Last year I completed the conversion of my standard wing RV4 to a custom tapered wing. For those interested, here are some findings from the project:
Why: I wanted to increase the range of the plane to complete my most frequent trip nonstop. I had previously increased the fuel capacity by extending the standard tanks a couple ribs outboard to hold 27 gallons each. I felt that increasing the capacity further was a bit “aggressive” without conducting a structural analysis and I wanted to know I had the ability to land with the wing fully fueled (since there is no “fuel dumping” provision) should a trip have to be terminated much earlier than expected.
Since engineering was going to be done I decided (although it would add a lot of work and complexity) to also taper the wing. The decision to taper the wing was mostly for aesthetics. I enjoyed my standard RV4, but when I looked to the sides, I always had the sensation I was flying a ping pong table.
What was done: The new wing was built using the same airfoil and design/construction parameters as the original and simply incorporating a 64% taper. The chord at the tip and spar height at the tip are 64% of what they are at the root.
Washout of 1.5 degrees (IIRC) was incorporated to retain good stall characteristics with the taper. The span was increased 1’ on each side from a span of 23’ to 25’. The increase in span was a somewhat arbitrary attempt to recover some of the lost wing area. This resulted in a 10% loss in wing area instead of an 18% loss which would have occurred with the taper if not increasing the span. A secondary objective was to reduce the roll rate to make the plane a bit better for flight in IMC and it was also thought this increase in span would slow the roll rate.
The wing skin does not change from .032 to .025 midspan, but instead a single sheet of .032 runs the entire length of each wing. Increasing the skin thickness in the outboard section was an attempt to maintain torsional stiffness of the wing in the outboard section where the cross section (ribs) was getting pretty small, and the one piece skin gives it a clean look. Similarly the aileron skins were increased to .025 (I think it’s .016 on early RV’s like mine and maybe .020 on later ones???) The flap skins were also increased to .025 The increase in control surface skin thickness was an attempt to increase the TAS limit of the aircraft and prevent aileron cracking that has reportedly occurred in some early RV’s. In regard to higher TAS limit: The “published red line” (Vne) of the RV4 (and some others I think) is 210 MPH IAS. Vans seems reluctant to specify an altitude to which that IAS is “safe” but did write an article (I think it was related to turbo charging RV’s) about the dangers of high TAS that can be obtained at higher altitudes. I’ve always powered back significantly in descents from altitudes in the teens to keep the TAS down to a “reasonable” limit. I had hoped to fly the new wing at higher TAS and keep (at least more) power on in the decent. Therefore the ailerons were balanced to 100% static, whereas on the previous wing they were balanced to something less than that (The water pipe at the leading edge of the aileron is not quite heavy enough to achieve 100% static balance) More on TAS testing below.
Outboard of the main fuel tanks are aux tanks which run from the main tanks out to the fiberglass wingtips. The aux tanks are of the same basic design and construction as the main tanks which are of the same construction as the original RV-4 except that they incorporate the Z-brackets of the RV8 fuel tanks. The fuel capacity is a total of 72 gallons. All four tanks are individually plumbed to a 4 position fuel valve which replaced the old 2 position (plus off) fuel valve.
The RV-8 aileron bellcrank was used as it is a bit more “straightforward” than the original RV4 bellcrank. The bellcranks, the fuel caps, the internally threaded wing tie down ring receivers and the 4 spar splice plates at the aircraft centerline are the only Vans parts applicable to this wing.
Design parameters: The wing was designed for a gross weight of 1942 Lbs at +4.4 / -1.75 (though the RV-4 landing gear and motor mount to firewall attachments likely are not adequate, at least for landing, at this weight).
Due to the aforementioned measures to increase torsional stiffness and flutter resistance, Vne testing was intended to proceed up to 231 MPH IAS at a PA of 10,000 feet. 231 MPH is 110% (the standard for Vne testing) of the “published red line”b of 210 MPH. This would yield a TAS of about 280 MPH.
The Results:
Weight and Balance: The tapered wing weighs 13.5 (each wing so 27 LBS for aircraft) more than the rectangular wing I removed. 7 Lbs of this weight comes from changing the outboard leading edge section into an Aux fuel tank. If the Aux tank was omitted the increase in weight over the original wing would be only 6.5 Lbs. The spar weighs 8 Lbs more (each side) than a standard RV4 spar. This is because the spar caps (stacked and staggered like the original) are all a bit longer than the original spar caps. The wing skin weighs about the same as before because the thicker (.032) skin is offset by their being less skin area. There is about 1.5 LBS of weight savings in the smaller ailerons and other components in the wing.
The CG envelope is narrower on the tapered wing. However the aft limit remains in the same place, it is the forward limit that has moved aft. Since RV4’s are never loaded near the fwd limit, this is of no consequence. The CG of the wing itself is slightly fwd of that of the original wing so this moves the empty CG of the A/C forward just a bit. How much? The shift in CG allows for having a back seat occupant that weighs 6 lbs more than with the original wing (all other factors being the same of course) OR 3 lbs more in the aft baggage compartment than would have been allowed with the original wing. So this difference is pretty minimal.
Performance: (The same engine, propeller, instruments, etc. were used to collect test data on each wing, only the wing was changed. Of course the pitot tube had to be installed on the new wing and there is a possibility it was not installed at exactly the same angle and alignment as before.)
Cruise speed increased by 4 Knots. (Measured by GPS in multiple directions on multiple occasions)
Stall speed was unchanged as indicated by the same ASI as on the original wing. (Had I not increased the span I expect stall speed would have increased a few knots).
Stall characteristics are (as close as can be perceived) unchanged.
Rate of Climb: This was expected to improve with the higher aspect ratio, especially at higher altitudes. It probably did, but this was hard to measure. A climb test was done both with the original and tapered wing with the same weight and CG and nearly the same OAT. Days were chosen that had a stable lapse rate (stable air). However this data collection was not extensive and repeated on multiple days. The resulting data has some obvious “noise” in it which may be greater than the differences being measured. The data did show a modest increase in ROC, especially above 12,000’ PA but the I feel the collection of data was not extensive enough to support and claim. Computer modeling/testing would probably yield more accurate data than the actual flight testing provided.
Vld / Glide Angle: The higher aspect ratio undoubtedly improved the glide performance of the airplane, but again, the data collected in flight testing is not extensive or “clean” enough to support any performance claim.
Vne/Flutter testing: Static torsional loading and measurement of the original wing and the tapered wing were performed and the data indicated the tapered wing had slightly more resistance to aileron induced flutter than did the original wing. The stiffness of the tapered wing was actually less (as to be expected with its reduced cross sectional area) but the aileron is closer to the spar so the torsional loads applied by the aileron (especially this smaller aileron) are less.
As stated above flight testing was intended up to 231 MPH IAS at 10,000’ PA. Somewhere just a bit below that mark (I don’t remember the exact number and have not yet received the flight report from the test pilot that would contain this exact data) there was a limit cycle oscillation of the elevator and further testing to higher speeds was aborted. I understand that the RV4 empennage on the Harmon Rocket has been tested to speeds considerable higher than this , but my empennage did not exhibit such robust characteristics. Perhaps it is due to mine being an early empennage consisting of .016 instead of .020 control surface skin, or too many flight hours on the trim tab hinge, or???? Refitting a new empennage may occur in the future, or at least re-skinning of the control surfaces, and thence further Vne testing will be conducted to the original intended parameters.
Flight testing was conducted at the design gross weight of 1942 Lbs at +4.4 G’s (Negative G testing was not conducted as the aircraft is not equipped with inverted systems) and the test pilot reported no signs or sounds or reaction of any type from the wing. He did report that when he pulls his RV to its design limit it makes a lot of “ugly noise”.
Spin testing was not conducted and the aircraft is placarded “Intentional Spins Prohibited” (due to the increase outboard mass of the fuel tanks, even if they are empty) but extensive stall and accelerated stall testing was conductive and the aircraft exhibited no propensity to enter a spin and normal responses to spin recovery inputs.
Roll and Yaw coupling remain (so far as can be perceived) the same as with the original wing. That is to say that roll inputs generate very little yaw tendency and yaw inputs create very little roll.
The much touted “balanced control feel “ of the RV (the fact that the “weight” of the stick forces in roll are about the same as the stick forces in pitch) was considerably degraded. As expected (with smaller ailerons) the roll forces are considerably less than they were before, but the pitch forces are unchanged. This parameter was not measured/quantified with an instrument, but it is obvious to the pilot that the roll forces are much lighter than the pitch forces.
The “typical regimen” of positive G aerobatic maneuvers was performed and the test pilot (familiar with RV aerobatics) stated that he found himself wishing the pitch forces could be reduced to match the aileron forces. He reported that (albeit subjective) it was in the aerobatics that he really felt it flew any different than a “standard RV-4”. In the previous test flights (except for the reduced roll forces) it felt like a “normal RV” but in aerobatics it behaved much more “elegantly” and that he teaches his RV aerobatics students that if they get into trouble (too fast) on the downline they can “escape” by pulling G’s and the wing loads up with drag and slows down. It was his perception that the tapered wing did not exhibit this increased drag at high AOA and therefore demanded more caution in aerobatics but would probably compete well against other RV’s at Reno – now Roswell…..
Roll Rate: Roll rate was measured on the original wing to be 120 degrees/second. This testing was measured only in rolls to the left, an oversight I suppose. The tapered wing roll rate (Aux fuel tanks empty) to the left was measured at 81 degrees/second. Oddly the roll rate to the right was measured at 101 degrees/second. The degree of difference was an unexplained surprise and the fact that it rolls faster against engine torque than with it is also unexplained. In any case, the slower roll rate expected and hoped for (for flight in IMC) appears to have been achieved. The lower stick forces in roll are not really desired for IMC flight and bellcrank modification may be implemented to remedy such.
Fuel imbalance testing was conducted up to 9 gallons of imbalance of the Aux (Outboard) tanks. At a 9 gallon differential quantity in the Aux tanks the imbalance was (surprisingly) almost imperceptible in flight. However the takeoff roll was getting pretty tricky as the increased weight on one landing gear made it somewhat difficult to maintain directional control and further imbalance testing was deemed unnecessary.
Aesthetics, according to most observers, is improved.
Range was obviously extended, as long as you carry a brief relief pee bag…..
Why: I wanted to increase the range of the plane to complete my most frequent trip nonstop. I had previously increased the fuel capacity by extending the standard tanks a couple ribs outboard to hold 27 gallons each. I felt that increasing the capacity further was a bit “aggressive” without conducting a structural analysis and I wanted to know I had the ability to land with the wing fully fueled (since there is no “fuel dumping” provision) should a trip have to be terminated much earlier than expected.
Since engineering was going to be done I decided (although it would add a lot of work and complexity) to also taper the wing. The decision to taper the wing was mostly for aesthetics. I enjoyed my standard RV4, but when I looked to the sides, I always had the sensation I was flying a ping pong table.
What was done: The new wing was built using the same airfoil and design/construction parameters as the original and simply incorporating a 64% taper. The chord at the tip and spar height at the tip are 64% of what they are at the root.
Washout of 1.5 degrees (IIRC) was incorporated to retain good stall characteristics with the taper. The span was increased 1’ on each side from a span of 23’ to 25’. The increase in span was a somewhat arbitrary attempt to recover some of the lost wing area. This resulted in a 10% loss in wing area instead of an 18% loss which would have occurred with the taper if not increasing the span. A secondary objective was to reduce the roll rate to make the plane a bit better for flight in IMC and it was also thought this increase in span would slow the roll rate.
The wing skin does not change from .032 to .025 midspan, but instead a single sheet of .032 runs the entire length of each wing. Increasing the skin thickness in the outboard section was an attempt to maintain torsional stiffness of the wing in the outboard section where the cross section (ribs) was getting pretty small, and the one piece skin gives it a clean look. Similarly the aileron skins were increased to .025 (I think it’s .016 on early RV’s like mine and maybe .020 on later ones???) The flap skins were also increased to .025 The increase in control surface skin thickness was an attempt to increase the TAS limit of the aircraft and prevent aileron cracking that has reportedly occurred in some early RV’s. In regard to higher TAS limit: The “published red line” (Vne) of the RV4 (and some others I think) is 210 MPH IAS. Vans seems reluctant to specify an altitude to which that IAS is “safe” but did write an article (I think it was related to turbo charging RV’s) about the dangers of high TAS that can be obtained at higher altitudes. I’ve always powered back significantly in descents from altitudes in the teens to keep the TAS down to a “reasonable” limit. I had hoped to fly the new wing at higher TAS and keep (at least more) power on in the decent. Therefore the ailerons were balanced to 100% static, whereas on the previous wing they were balanced to something less than that (The water pipe at the leading edge of the aileron is not quite heavy enough to achieve 100% static balance) More on TAS testing below.
Outboard of the main fuel tanks are aux tanks which run from the main tanks out to the fiberglass wingtips. The aux tanks are of the same basic design and construction as the main tanks which are of the same construction as the original RV-4 except that they incorporate the Z-brackets of the RV8 fuel tanks. The fuel capacity is a total of 72 gallons. All four tanks are individually plumbed to a 4 position fuel valve which replaced the old 2 position (plus off) fuel valve.
The RV-8 aileron bellcrank was used as it is a bit more “straightforward” than the original RV4 bellcrank. The bellcranks, the fuel caps, the internally threaded wing tie down ring receivers and the 4 spar splice plates at the aircraft centerline are the only Vans parts applicable to this wing.
Design parameters: The wing was designed for a gross weight of 1942 Lbs at +4.4 / -1.75 (though the RV-4 landing gear and motor mount to firewall attachments likely are not adequate, at least for landing, at this weight).
Due to the aforementioned measures to increase torsional stiffness and flutter resistance, Vne testing was intended to proceed up to 231 MPH IAS at a PA of 10,000 feet. 231 MPH is 110% (the standard for Vne testing) of the “published red line”b of 210 MPH. This would yield a TAS of about 280 MPH.
The Results:
Weight and Balance: The tapered wing weighs 13.5 (each wing so 27 LBS for aircraft) more than the rectangular wing I removed. 7 Lbs of this weight comes from changing the outboard leading edge section into an Aux fuel tank. If the Aux tank was omitted the increase in weight over the original wing would be only 6.5 Lbs. The spar weighs 8 Lbs more (each side) than a standard RV4 spar. This is because the spar caps (stacked and staggered like the original) are all a bit longer than the original spar caps. The wing skin weighs about the same as before because the thicker (.032) skin is offset by their being less skin area. There is about 1.5 LBS of weight savings in the smaller ailerons and other components in the wing.
The CG envelope is narrower on the tapered wing. However the aft limit remains in the same place, it is the forward limit that has moved aft. Since RV4’s are never loaded near the fwd limit, this is of no consequence. The CG of the wing itself is slightly fwd of that of the original wing so this moves the empty CG of the A/C forward just a bit. How much? The shift in CG allows for having a back seat occupant that weighs 6 lbs more than with the original wing (all other factors being the same of course) OR 3 lbs more in the aft baggage compartment than would have been allowed with the original wing. So this difference is pretty minimal.
Performance: (The same engine, propeller, instruments, etc. were used to collect test data on each wing, only the wing was changed. Of course the pitot tube had to be installed on the new wing and there is a possibility it was not installed at exactly the same angle and alignment as before.)
Cruise speed increased by 4 Knots. (Measured by GPS in multiple directions on multiple occasions)
Stall speed was unchanged as indicated by the same ASI as on the original wing. (Had I not increased the span I expect stall speed would have increased a few knots).
Stall characteristics are (as close as can be perceived) unchanged.
Rate of Climb: This was expected to improve with the higher aspect ratio, especially at higher altitudes. It probably did, but this was hard to measure. A climb test was done both with the original and tapered wing with the same weight and CG and nearly the same OAT. Days were chosen that had a stable lapse rate (stable air). However this data collection was not extensive and repeated on multiple days. The resulting data has some obvious “noise” in it which may be greater than the differences being measured. The data did show a modest increase in ROC, especially above 12,000’ PA but the I feel the collection of data was not extensive enough to support and claim. Computer modeling/testing would probably yield more accurate data than the actual flight testing provided.
Vld / Glide Angle: The higher aspect ratio undoubtedly improved the glide performance of the airplane, but again, the data collected in flight testing is not extensive or “clean” enough to support any performance claim.
Vne/Flutter testing: Static torsional loading and measurement of the original wing and the tapered wing were performed and the data indicated the tapered wing had slightly more resistance to aileron induced flutter than did the original wing. The stiffness of the tapered wing was actually less (as to be expected with its reduced cross sectional area) but the aileron is closer to the spar so the torsional loads applied by the aileron (especially this smaller aileron) are less.
As stated above flight testing was intended up to 231 MPH IAS at 10,000’ PA. Somewhere just a bit below that mark (I don’t remember the exact number and have not yet received the flight report from the test pilot that would contain this exact data) there was a limit cycle oscillation of the elevator and further testing to higher speeds was aborted. I understand that the RV4 empennage on the Harmon Rocket has been tested to speeds considerable higher than this , but my empennage did not exhibit such robust characteristics. Perhaps it is due to mine being an early empennage consisting of .016 instead of .020 control surface skin, or too many flight hours on the trim tab hinge, or???? Refitting a new empennage may occur in the future, or at least re-skinning of the control surfaces, and thence further Vne testing will be conducted to the original intended parameters.
Flight testing was conducted at the design gross weight of 1942 Lbs at +4.4 G’s (Negative G testing was not conducted as the aircraft is not equipped with inverted systems) and the test pilot reported no signs or sounds or reaction of any type from the wing. He did report that when he pulls his RV to its design limit it makes a lot of “ugly noise”.
Spin testing was not conducted and the aircraft is placarded “Intentional Spins Prohibited” (due to the increase outboard mass of the fuel tanks, even if they are empty) but extensive stall and accelerated stall testing was conductive and the aircraft exhibited no propensity to enter a spin and normal responses to spin recovery inputs.
Roll and Yaw coupling remain (so far as can be perceived) the same as with the original wing. That is to say that roll inputs generate very little yaw tendency and yaw inputs create very little roll.
The much touted “balanced control feel “ of the RV (the fact that the “weight” of the stick forces in roll are about the same as the stick forces in pitch) was considerably degraded. As expected (with smaller ailerons) the roll forces are considerably less than they were before, but the pitch forces are unchanged. This parameter was not measured/quantified with an instrument, but it is obvious to the pilot that the roll forces are much lighter than the pitch forces.
The “typical regimen” of positive G aerobatic maneuvers was performed and the test pilot (familiar with RV aerobatics) stated that he found himself wishing the pitch forces could be reduced to match the aileron forces. He reported that (albeit subjective) it was in the aerobatics that he really felt it flew any different than a “standard RV-4”. In the previous test flights (except for the reduced roll forces) it felt like a “normal RV” but in aerobatics it behaved much more “elegantly” and that he teaches his RV aerobatics students that if they get into trouble (too fast) on the downline they can “escape” by pulling G’s and the wing loads up with drag and slows down. It was his perception that the tapered wing did not exhibit this increased drag at high AOA and therefore demanded more caution in aerobatics but would probably compete well against other RV’s at Reno – now Roswell…..
Roll Rate: Roll rate was measured on the original wing to be 120 degrees/second. This testing was measured only in rolls to the left, an oversight I suppose. The tapered wing roll rate (Aux fuel tanks empty) to the left was measured at 81 degrees/second. Oddly the roll rate to the right was measured at 101 degrees/second. The degree of difference was an unexplained surprise and the fact that it rolls faster against engine torque than with it is also unexplained. In any case, the slower roll rate expected and hoped for (for flight in IMC) appears to have been achieved. The lower stick forces in roll are not really desired for IMC flight and bellcrank modification may be implemented to remedy such.
Fuel imbalance testing was conducted up to 9 gallons of imbalance of the Aux (Outboard) tanks. At a 9 gallon differential quantity in the Aux tanks the imbalance was (surprisingly) almost imperceptible in flight. However the takeoff roll was getting pretty tricky as the increased weight on one landing gear made it somewhat difficult to maintain directional control and further imbalance testing was deemed unnecessary.
Aesthetics, according to most observers, is improved.
Range was obviously extended, as long as you carry a brief relief pee bag…..
