Motorcycle Aero Question

[Grant-53]

Fineness ratio is the length divided by the width or diameter a streamline body. The flow behind the rider can be cleaned up considerably with a well designed tail fin. The optimum shape is a gentle curve rather than a straight angle. In your case the max. width may need to include the battery boxes.

[aerohead]

When using a (vertical) airfoil as a template,unlike a (horizontal) streamline body of revolution, which can be truncated (K-form),the wing should have as complete an aft-body as possible,even if the slope is too great.

Dornier's truncated wing patent of 1920 has been bandied about,out of context,as an argument in favor of wing section truncation,but it should be limited to no more than around 5% of chord, or the drag will escalate geometrically.

[Frank]

Originally Posted by aerohead

When using a (vertical) airfoil as a template,unlike a (horizontal) streamline body of revolution, which can be truncated (K-form),the wing should have as complete an aft-body as possible,even if the slope is too great.

Why? Something to do with ground effect? Isn't air moving around the wing the same regardless of orientation? (I think you're saying that a horizontal airfoil acts differently than a vertical one, is this correct?)

[freebeard]

It doesn't act differently, and it's not ground effect.



It's a plane of symmetry.

[aerohead]

Originally Posted by Frank

Why? Something to do with ground effect? Isn't air moving around the wing the same regardless of orientation? (I think you're saying that a horizontal airfoil acts differently than a vertical one, is this correct?)

*Since the MC is narrow compared to its height,the convention is to streamline it using plan-view taper,which puts it in the domain of an 'uprighted,' 2-dimensional flow, symmetrical airfoil.
*The airfoil will essentially behave the same,whether horizontally as an 'elevator',or vertically as a 'rudder.'
*The part about having a full tail comes from empirical results from wind tunnel research.
*Truncated wing sections suffer enormous drag increase when their aft-bodies are truncated.
*And if they are truncated,they need to be 'closed' back their.One of the LSR motorcycles I just saw at Bonneville was tested at Darko Technologies wind tunnel,and it was found that the original open tail was a no-no as far as drag was concerned.
*Here's the mathematical relationship for wing truncation drag.You can arbitrarily 'build' some different sections in your calculator and see what the drag would be.

*And hint-hint,this year at Bonneville,a handful of LSR streamliners had finally adopted Walter Korff's full-tail boat tail,with clamshell doors over the parachute tubes,dating to Korff's 1965 Summer's Brothers' 'GOLDENROD'.of 555-mph design velocity.

[Frank]

Very interesting! Coincidentally, I've been looking at streamliners; it's hard to see the back-end. Most of their "photo op" pictures seem to be from the front or side. I was wondering how they were handling the parachute tubes. Most I've seen are just "open".

Thanks for the info, something else for me to play with!

[freebeard]

This appeared to be a 'chute attached directly to the 3rd member. It consisted of a cast aluminum lid off a pressure cooker or something, a hinge at the bottom and a simple choke cable release you can see clockwise from 12 o'clock.

/Check the underslung frame

[Frank]

Hi Guys,

I'm resurrecting another old post because I'm going to do something on this project this year (I really am!). The motorcycle configuration has been changed to eliminate the saddlebag batteries at the rear. 2017 Frank1 by Frank

In reading older replies I have a question. Typical motorcycle aero rear fairings that you can purchase are truncated (but closed). In post #15, aerohead mentions that truncation should be limited to no more than 5% of chord, or drag will increase exponentially. In an earlier post (#4), freebeard proposes Mair as an alternative which seems to say that drag won't be that bad (at least, compared to having nothing). I'm wondering if anyone has any further insights.

I going to do my best to close both sides but rules limit the max length extending past the rear tire to 10". Am I better off planning to truncate (with shallower slope) or closing down to a point (with greater slope)? I appreciate any insight.

BTW, I've gone >200 mph several times in this configuration. I'm now running a lower voltage pack and I want to maximize speed with about 100 less hp.

[freebeard]

Google Search Results - EcoModder.com:Faschenfeld+tearing+edges


https://www.motornature.com/wp-conte...by-Coupe-2.jpg

According to various authorities, optimum would be a tearing edge and a concave bowl. The Cobra truncation was angled toward the ground, this may not apply to a motorcycle.

[Grant-53]

Tests done on high performance bicycle tube aerodynamics yields a 5:1 airfoil with Kamm tail reducing to a 75% length body. This seems the best combination of low drag and minimum cross wind. This is at a lower speed. If crosswind is not an issue, use a 3:1 body and cut at the maximum legal length. The cross section would be a heart shape with fairing behind the helmet.