Can I have a few comments on this ?

[PeterS]

I hate to admit it dcb but that would be FAR beyond my capabilities, the model is underway and I'll arrange some kind of a scale wind tunnel to see the air flow .I'm still quite worried about the loads from side winds ,especially those from 10 or 20 degrees off the nose , but I can't work out how to test those loads .

What I've drawn is a lot like Allert's model ,but considerably higher and with a higher Centre of Areas /Pressure and a higher Centre of Gravity .There isn't much I can do about that as I have to just design a shell to fit over a normal upright bike and I also want to use a 10 hp air cooled diesel ....which rules out the more complex FF scooters and pretty well requires a standard bike with a pre unit gearbox .

The diesel is because a diesel has about 30% efficiency versus 20 or 22% for a petrol /gasoline engine .Unfaired Enfield diesels are returning 130 mpg US (1.8 litres /100km)so a faired version should be very economical .I also produce biodiesel so it is my preferred fuel.

[Grant-53]

Simplest method for shaping the shell might be using a spreadsheet 3D graph. Most designs start with a flat bottom and curved top which generates lift. Use a profile of an inverted wing NACA shape. The top view would be a 5:1 foil with a Kamm tail. The nose cross-section should be as rounded as possible to reduce the Cd and area exposed to cross wind forces. Angle of attack of the body should be at negative 3 degrees. The windscreen should reach to the shoulder and a center section blend to the top of the helmet front and rear. Use rigid foam insulation to pad the shoulders and hips. An inertial reel lap belt mounted to the frame is worth the effort. Give some thought to internal air flow to the engine. Ceiling tile material and rubber mounts should help the noise and vibration problem. Good mirrors and marker lights help you to see and be seen. If you want to do some wind tunnel testing try a window fan and a 55 gal drum (or the metric equivalent Good luck mate!

[PeterS]

http://pic40.picturetrail.com/VOL282.../398090017.jpg

Hi Grant , thanks for the tips .I'm unsure about the practicality of an inverted NASA wing shape ... could you link to something like that ? I can't visualise it .I was planning a up curve on the underside of the fairing .... is that what you mean?

I was planning to use winglets to offer it bit of downforce .I wonder if they would be sufficient ?

''The nose cross-section should be as rounded as possible to reduce the Cd and area exposed to cross wind forces. Angle of attack of the body should be at negative 3 degrees''.
Could you clarify that ? The rounded nose is self explanatory but not the 3 degree angle of attack.Sorry ,but this field is rather new to me and the learning curve is steep .I find stuff online but the difficulty is understanding it !

I've found some very good sound deadening foam used in the marine industry .It appears very good ,so I'm planning a double layer of that inside a fiberglass nacelle .The engine is a standard 10 hp stationary with forced air through it's shell .It should be OK as long as there is a path for air in and out .A digital pyrometer might be a good idea, especially as I have a spare one ,I'd hate to cook an engine .

I'm hoping for 185 mpg US at 50 mph,a top speed of 70mph,not up to Allert's standards but still good .

For that I need a cD of .45 .I wonder if that is possible?

All and any advice is very welcome !

[Frank Lee]

I think Grant is thinking of bodies that are predominantly WIDE like cars. With bike bodies being so narrow I suspect the whole "inverted airfoil" and "3 degree" things don't apply.

[redyaris]

Grant-53 could you provide references for you comments on inertia lap belts on motorcycles and motorcycle body shapes? Your comments on body shape are inconsistent with Hucho et all... being strapped onto a vehicle without a protective shell is a perscription for injury or death, in an accident!

[PeterS]

I think they relate to my fairing shell pictured at the top my post 43.

[PeterS]

Good info here .
Aerodynamics | FF Web

It is widely believed that bodied PTW's are susceptible to sidewind disturbance. This is sometimes cited as the reason for their ban in motorcycle racing. Sidewinds may disturb an FF if it has the 'wrong' characteristics but it is actually quite easy to achieve excellent cross-wind stability.

Sidewinds act on the whole vehicle, tending to roll it, on it's contact patches, out of the wind. This effect is modified by the shape of the vehicle and normal PTW steering geometry.

The shape of the vehicle has two effects. If the shape has more side area behind the CG than in front of it, it will "weathercock", turn into wind, and this is a desirable effect. The other effect is that if the side area is high, especially at the front, it will increase the tendancy of the vehicle to roll out of the wind. This is an undesirable effect.

This is the basis of the 'notched nose' shaped used in the Voyager and Cmax designs.
(Old and new | FF Web).
In the course of development it also became clear that flow separation devices damp and reduce these 'shape' related effects.

PTW steering geometry modifes the effect because the steering rotates away from a sidewind, the trail element in the geometry providing the lever. This is the same effect as normal counter-steer. The vehicle will then roll into the wind with gyro and tyre-coning effects resisting the wind-induced counter steer to provide self-balance at some resultant angle to the wind.

This effect can be modified by bodyshape - countered by a high frontal side area, or assisted by a suitable tail area.

It can also be modified by PTW geometry. Dynamic trail provides directional stability, so large trail figures will generate a higher resistance to deflection of the steering by a given sidewind - and reduce the steering angle of a given deflection. High trail figures therefor reduce the automatic counter-steer effect.

Conversly, low trail figures generate a relatively large steering rotation for a given sidewind, and the dynamic trail effect is less. Therefor short trail will produce steering that is more reactive to sidewinds, and this allows adjustment of the speed and energy of the self-balancing effect.

HCS systems readily tolerate rapid self-balancing, which minimises the detectable disturbance and is desirable.

[PeterS]

To summarise,

Tail area helps automatic turn into wind. Good.
High nose side area helps roll out of wind. Bad.
High trail reduces and slows, while Low trail increases and speeds, sidewind-induced counter-steer.
Fast sidewind-induced counter-steer reduces disturbance.

Short trail, small nose side area, and a good tail side area (at least as much as the nose) will produce a strong response, rolling firmly into a side wind. When flow separation features are added the result can be good 'self-correction' which does not intrude on the driver except in truely heroic winds.

For reference, the early Banana bodywork, without a tail, could be blown out of winds, but with later bodywork, including a tail and nose separation features, it did not, self-correcting adequately.
(Royce in the Banana | FF Web)
(Banana with new skin 1999 | FF Web)
Aerodynamic Efficiency

low drag, is usually regarded as the main reason for bodywork. In practice it has a minor role in the efficiency gain provided by the FF layout. At road vehicle speeds, simple frontal area is the main determinant of drag.

Almost all FFs that run as well as the vehicle they share an engine with, return fuel efficiency gains of around 20%. Many, especially high powered ones, achieve similar increases in top speed. Even 001, where drag was hardly considered, comfortably out-performed the Ducati motorcycle. It's always worthwhile keeping frontal area in mind when designing bodywork.

There are detailed ways to improve efficiency. None of these are mysterious. Smooth, clean surfaces and separations, slow surface direction changes, filled voids especially behind the rider, can add up to quite surprising fuel efficiencies - when the vehicle is being run fast enough for these details to matter.

The actual nose shape is least important for efficiency, although it must be carefully shaped to 'set-up' the separated airlfow over the rest of the vehicle.

Fat Jogger was optimised for efficiency more than any of the other shapes. In addition to it's clean lines it also has a sharply cut off 'Kamm Tail' and the resultant low pressure bubble is filled by the hot radiator outlet flow
(03 and FJ | FF Web). The side and centre stands are part of the shape, with the centre stand, when retracted, forming the 'chin' of the radiator inlets.

It impossible to say whether this detailing is 'worth' the effort. Although it has reached 90 mpg it has also returned worse figures than Ian's Production Voyager travelling together Vagaries of old engines and states of tune probably have a greater impact. However the three Voyager shapes are all fairly clean and a computer generated prediction of the Cd for 002 was .3. This level of efficiency is clearly worth having.

[PeterS]

Indifference

Extreme efficiency can lead to extreme problems. In general the cleaner a shape and the closer the airflow over it, the more sensitive it will be to disturbances in that airflow.

For FFs these problems chiefly amount to sensitivity to turbulence, where the vehicle is noticeably buffeted by turbulence, typically on motorways, at speed, in traffic. Ideally an FF should be “Indifferent” to these disturbances.

Poor indifference is caused when attached (lamminar) airflow over a surface changes direction or pressure. Sideways lift generated across a smooth nose may be balanced by sufficient tail area but the sudden cessation, or reversal of the airflow will instantly cancel the side load and this will be felt in the steering or attitude of the vehicle.

This is avoided, on cars as well as FFs, by enforcing separation of the airflow from the surface at chosen points, rather than allowing the separation line to wander about the surface generating unpredictable lift effects.

At the front this is usually done almost immediately, certainly within 500mm of the leading edge. 001 used immediate separation devices on all the leading edges and some of these sharp direction changes, can be seen in the picture.
(on the track | FF Web). 002 had less separation on flat surfaces, as nose lift was not seen as a problem but has clearly visible channels running up each side of the nose to enforce separation of any airflow across the nose surface
(002 | FF Web). The ledge in front of the lights is intended to enforce separation of the flow over the lights and features on all the Voyagers in various forms.
(FJ original nose, plus Production nose, for comparison. | FF Web)

002 however had a minor indifference problem. Strong turbulent side winds could be detected as tail buffeting. The probable reason for this became apparent when wool tufting tests were studied. 002 has no separators on the upper tail for cross flowing air. It's tail ridge is a gentle curve. .
(002 | FF Web)

The production Voyagers are a simple step on from 002. There is more tail area, chiefly due to the use of slides on the head fairing and a complete head fairing. The separation feature on the nose is somewhat more 'styled' and the nose is shorter due to packaging improvements in the basic design. The result was very encouraging with excellent stability and indifference, while efficiency seems as good as any other of these shapes.
(03 | FF Web)

Enforced separation of flow across the tail is just as important as across the nose. 001 and FJ both use separation enforcing edges on the upper edges of the tail. Subsequent study revealed that other vehicles have suffered from cross-flow generated tail lift and buffet and this device is a fairly common solution. The sharp upper edge of FJ's tail may be extreme but some device like this is important.
(Banana and Fat Jogger | FF Web)
FJ's nose shape paid only lip service to enforced separation. The shut line gap of the nose opening panel was supposed to bleed air into the airflow over the nose to provide separation. It's pressurised by the air entering the headlight opening. There is no evidence that this works and indifference was initially so bad that strip separators were retro-fitted, very crudely, to the nose.
(Royce | FF Web)

Indifference was still only moderate in some conditions and a new nose was made with explicit separation features and other changes.
(New Nose | FF Web). This shape improved indifference and flow into the radiators.

Finally the plan view of the vehicle should be considered. If the shape tapers too sharply after the point of maximum width then there may be too much separation towards the tail leading to poor indifference in some conditions. FJ has a slight problem in this respect, whereas the fatter-tailed production shape is markedly more indifferent. Remember the airflow will separate below 2 degrees positive angle of attack, so the tail should be almost parrallel untill the cut-off (Kamm-style) or whatever final separation point is chosen.

[PeterS]

This seems particularly relevant to what I'm trying to do .... I'm not after a record breaker but a bike that can be ridden in traffic and that means turbulent air .

Indifference

Extreme efficiency can lead to extreme problems. In general the cleaner a shape and the closer the airflow over it, the more sensitive it will be to disturbances in that airflow.

For FFs these problems chiefly amount to sensitivity to turbulence, where the vehicle is noticeably buffeted by turbulence, typically on motorways, at speed, in traffic. Ideally an FF should be “Indifferent” to these disturbances.

Poor indifference is caused when attached (lamminar) airflow over a surface changes direction or pressure. Sideways lift generated across a smooth nose may be balanced by sufficient tail area but the sudden cessation, or reversal of the airflow will instantly cancel the side load and this will be felt in the steering or attitude of the vehicle.

This is avoided, on cars as well as FFs, by enforcing separation of the airflow from the surface at chosen points, rather than allowing the separation line to wander about the surface generating unpredictable lift effects.