Hartstoc
Well Known Member
A recent thread I posted on the importance of accurate W&B docs generated a lively discussion and lots of valuable shared experience from veteran RV builders and pilots, so I thought it might be worthwhile to expand the discussion in a few other directions. If this one gets a similar response, the next will be about the temptation to nudge the GW limit upward a bit during the registration process. Stay tuned.
This thread makes reference to the "selfie-quote" In my VAF signature, which seems to have ruffled a few feathers already. Is "RV-Fun is inversely proportional to RV-Weight" a valid statement? Should it be of concern to all RV builders? It may be a bit cryptic, but I think it IS valid for any given RV configuration of engine, prop, and instrumentation.
I'm not an aerodynamicist, and my hard knowledge here is dwarfed by my ignorance, but I do have a limited qualification to bring up the subject based on my experiences as rookie CAFE Foundation test-pilot/author or flight-engineer/volunteer for most of the CAFE APR series published in Sport Aviation during the 1990's, which included four RV models. During these APR's, we measured stick force gradients, explored stall recovery behavior, and flew a structured handling qualities protocol in multiple loading configurations for each aircraft tested, and the overall experience was a real eye opener.
Actually, I'm betting that you RV-vets will have more interesting things to say than me here, so I'll make just TWO points and cut this loose-
1-Absolute Weight- The rewards for lower empty weight are improved performance at any given load and a higher useful load, both more fun in my book. Fortunately, one strength of all Van's aircraft is that they are intrinsically light in weight if built according to the plans, but the designs are also quite sensitive to excess weight for reasons too complex(and too over my head)to fully address here. Ending up with a finished aircraft that is within Van's guidelines requires incredible discipline every step of the way. Weight goes on a little at a time, and the temptation to add more features and equipment because each will "only add a few ounces" is a slippery slope. Those of you just starting to build are in a position to adopt a very strict attitude about weight and maintain it through the whole process. Is it really worthwhile to prime all those inner surfaces on an airplane that will be babied in a hangar? The tired old saw has some truth: "If you are thinking about adding something to your airplane, toss it in the air. If it does NOT fall to the ground, then it is OK to install it." Every pound saved adds 8-15 miles to your range, depending on the model.
2- Polar Mass- Of course, everything you put on that airplane is subject to gravity, but there can be very compelling reasons to do so. Not everything has the same impact, though. Comfy seats are SO nice to have, and actually not a bad place to splurge a little because they are practically inside the CG range. A CS prop will be a must-have item on the RV-7A I am looking to buy, and the impressive speed range of most RV's simply cries out for CS, but there is a real price beyond dollars to be paid. Unfortunately, the gold-standard Hartzell is heavy and as far forward of CG as you can get. Flying with a forward CG requires extra down-force from the tail, which requires additional up force from the wing, etc.,etc., a drag-inducing positive feedback loop that, carried to the extreme, can compromise control authority in flair, reduce cruise performance efficiency, and make the aircraft sluggish and unpleasant to fly. The lightest weight solution to these problems is a bit of lead as far back in the tail as you can get. Now you have a better balanced airplane for control authority and efficiency, but one that handles a little bit less like a mid-engine sports car and a little bit more like a dumbbell. The prop and ballast have slightly increased resistance to control in the pitch and yaw axis. A heavy paint job would do the same, but with a price in category one too. A very strong argument can be made for saving even 8-10 pounds at the nose with a composite prop if, like me, you must have CS.
Or maybe you are ready to give up the CS advantages for a nice, light FP prop and maybe even a lighter engine. Great, you have just solved the polar mass problem and will have an incredibly nimble airplane that is a delight to fly. Now you must REALLY pay attention to the CG, limit luggage, hang everything you can toward the front, and really fuss over making the paint job light, or even go without, or you may find yourself needing to add one of those 20 pound crush plates behind that lightweight prop. (Or hey- maybe I SHOULD consider a whirlwind? Sheesh!) Every choice has a consequence.
Mind you, I'm not here to be critical of anyone's particular choices, many think added luxury is worth any price. I've just seen disappointment on the faces of too many builders learning the true weight of their aircraft for the first time. The important thing is to make these choices consciously and avoid regret. Not putting something on is a lot easier than taking it off later.
Link to my previous thread mentioned above:
http://www.vansairforce.com/community/showthread.php?p=1204252#post1204252
This thread makes reference to the "selfie-quote" In my VAF signature, which seems to have ruffled a few feathers already. Is "RV-Fun is inversely proportional to RV-Weight" a valid statement? Should it be of concern to all RV builders? It may be a bit cryptic, but I think it IS valid for any given RV configuration of engine, prop, and instrumentation.
I'm not an aerodynamicist, and my hard knowledge here is dwarfed by my ignorance, but I do have a limited qualification to bring up the subject based on my experiences as rookie CAFE Foundation test-pilot/author or flight-engineer/volunteer for most of the CAFE APR series published in Sport Aviation during the 1990's, which included four RV models. During these APR's, we measured stick force gradients, explored stall recovery behavior, and flew a structured handling qualities protocol in multiple loading configurations for each aircraft tested, and the overall experience was a real eye opener.
Actually, I'm betting that you RV-vets will have more interesting things to say than me here, so I'll make just TWO points and cut this loose-
1-Absolute Weight- The rewards for lower empty weight are improved performance at any given load and a higher useful load, both more fun in my book. Fortunately, one strength of all Van's aircraft is that they are intrinsically light in weight if built according to the plans, but the designs are also quite sensitive to excess weight for reasons too complex(and too over my head)to fully address here. Ending up with a finished aircraft that is within Van's guidelines requires incredible discipline every step of the way. Weight goes on a little at a time, and the temptation to add more features and equipment because each will "only add a few ounces" is a slippery slope. Those of you just starting to build are in a position to adopt a very strict attitude about weight and maintain it through the whole process. Is it really worthwhile to prime all those inner surfaces on an airplane that will be babied in a hangar? The tired old saw has some truth: "If you are thinking about adding something to your airplane, toss it in the air. If it does NOT fall to the ground, then it is OK to install it." Every pound saved adds 8-15 miles to your range, depending on the model.
2- Polar Mass- Of course, everything you put on that airplane is subject to gravity, but there can be very compelling reasons to do so. Not everything has the same impact, though. Comfy seats are SO nice to have, and actually not a bad place to splurge a little because they are practically inside the CG range. A CS prop will be a must-have item on the RV-7A I am looking to buy, and the impressive speed range of most RV's simply cries out for CS, but there is a real price beyond dollars to be paid. Unfortunately, the gold-standard Hartzell is heavy and as far forward of CG as you can get. Flying with a forward CG requires extra down-force from the tail, which requires additional up force from the wing, etc.,etc., a drag-inducing positive feedback loop that, carried to the extreme, can compromise control authority in flair, reduce cruise performance efficiency, and make the aircraft sluggish and unpleasant to fly. The lightest weight solution to these problems is a bit of lead as far back in the tail as you can get. Now you have a better balanced airplane for control authority and efficiency, but one that handles a little bit less like a mid-engine sports car and a little bit more like a dumbbell. The prop and ballast have slightly increased resistance to control in the pitch and yaw axis. A heavy paint job would do the same, but with a price in category one too. A very strong argument can be made for saving even 8-10 pounds at the nose with a composite prop if, like me, you must have CS.
Or maybe you are ready to give up the CS advantages for a nice, light FP prop and maybe even a lighter engine. Great, you have just solved the polar mass problem and will have an incredibly nimble airplane that is a delight to fly. Now you must REALLY pay attention to the CG, limit luggage, hang everything you can toward the front, and really fuss over making the paint job light, or even go without, or you may find yourself needing to add one of those 20 pound crush plates behind that lightweight prop. (Or hey- maybe I SHOULD consider a whirlwind? Sheesh!) Every choice has a consequence.
Mind you, I'm not here to be critical of anyone's particular choices, many think added luxury is worth any price. I've just seen disappointment on the faces of too many builders learning the true weight of their aircraft for the first time. The important thing is to make these choices consciously and avoid regret. Not putting something on is a lot easier than taking it off later.
Link to my previous thread mentioned above:
http://www.vansairforce.com/community/showthread.php?p=1204252#post1204252
Last edited: