This page tries to answer the question
What clearance angle should we use?
In bevel-down planes with a bed angle of 45 degrees and a 30 degree included angle on the blade there will be a 15 degree clearance angle.
For one particular blade, I think increasing the included angle by only one degree does much to improve blade performance if edge retention or chatter are problems. However, you cannot increase the included angle too far or the clearance angle will be too small.
In theory the minimum clearance angle is zero degrees, corresponding to an included angle equal to the bed angle. But if you did this then after the first few shavings are made and the blade edge crumbles and dulls then the cutting edge will be above the bevel surface and you will not produce another shaving because you will not be able to push down the wide bevel to compress the wood enough to get the cutting edge into the wood. The dull curved blade edge would want to rise in the cut making this even harder. Also you must drag the bevel it across the wood with great pressure and friction. Even when the blade is freshly sharpened you would only be able to start a cut at the edge of a board. Trying to start mid board would require a lot of pressure.
Because the plane blade extends below the sole, it is slowly worn down just like in honing but here the wood is the abrasive. The worn area would be parallel to the surface of the wood and sole of the plane. Now you have the problem described above for an intentional zero clearance angle. Brent Beach suspects that when this worn area gets large enough the blade feels dull. So a large clearance angle might reduce the frequency of sharpening because more blade length must be worn off before the wear bevel is wide and difficult to push into the wood. However since the included angle is less the blade length will reduce faster with use. In practice, if small clearance angles do not work, try a larger clearance angle. Large clearance angle means small included angle. A blade with a small included angle is like a little diving board sticking out there and might vibrate, producing chatter on harder woods. This is all a game of balancing pros and cons.
Imagine a blade with a large included angle. Now wear the first 0.001" off the tip. The cutting edge is now much thicker. Now imagine a very small included angle and dull the first 0.001" off the tip. The cutting edge is not much thicker and the blade would not seem so dull because it is still easy enough to push down and force the cutting edge into the wood.
Leonard Lee writes in The Complete Guide to Sharpening about the blade's included angle "Anything between 30 and 35 degrees is quite acceptable. If you go much lower than 30 degrees you encourage blade chatter; if you go much higher than 35 degrees any wear dulls the blade much faster, but, more significantly, you reduce the relief angle unacceptably, particularly on planes with a 45 degree bed. Bevel-down planes as a group represent an exception to the rule that blades should be sharpened at the lowest angle consistent with edge retention, because the bevel angle has no bearing on the cutting angle. A sturdy edge is wanted, and a basic grind angle between 30 to 35 degrees will give you good blade stability and the least chatter."
It is too bad Lee doesn't say what he thinks the relief (clearance) angle is for. He has implied it is involved with more than how quickly the blade dulls as Beach has suggested.
From the Lee Valley bench plane instructions: "The Veritas bench plane blade comes honed ready for use, with a 30 primary bevel and a 35 micro-bevel. This configuration provides a strong, long-wearing edge that can be quickly honed many times before the primary bevel needs to be reground. The 35 micro-bevel leaves a 10 relief angle; more than adequate to accommodate the springback of the wood fibers."
Choosing a clearance angle is a trade off between included angle for edge toughness/retention and producing desired surface finish for a long period of use, supportive geometry to avoid chatter, clearance angle for a prolonged feeling of sharpness and according to Lee Valley avoiding wood fiber springback. For a particular task (type of wood and grain) there will be an optimum clearance angle for your assessment of the correct balance of the tradeoffs. There probably isn't one optimal clearance angle for all tasks.
Perhaps we need a new Leonard Lee type rule that maintains quality plane performance and wood surface but minimizes sharpening frequency.
For bevel down planes the blades should be sharpened at the lowest included angle consistent with edge retention and chatterless performance.People seem happy with 15 degree clearance angle on bevel down planes. I currently use a double bevel with a 28 degree primary bevel and 30 degree microbevel. I will use that until I encounter a wood that makes my blade chatter. Then I will either start using a single bevel, decrease the clearance angle and sharpen more often or get a thicker blade made of better metal.
Thanks to Brent Beach for feedback and suggestions about this page.
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