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Idea

donstim avatar image
donstim suggested donstim edited

Reveal all secret hardcoded friction and braking coefficient tables

According to the SDK:

Several empirical/experimental tables are set in hard in the code to provide friction and braking coefficients Cside friction, Crolling resistance and Cbraking resistance depending on:

  • the type of contact (eg: wheels, skids, scrape points, etc...)
  • the type of surface (eg: concrete, snow, grass, etc...)
  • the surface condition (eg: normal, wet, icy, etc...)


Please reveal the values contained in these tables and allow developers to change them. Also, provide information on the source of the values currently being used.

aircraftflightmodeldocumentation
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Krazycolin avatar image
Krazycolin commented

yes please.

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donstim avatar image
donstim commented donstim edited

I'm not sure if I should start a new topic or just add to this one, but here goes...

It seems that more information has been added to the SDK regarding ground friction modeling, including the addition of friction coefficients used for "static" and "dynamic" friction at low and high speeds on dry and wet runways. However, this has raised some additional questions needed for me to better understand this model.

It is stated that the coefficients are expressed as in the coulomb model. That's all well and good, but the coefficients provided are for 2 surfaces sliding against each other, not that of a rubber tire rolling over a surface. The equation provided does have a "less than or equal to" sign for the friction force determined from the friction coefficients, but there is no explanation given for how the "less than" force is computed.

There is also a statement that using this model, "weight does not play a significant role on tarmac." I may not be understanding what you are trying to say here, but weight (or more accurately, the normal force) plays a very significant role on the friction force generated between the tires and the tarmac (or any other surface). You even have it correctly represented in your friction force equation. Why do you think jet transports have very effective lift dumping systems (ground spoilers) in order to get more normal force applied as quickly as possible? It is not the drag produced by these devices that is most important in achieving shorter stopping distances. It is the reduction in lift, and hence, increased normal force that results in higher braking friction force being generated.

For modeling of an airplane moving over a surface on the ground, one is not normally concerned about "static" and "dynamic" friction, but rather "rolling" and "braking" friction. Static and dynamic friction may come into play when the vehicle is stopped with the brakes on, preventing the wheels from turning. In that case, you have to consider a surface (rubber) resisting slippage against another surface (tarmac). There is an equation for rolling friction given, but it is a summation of the rolling and braking resistance coefficients multiplied by the strut force.

Perhaps there is more (hopefully a lot more) to the ground friction force model than has been provided in the SDK? For a rubber tire rolling on a surface, there is very little resistance. This is what is referred to in the aviation industry as "rolling friction." For example, for a transport category airplane, the rolling coefficient of friction is on the order of 0.0165.

For non-skidding braking on a dry surface (e.g., an airplane with an effective anti-skid system), the maximum braking friction does not vary much with speed and is on the order of 0.40. For a wet or snow covered surface, the maximum braking friction does vary greatly with speed and is (very roughly) about 50% and 25% of the dry braking friction, respectively. On ice, the maximum braking friction is about 0.05 to 0.08, depending on speed. Thus, a non-skidding braked wheel on a dry runway cannot achieve the friction coefficients in your table even though skidding drastically reduces the effective braking coefficient.

Using your current model makes it difficult to accurately model both the taxi movement situation (rolling friction) and the landing or rejected takeoff braked distances.

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