Maximum angle for boat tail?

[JackMcCornack]

bombloader, there are a lot of factors (and a lot of centers of pressure) in pitch (unless the wing is Hershey-bar shaped, the Cp of the airfoil is different from the Cp of the wing...which is different from the Cp of the complete aircraft), but as far as yaw stability goes, Bicycle Bob has got it:

Originally Posted by Bicycle Bob

A major problem with stability is that a streamlined shape of constant density will have the center of pressure, or lift, ahead of the center of gravity, so it naturally wants to tumble.

A ground vehicle viewed from the top is nearly always symetrical, and a symetrical airfoil has (for all practical purposes) its Cp at 1/4 of its overall length. To be (aerodynamically) yaw neutral in side gusts, the CG and Cp need to be in the same place (viewed from above), and man that's tough without adding some area behind the CG.

I think most aircraft designs (most of mine for sure) don't have an overabundance of yaw stability--enough to get the job done is the goal; too much adds drag for no reward and makes the controls heavy--but the books are full of aircraft which had fuselages modified for better streamlining and suddenly found they needed more vertical fin. I suspect the same is true for automobiles; the better the streamlining, the more challenging the stability issues.

[bombloader]

I think I was little confused by Bicycle Bob's post. He was referring to Cp of the car in the yaw direction, I took it to mean in pitch which is the only way I heard it talked about in pilot training. Got it now So to make things clear, I was talking about what aircraft are designed to with the CP and CG in pitch for stability. Bicycle Bob, you were talking CP in yaw correct? Anyway, I still think I'm correct about the vertical fin though. Two reasons:
1) Directional stability(yaw) in a car comes from traction. Even race cars producing downforce are just trying to generate more traction. Therefore, CG in relation to wheel placement is probably more important than CG related to CP.
2) Observationally, I've noticed that the poorest handling cars in high winds are typically aerodynamically dirty ones like my Jeep. Cleaner vehicles seem to shrug of winds better.
Take my my nonexpert opinion for what its worth, it would seem strange if a well designed boattail actually decreased stability. If I had the problem, I'd probably question whether I actually achieved the drag reduction I aimed for.

[JackMcCornack]

Good points, bombloader--good enough that I hereby revise my suspicion. I now entertain the reverse suspicion: I now suspect that streamlining in general improves automobile yaw stability because it reduces the aerodynamic component in the overall stability equation. Also, a boattail will improve yaw stability (presuming the rest of the body is left alone) because its area (and influence) are behind the CG, and there are other mods that will both reduce drag and improve yaw stability (e.g as you've noted, a small and rounded nose will shrug off a side gust better than a big boxy Jeep nose).

Of course, if your wheels leave the ground then the aero component gets all the votes. There were Mercedes racers at Le Mans (or some LMP event...I think it was Le Mans) that made some spectacular flights, and I'm not saying "flights" loosely, not like "He mised the turn and flew off the track," I'm saying lift greater than weight, the car was picked up off the track by aerodynamic forces and flew to the scene of the accident. There's not much risk of that at ecomodding speeds so I doubt we'll get ourselves into much trouble with our streamlining efforts.

[bombloader]

I know of the incidents with the Mercedes racers. I'm pretty sure it was at Lemans. If I remember correctly, they figured out that the car did generate lift if it was given a certain angle of attack. It seems that the cars hit bumps where the accidents started that caused them to do just that. Nobody noticed in the wind tunnel testing because the car sat on flat surface during all the testing. It seems like it really changed race car aerodynamics, guys got to thinking about what would happen to the cars aero in all kinds of circumstances, not just nice flat tracks. Not sure if that's what caused stock cars to develop their automatic roof spoilers, but I do know they've had them for a while. Of course, because their shape is fairly limited by rules, they can't change the entire car if it generates lift. So they just kill the lift if the car starts to get airborne.

[evolutionmovement]

That was LeMans. They removed the bumps after that.

[Bicycle Bob]

"A tea-tray will fly if you give it enough power."
- Popular saying of the aviation pioneers.

The first rash of blowover accidents involved hydroplanes. Then they appeared on drag strips. Race cars have always flown at the Nurburgring, but Le Mans is faster. Crosswind stability has always been a huge challenge for fully-streamlined bicycles. The four hazardous factors are speed squared, size, light weight, and attached flow on the low-pressure side. On a car, a rounded nose can reduce the effective aspect ratio of the unintended airfoil, while the more upright velomobile is much worse off. The size and height of a SUV would work against it.

In designing velomobiles, I try for enough balance that if it hit a patch of wet ice in a strong crosswind, it would get pushed sideways, but stay straight. In situations where there is more traction, that helps ensure equal slip angles at front and rear, so that gusts don't require much steering correction. To reduce the absolute force, it might be worthwhile to design shapes that barely maintain attached flow on the side in still air, so that they stall before generating much force, but that can also move the cp as it raises the cd.

Aye, BL, I was on about yaw, with the sideslip example. Most aircraft do trade drag for stability, but the canards get lift at both ends.

[aerohead]

Originally Posted by Bicycle Bob

I'm still waiting for the reference on that "ideal" 2.5: 1 teardrop. My many other sources all say that trying to proceed from 5:1 to 3:1 will almost inevitably cause separation. And what if there is any modification to the forebody shape, for wheels or other practical considerations?

Bob,the reference is from Hoerner,it's from airship hull research.For a frontal area-based drag coefficient,the fusiform body of revolution,with 1/3 forebody,and 2/3 aftbody and thickness ratio(length/diameter) of 2.5:1 has Cd 0.04 in free air.An image of this form appears in Hucho's book and is sourced from Horner's book of 1951,Table-7.2,I believe.I'll try to get this stuff online this coming Saturday.

[aerohead]

Originally Posted by theycallmeebryan

I was curious as to how the idea of (4 length : 1 Height) ratio would apply to my Ninja 250. So i drew a diagram in photoshop to figure it out.



I took into consideration where my head would be for the highest point.
The lighter fairing meets the 4 length : 1 Height ratio completely.
It would be 15.4 feet long!!
The dark fairing is perhaps a more realistic and streetable approach.
It would be a realistic 8.55 feet long.

Not too far off from what the prometheus EV ninja looks like.

The template isn't intended for motorcycles per se.You would never survive a sidewind gust.You'd simply be blown off the road or into the next lane,vehicle,guardrail,fence,tractor-trailer,etc..You get the picture.------- My oldest brother witnessed a rider decapitation this way in Los Angeles.Trikes have an advantage with respect to streamlining,as they provide some roll moment resistance impossible to achieve with a 2-wheeler.

[aerohead]

Originally Posted by pvan93

Try googling NACA and report 291 for a start. The lowest drag came from a Fineness ratio of 2. A NACA search for airforms and struts also brings out some interesting information that might be applicable to cross winds.

A referance somewhere (may have been here) said the nose should have the shape of a 2 to 1 ellipse, while the tail should have a shape of a 3 to 1 ellipse. Anyone know? Just bought a trailer for my boattail. Would like to verify the 3 to 1 ellipse referance before I build anything.

Struts and wings are two-dimensional forms,and not really valid for "bodies"..Sylph uses an "airfoil" as the basis of it's body,however,it's so short(height) it's more a body-of-revolution as the trikes fuselage.It was chosen for the position of it's "thickness",for the mechanicals it was going to enclose( HONDA Gold Wing ).--------- As for a nose,below 250 mph,a prolate hemisphere( 1/4-section) is adequate.------------ The ellipse is good,the airship tail a bit better.

[aerohead]

Originally Posted by Bicycle Bob

Well, that was tedious, digging out a 1927 study, but it says at NACA UK Mirror report description page
that the lowest drag was on the 4.62 fineness ratio. This was at a very low Reynold's number, which may account for all the Zeppelins being much finer.

I see that figure 13, presumably the source of this discussion, does not seem to agree with figures 7 and 9

Hoerner shows the smaller fineness ratios to be of lower drag.As airships,these forms are less "stable" in flight,and capable of inducing air-sickness in the crews.The Paul Jaray-designed Zeppelins are of the "lowest" fineness ratios compared to their siblings,and faster on a frontal area/horsepower basis.-------------- The US NAVY ZMC all-clad airships,of 2.8:1 are of the fastest for frontal area/horsepower.------------Also,the US Naval Academy's SQUID,human-powered submarine,with thickness of 2.5:1 was the World's fastest in the mid-90s.