F1 in Schools Help - basizeland's project

[Ciro Pabón]

Well done, REA. What an eye for regulations! Do you work at Ferrari, McLaren or Renault? If you don't, at least you should become a steward.

Well, as bazanaius states, it's better NOT to have anything that creates eddies than trying to "smooth" them.

I'm not sure about the "whole" point: for example, is not there a way to minimize turbulence behind the car, or to increase pressure at the back, by some kind of "incomplete" diffuser (a section behind each sidepod) or by using something else?

At least, I think (like the noob I'm in aerodynamics) that the aerodynamic regime of the canister cover has to be similar to golf balls, so, if diffuser won't work, what about dimples on its surface? Here you have the very basic ideas the forum was able to put together about them, dimples: viewtopic.php?f=6&t=6649

Flyn, I didn't know I could violate bastard laws. It sounds to me like an argument for a cheesy black novel...

Anyway, I have the same grasp of the fairing as A13x_f (both of us are equally insecure about the truth of our statements). I also believe that any fairing around the canister exit will help to "suck" a larger volumen. I also think that's the general idea behind turbofans: to impulse more air at a lower speed, which gives you more efficiency

I have the confuse notion that it's something like (but different ) the way in which, by increasing the size of propellers and decreasing their rpms you increase greatly the efficiency of ultralight delta wings engines.

Actually, I'm completely sure that in primitive delta wings the tips of the propeller worked at speeds higher than the speed of sound, so, by forcing them to turn at a lower speed (with a gearbox) and by increasing its size, the propeller worked in a more efficient regime, where its tips did not go over the speed of sound.

So, I think the fairing is used in a similar role (larger volume of air impulsed, less speed). Check this slides I googled quickly: the titles in italic point to the relevant part in each slide.

The difference in energy transfer rate...


Low pitch propeller - high performance...


The overall net trust depends in part...


The efficiency can be improved...


Here is the link: http://www.physics.hku.hk/~phys0611/3Te ... flight.pdf

Other relevant phrases in that document: "Propellers have greater efficiency than jets" (thus the idea of the "turbofan" car), "For subsonic aircraft the jet is normally accelerated by means of a simple fixed converging nozzle" (it gives you and idea of the kind of nozzle, if it can be added complying with regulations) and "Wing planform - Elliptical: ideal shape, the lowest drag" (which gives me the idea of elliptical wings for your car, altough if the AoA is zero, I don't know).

About the nozzle, I'm happy about the positive dictum of bazanaius. The problem, as basizeland states, is that the car has to be made of one piece: how in heaven do you insert the CO2 canister into its cover under that condition? I think that if the nozzle is separated from the "body" of the cover, the reaction force on the nozzle is greatly diminished.

After reading flyn comment about drag "generated" by lift, I agree with basizeland: the friction of the wheels is probably small (that is, wheels are more efficient for carrying the weight than wings), so I also think now that the energy savings you would get by getting the car airborne are overwhelmed by the extra drag, as flyn says.

I don't know if the support our forum could give to you will give your team points in the "sponsors acquired" scores, but, please, guys, keep the ideas flowing. Very nice thread, from my point of view. It beats the "scandal" threads on any given day. I think I'll start reading about the magnus effect on rotating wheels. Isn't that effect what "carries" more air throught the canister fairing (even without a fan, as A13x_f states?).

As usual, I apologize for the length of my postings and the awkward wording... sometimes directly translated from spanish. I know some people don't like 2,000 words posts... the wimps.

basizeland: I think flyn is talking about the AoA (Angle of Attack) of the wings. Make them neutral, to create no lift and thus no increase of drag. They are in the car just for the "beauty" of them... flyn says that a "flying car" or a "downforce car" would have larger drag. You know, downforce in F1 comes at the price of a large Cd.

[bazanaius]

@A13EX_f:

This idea is called entrainment, and I agree it's worth looking at for sure. You could rig up a test where you have various nozzles held at a set distance from the jet outlet, and fire co2 jets - then measure the force generated (put the jet on a sled, with a laod cell or something) to establish qualitatively which nozzle seems best.

@baseizeland:

indeed, any flow conditioners upstream will act to stabilise the flow and give you a more laminar flow at the leading edge of the model. Just make sure you don't go overboard and reduce the flow rate too much - you'll need a reasonable airspeed to replicate the reynolds numbers you'll see when you're accelerating your car. Do you know the typical time to 20m? this will give you an idea of speeds and the accelerations you'll be seeing. It will also tell you whether your belt sander is quicker or slower than the speeds you'll see on the track.

AoA stands for angle of attack, and is a measure of the angle between the chord line of an aerofoil and the direction of the free stream it sits in. A neutral aerofoil at and AoA of 0 degrees produces no lift. A flat plate at 10degrees AoA will generate lift, but with a component of drag. A lifting section aerofoil will generate lift at 0 AoA etc. etc.

So streamlining something is effectively adding a neutral profile to it, and placing it at 0 AoA. The shape of a streamlined object is designed to prevent those flow features we discussed previously, like separation (and the associated form drag). So long and thin, tapered (air doesn't like turning sharp corners) is good - this is what flynfrog refers to for your fairing on the CO2 cylinder. Question is - how long and how thin? A measure of this is called the 'fineness ratio' (more googling fun). If you make the length of your object too great you eventually generate excessive amounts of 'skin friction' which is caused by the visous forces generated as air passes over the 'wetted surfaces' of your shape. There is a range of fineness ratios that balance these two types of drag for different shapes.

as a general rule you'll want to keep your car as horizontal as possible - if it wheelies not only will your rocket then be adding a vertical component (decreasing the force accelerating you down the track) but you'll also create a nice flat plate to act as a sail to slow you down I think this can be acheived with statics/dynamics however rather than resorting to aero DF. Calculate where your CG of the vehicle (incl cannister) is, and compare that to the line of action of the force. Which way does your care want to rotate when you look at it side on? Can you make your car underweight and add ballast to put the CG where you want it?

You've got a year to do this right?

[bazanaius]

just googled entrainment and didnt find what I want so here's a demo:



the 'augmented' (changed) thrust is due to an increase in the mass flow rate of fluid through the nozzle. it will depend on the spacing of the nozzle from the outlet, and the size of the nozzle. PS image above is a cross section through jet (left) and nozzle (right).

depending on the regs you could do it in a number of ways - if the wings can be put on after the cannister is inserted, you simply make a 'different' wing mount that is the nozzle If you can't, is there anything saying the wing has to be in a certain place at the rear? Could you mount your areofoil on a front cover for the cannister housing? What shape is the cannister? if it has a streamlined face just secure it in through the front, with no cover? etc. etc. up to you

[Ciro Pabón]

Bazanaius, I read at the F1 in Schools site that the typical time is around 1 second for 20 m (that's how I reached the 80-100 kph top speed estimated figure I gave before, for us to have a general idea of the aero regime).

The world record for the run is around .98 sec, I think. That's the "pole" we have to beat...

What do you think of the idea of elliptical wings to minimize wingtip vortices? (I edited my previous post and added that suggestion while you were writing yours). I don't know what the regulations say about that idea.

Basizeland, have you checked the previous threads given in the "Similar Threads" section, at the bottom of this page? They include some ideas by manchild, a very clever ex-member of this forum. We've had at least five threads on this subject (but I think this is the best so far ).

[basizeland]

I have thought about the dimples idea but never researched it enough to see what type of gain i would achieve because it would be difficult to machine and sand, but if it was going to make a reasonable difference it would be worth trying.

With the nozzle I was thinking that I could have it hinged to the car body so that i could lift it up to put the CO2 canister in and then drop it back down to run the car.

The quickest time that our car has set is 1.045 seconds which works out to be 19 meters per second but that is the average and the car is being propelled between half way and 3 quarters of the way down the track and then it rolls to the finish so we achieve our highest speed towards the start of the race which would be higher than 19 m/s.

I will have to look into where the CG of the car is because i think from looking at the car that it is going to want to wheelie down the track which will create a flat surface to slow the car down.

The rear wing must be behind the centre line of the rear axle and above the rear wheels so the only way that i could have the rear wing at the start of the canister housing is to move the rear wheels forward which would shorten the wheelbase making the car very unstable. You also have to have a canister housing that is at least 3.5 mm thick with a 0.5mm tolerance.

The elliptical wings idea seems interesting

[A13EX_f]

just googled entrainment and didnt find what I want so here's a demo:

thats exactly what i was trying to explain. and what the aussies car seems to be trying to achieve. it would defiantly be worth testing that theory although how "legal" it is I dont know.

[basizeland]

Yeah in theory it should improve the performance of the car and I want to find out whether they got docked points for it because you are allowed to break the rules but you get marked out of 50 for the rules so if everything is correct you get 50 points but some teams will sacrifice a point or 2 to improve their car performance although there are some rules that if broken you get a .5 of a second added to your times so as long as it doesn't gives us a time penalty it might be worth trying it and losing a mark to improve the performance of the car.

[bazanaius]

I think the end plates on the front wing is a good idea for ensuring you break the barrier immediately, and so simply a convex nose, with endplates sticking up to break the beam would probably be my first hunch. At the rear, why not run a central rudder rather than endplates, and then make the rear foil elliptical (in shape I presume, rather than lift distribution? Just to make sure we're not falling into an aircraft control chat!) in plan view to, as you say, try and minimise vortex generation.

basizeland - end plates are generally added to wings to improve the performance of the aerofoil at its tips. they prevent the lower pressure area above the wing from drawing air up and over from below, which would decrease the suction (lift) on the outboard edges (indeed, tip stall, or loss of lift at the tips isn't fun as it often doesn't occur simulatneously on both wings of a plane, creating a large rolling moment!).

In this case we're looking at as much of a neutral section as possible for your rear wing, and so the lift loss problem isn't really there - the vertical fins are now for yaw stability. A single rudder does this with less drag.

re dimples - obviously many previous discussions on this forum about these. I also found this nice link

http://wings.avkids.com/Book/Sports/ins ... lf-01.html

dimples trip the flow to turbulence, which has a bit more energy and so separates later on a bluff body, decreasing the form drag. In our case we are trying to minimise form drag by streamlining, which acheives the same goal but without tripping to turbulence. Therefore I think dimples might be overkill, and as you say, would be a lot of extra work and machining time.

B

B

[bazanaius]

just thought, if you're looking at vertical rudders, why not hang your nozzle off the bottom of an extended (rearwards) rudder? the rear wing then sits on top of the cannister as you have now, without having to have an additional support below the wing.

[flynfrog]

I think you would be better off not using a nozle they are pretty hard to make correctly and you would see little gain.. Focus on drag and mass reduction your biggest pay off will come through these.

How are you axles riding on bearings or in straws?

[bazanaius]

I think you would be better off not using a nozle they are pretty hard to make correctly and you would see little gain

i think this depends on time and the facilities available to you. If you can RP at school then you can manufacture anything you can CAD, and if you make it part of the rear wing I believe that's allowed in the rules.

Agreed, finding the correct shape and geometry could be a long job - although if you can RP, you could maybe make and test 5 in a day? Making the test rig maybe another day? this would at least give you an insight into whether it's worth pursuing? As far as little gain, I've seen augmentations of 2.5+ with pulse jets...

[mep]

When I saw Ciros picture of the fan-engine I immediately thought about a jet pump.
Unfortunately I waited half a day to post it and now bazanaius was faster than me with the same idea. =D>

I would just move the inlet more to front, so that the low pressure zone will suck the engine trough the air
and I would add a diffusor behind it formed like a laval nozzel.



The air volume moved with such a arrangement is quite high.
You could expect something between 60% - 90% air/CO2.
I took that figure from some jet pumps you can buy.
I don't know whether you will get any kind of thrust,
but it is really worth a try.
When it is not working with CO2, try it with any kind of burnable gas.
In fact, that would be even more interesting.

[flynfrog]

You are forgetting the nozle is the hole the pin punches in the co2 container. Also MEPs pump still wont have more energy than the CO2 contains it may move more air but at a lower speed. The pesky laws again.

[mep]

True, I must admit I have the feeling that it will not help to increase thrust. It looks a bit like a perpetuum mobile. (means not working)
Maybe you are able to degrease drag with the insucked air, so still worth a check.

[Ciro Pabón]

just thought, if you're looking at vertical rudders, why not hang your nozzle off the bottom of an extended (rearwards) rudder? the rear wing then sits on top of the cannister as you have now, without having to have an additional support below the wing.

=D> This could be called Bazanaius Inverted Shark Fin...

I haven't thought about CG. The body acting as a stabilizer is also a good idea.

The following comments apply only if ballast is not enough to nullify the torque caused by the CG being below the canister axis (that is, if the car tries to do a wheelie, even with the front end loaded with ballast):

What if instead of using only the body of the car for stability, and given the rules restrictions, you use the front wing as the "primary" stabilizer?

I mean, if you use the whole surface of the car, don't you create more drag than if you use a smaller NACA section at the front to achieve the same effect?

I suppose this question is hard to answer: the "continous" small push from the front wing must be compared with the "haphazardly" push of the body. If the car is inherently unstable, that is, if the body is continously pushing down throug the whole run, perhaps it's better to use the front wing.

I think bazanaius is right: the body has to be flat, for added stability. But perhaps the "minimum" stability can be provided by the front wing (pushing downwards to avoid the car from flipping up).

If the variations in push (or in friction or in "whatever happens") overcome the "primary" stabilizer (the front wing), then the body "kicks in", keeping the car on the track and minimizing friction with the guidelines under the car.

Now, I´d presume that smaller drag of a wing as compared with a body is one of the reasons for having stabilizers in airplanes whose body is also stable...

Actually, perhaps you could use both wings to stabilize the car in the XY plane: front one with a "regular" NACA and rear one with an inverted profile, so you can use "flatter" profiles with smaller lift.

So I have another question to test bazanaius and flynfrog patience: what's better, two profiles, one profile or none (that is, only the body)?

Or, to put it in other terms, what's better at this speed, a flatter profile with more area or a steeper one with less area? What's the best wing load, if that's the proper term? I repeat, the previous discussion only applies if there is a need to use the car surfaces to stabilize it. It's always about ballast, I guess...

You are forgetting the nozle is the hole the pin punches in the co2 container. Also MEPs pump still wont have more energy than the CO2 contains it may move more air but at a lower speed. The pesky laws again.

Well, of course a laval nozzle should be laying right at the edge of the hole in the container. Good point. The whole idea of the nozzle is that it's more efficient than having just a "combustion chamber" (in our case, the hole in the canister). For the nozzle to work as a Laval you have to adjust the nozzle against the hole with some kind of seal.

However, if you don't have a good seal, like when you use a fairing, or part of the CO2 flow to impulse a propeller, you are "sucking" more air into the CO2 flow, isn't it? That's why we talk about the "proper speed" of air.

I don't think that considerations about that "proper" speed of the "impulsed" air are a "perpetuum mobile" idea. What I've understood of what I've read is that you have different efficiencies depending on the speed of the craft.

At low aircraft speeds it's better to use a larger volume with low air velocity than at higher craft speeds, where you want a faster flow with less volume (if I'm not mistaken, flynfrog), even if the total push is the same in those two situations.

Of course, the turbofan idea is a bit crazy, it's better to use a fairing, as mep and myself have suggested (if we're not mistaken, again).

For example, I think that the reason for smaller passenger planes to have turbofans instead of turbines is that propellers are are more economic at their relatively low speeds.