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They are two different wings. The ES has a wing loading of around 22lbs/sq Ft and the IV has a wing loading around 34lbs/sq Ft. The ES has a rear mounted spar and a short stub front spar (I know that’s wrong nomenclature). The IV has a traditional front spar. The IV also has very little dihedral whereas the ES has significant dihedral. The IVs wing is very thin. Two very different animals.


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George Wehrung wrote: ↑Thu Aug 17, 2023 6:21 pm They are two different wings. The ES has a wing loading of around 22lbs/sq Ft and the IV has a wing loading around 34lbs/sq Ft. The ES has a rear mounted spar and a short stub front spar (I know that’s wrong nomenclature). The IV has a traditional front spar. The IV also has very little dihedral whereas the ES has significant dihedral. The IVs wing is very thin. Two very different animals.

But both say RXM5-217 at the root transitioning to NACA 64-212 at the tip. Seems like actual and potential differences are: (1) wing loading (which is really just the weight they support); (2) amount of dihedral (apparently more for the ES); (3) amount of washout??? (4) rate at which the airfoil transitions from RXM5-215 to the NACA 64-212???

I do understand that dihedral has a significant effect on the rate at which the bird wants to return to wings level. Someone somewhere must know whether that's the main issue, but in any case, it's apparently clear from experience the IV is more critical, and even so, the ES wasn't able to obtain a standard airworthiness certificate under Part 21, although I don't know those details. Presumably someone knows those, too.

When you see them side by side you will notice that they’re completely different. The IV wing is shorter width by 3’ per side, shorter chord length beginning at the wing root, and the wing is much thinner. The ES wing looks like a Hershey bar next to a IV even though it’s a very sleek wing. I am not an aerospace engineer but if I had to guess, the design for the wing would need some more aerodynamic twist to get a few more knots lower in the stall region and more elevator authority. But that comes at the cost of speed. Some builders have put stall strips on the wings. I have not. I like a clean wing.


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It make sense that the IV wing is much thinner given that is the same airfoil but smaller (so everything is proportionately compressed, including thickness at any % of span and % of distance from leading edge.). I am still puzzled why the stall is so much sharper. Maybe dihedral; maybe wing loading; maybe amount of twist. Martin Holman says in his book the IV has 2 degrees of washout (twist to get the roots to stall well before the tips). Maybe the ES has more? If not, what's left is dihedral and wing loading. Some experts somewhere would know.

My basic point is that, while I've never spun and never intend to, the handling of my ES seems great at stall. Carsten Sundin said the one he built was the same. I am just wondering why it's different with the IV given that it has a scaled down version of the same airfoil.

Dan,

Stall characteristics from airplane-to-airplane in the IV and ES aircraft can be wildly different. How the wing was built, especially early hand-built ones, and bodywork disparities both can cause large variations in both flight and stall qualities. That is the nature of the beast of experimental aircraft unfortunately.

Wing loading by itself can make a huge difference in stall characteristics. I have the most experience in this area with Evolutions. Having stall tested a large majority of the fleet, I can tell you that variable isn’t insignificant. When the gross weight increase was first approved to 4550 lbs from 4300 lbs we found that airplanes that once had benign stall characteristics became much harsher and unpredictable. Wing drop and altitude loss from the stall both changed measurably. Stall strip placement was more critical to reduce the severity of the stalls. Airspeed at the stall increased significantly also. This was all from a 5.8% increase in wing loading on a “friendly” airfoil compared to the IV/ES. Wing loading on the IV is upward of 30% higher than the ES.

As a data point on several Evos, starting at 4000 lbs airspeed at the stall break results in a full flap stall speed around 62 knots. Increasing to the max gross weight results in a full flap stall speed of 71 knots. A 13.8% increase in weight results in a 16.4% increase in stall speed. The bottom line is it isn’t linear, and this doesn’t take into account the noticeable changes in stability at the stall.

One other variable is the technique and parameter used to test the stall. The testing in Evos I discuss earlier was done within a narrow window of weight & CG, with a 1 knot-per-second speed reduction at idle power, coordinated, with an engine and prop setup correctly, at a given altitude in smooth air, and with flight control inputs within certain parameters. Change any of this and the results will likely be different. There have been many times in flight testing where I had to redo a stall because I had the rudder the tiniest bit uncoordinated. On the G900 we aimed to have the ball centered within a pixel or two. A quarter of a ball out of center would change the wing drop. Many pilots I’ve flown with can’t even really feel that difference, let alone the tiny parameters aimed for in flight testing,

Just because one person says their IV/ES stalls “great” or “has a large wing drop” doesn’t mean the next aircraft, or even the next stall in the same aircraft, will be anything like it. Take what you read on forums from people with limited experience with a grain, or perhaps heaping helping, of salt.

Thank you JC for that discussion. Lot's of complexities. Seems like wing loading can have a large effect not only the stall speed but also on sharpness of the stall. Good reason to avoid that part of the envelope especially when heavy.

In commercial maneuvers, my instructor said the person doing the check ride would expect at least the stall warning in my AOA indicator on some of the maneuvers (power off and on stalls, accelerated stall, end of chandelle). We went past that point on the stalls (got a slight nose drop) and went to that point on the chandelle. Nothing unusual in my ES.

Well written inputs JC and Ryan!


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