# Why does the helicopter Robinson R22's rotor blade have flat part as in the picture? What is it for?

This is Robinson Helicopter R22's rotor's blade. What is this flat part for as indicated by the red line? What is the aerodynamic benefit of putting it? There are many symmetric airfoils but no flat part like that.

• Are you sure about that? nasa.gov/image-feature/langley/100/naca-airfoils Many seem "flat" in that area. Nov 28, 2022 at 22:33
• For example, lower-right, 5th from the bottom. Nov 28, 2022 at 22:33
• @Steve, I checked the NASA's link, I didn't see any such flat extension. Moreover, it will extend separation/delay the upper wind meet the lower wind. The very basic of wing is the cord line is as short as possible. To meet the required wing area then it made long so the ratio of its long to its cord will be as big as possible. It is the very basic, it is the most efficient wing. Nov 28, 2022 at 23:20

The trailing edge part of an airfoil is the most "sensitive" part of it i.e. the part which changes the aerodynamic characteristics the most. That's why for example control surfaces like ailerons are placed there.

Backward extension of the trailing edge are normally used to decrease drag for free basically without modifications of the lift. The following plot from this NASA report shows $$C_l$$ and $$C_d$$ for several "Static Extended Trailing Edge" configuration. The plots to be compared are the ones labelled with a circle (baseline NACA 0012) wrt the ones labelled with an x (10% chord flat extension):

It can be seen that extending the trailing edge backward brings a small reduction in $$C_d$$ and a slight increment in $$C_l$$.

This report nicely describes why that happen. This report instead shows the (positive) impact of such a device for helicopter application.

I'm quite sure that also $$C_m$$ becomes more negative at high AoA but I couldn't find a plot to corroborate this claim.

• Actually, a bit different, but I accepted. The test in that report is extension for a fix wing acts as flap while in the R22's case is as a rotating wing. But as mentioned in the report that the TE extension increases the cruise efficiency, so for the rotating wing we probably may assume that it is being cruising. Nov 30, 2022 at 8:14
• In the report also a flat (i.e. without downward bending) leading edge extension is considered (the plots with the x) and this is basically what we see on the R22. So those plots can be totally used to understand the effect of the extension on the R22, which is a slight reduction of drag and increase of lift at high AoA. I'm quite sure that that extension modifies the pitching moment as well (less negative), but I couldn't find a plot for that. Nov 30, 2022 at 8:25
• @sophit: But why not just use a longer airfoil? Shouldn't that be even less drag then elongating it? Nov 30, 2022 at 21:36
• @AirCraftLover: here I've found another nice article about the topic and applied to helicopters Dec 1, 2022 at 9:30
• @U_flow: well, the increase in $C_l$ is more or less a fake since this plate extension increment the lifting surface and lift is proportional to it. Drag decreases because (according to the literature) the plate keeps the upper and lower boundary layers separated longer and that decreases drag. The report I linked in my answer contains an explanation about that process and some pictures as well. Dec 1, 2022 at 9:39

I assume the thin aluminum skin of the rotor is bonded at the trailing edge to keep the top and bottom together there. Due to the forces involved, that feature allows a larger area of close contact, maximizing the strength of the bond. That would be a poor place for rivets and aluminum bonding is notoriously picky to prevent long term failure so having a robust joint is critical, especially in the high stresses of a rotor.