working my way through the start of Benzing's Ali Wing book:

 

http://www.bookdepos...HpCwaAhJ28P8HAQ

 

It's quite good with loads of airfoils, but it is not a whole car aero book. .