---------------------------------------------------------- Europa-List Digest Archive --- Total Messages Posted Wed 01/10/18: 2 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 01:15 PM - Re: Are Vne and Va IAS or TAS? (Bud Yerly) 2. 03:58 PM - Re: Are Vne and Va IAS or TAS? (Pete) ________________________________ Message 1 _____________________________________ Time: 01:15:34 PM PST US From: Bud Yerly Subject: RE: Europa-List: Are Vne and Va IAS or TAS? Pete, Short answer: The U2 and coffin corner are at altitudes and airspeeds the Europa can neve r achieve... Therefore not relevant to mountain flying in a Europa. Or an y flight operation of a GA light propeller driven aircraft for that manner. The physiological limit of 25,000 feet without pressure breathing oxygen or cabin pressure and 50,000 foot limit without pressure suits, kind of li mits the Europa and its owners from fearing any of these effects. I prefer to believe the aerodynamicists on control design. Flutter is ofte n discussed and considered to be only the control surface moving by the non aerodynamicist, when the torsional rigidity of the wing, or control linkag e, or tab attachment, is the actual effect. Pilots feel vibration and repo rt it as flutter. From where, they don=92t know, it has to be found out. T AS is a component of the dynamic pressure or q = (1/2 density x V squared ) and the effect of q on the wing and the air speed itself can be explained below. A Europa in orbit at 25000 mph has no flutter problems as it has no air molecules. Without air molecules there is no air mass. No air mass, no aerodynamics. Mass of the air (viscosity, temp, etc. ) must always be c onsidered. Not just speed. Long answer: Re: Coffin corner. Coffin corner is operating between critical Mach number and Stall equivalen t airspeed. It is true that at 90 and 100 thousand feet, a subsonic slow f lying aircraft (even a glider in a dive I suppose) with proper propulsion ( uplift to get there and then dive) can get so high it is near the speed of sound even though it is near stall. Air liners at their operational limit can be there also as they cruise at .86 Mach or so. Supersonic designs are exempt from this. They just stall below transonic region at high altitude or fly supersonic and not stall at all at very high altitude. As you all know, Mach number is simply V/Va or the ratio of TAS speed divid ed by the speed of sound. The speed of sound is defined by the square root of specific heat of air x gRT with Temperature being the largest component of change. So its really cold at from 35,000-100,000 feet and the speed of sound is fairly constant in that range at about -55C or about 575KTAS. Air density of course is ex tremely small. So to get indicated airspeed high enough to fly (not stall due to lack of density), by default you must be getting closer to the Mach at the extremes of the atmosphere (from 45,000 you can feel the difference but much be much higher to see them close on a straight wing airplane). Example: 90 KIAS required to fly at the stall at 80,000 feet with a jet po wered drone, the true is 170 TAS, the mach is going to be about .3, so ther e is no technical coffin corner at 80K anywhere near the stall speed. Critical Mach is the Mach number where the normal shock wave forms on the a ircraft (normally the wing and fuselage juncture and the thickest part of a thick wing). In a U2 or T-33 that occurs about .86 Mach 45,000 feet, the indicated was 218 KIAS and 495TAS. Stall was about 120 KIAS. The cabin al titude was right at 25,000 so we couldn=92t go higher. Every subsonic airp lane has a different reaction to the large separated flow behind the normal shock wave. The U2 and T33 had hydraulic controlled ailerons so no flutte r or buzz possible on the wing. But the elevator control is severely affec ted by the separated flow of the shock wave. (Eventual loss of elevator co ntrol and nose pitch down.) The flexible wing design of some airplanes (drones, gliders, and even some powered planes) have very poor torsional rigidity (they bend leading edge u p and down easily with a vertical gust load or due to load deformation). I n straight and level non accelerated flight, if a normal shock wave forms o n the wing, the separated flow near mid chord, may cause the wing to twist, which changes the position of the shock wave, which changes the twist and a flutter will occur in the wing and the non boosted aileron will follow al ong, balanced or not. Turbulence exacerbates this problem due to the wings elastic effects. If only considering Mach crit, as the wing twists LE up (due to normal shock or turbulence), the lower side of the aileron will hav e pressure (due to normal flow) and the upper side not due to separated flo w and that can force the aileron upward, causing the wing to twist more and divergence begins until torsional rigidity finally reacts and the wing twi sts hopefully back down, changing the shock wave position to the lower surf ace and it all starts again. The stick will move of course so aileron flut ter is suspected when it is actually forced wing flutter. With aileron con trol input by the pilot at the same time, that twist effect can be quite la rge in flexible aircraft inviting very interesting effects. If operating a jet powered drone near the coffin corner, the pilot can=92t just pull up t o slow down (he=92s at the stall) and can=92t lower the nose to pick up air speed (he=92s seeing aeroelastic or Mach effects limiting control function) hence the name coffin corner. Only by deploying speed brakes and reducing power going to a lower altitude will recover the aircraft. Because the IA S at stall is a high TAS at very high altitude (50-100K), some have determi ned the affect of adverse aerodynamic affects to be because of TAS alone. That takes the TAS out of context and is a bit of a stretch to the actual p otential cause of their aeroelastic problems and the dampening that increas ed density may afford. It takes air to have aeroelastic problems. Re: Aeroelastic concerns: An aircraft built to be non rigid must be flown and tested very carefully a s any force outside of straight and level flight will have very serious eff ects. At very high altitudes, many high lift laminar airfoil designs have i ssues with the boundary layer separation and control effectiveness degradat ion occurs or wing twist due to the center of pressure shift the airfoil wa s not designed for (that is at weird Reynold=92s numbers ((a dimensionless number of 1.4 x density x V x length / viscosity of air that determines the boundary layer separation point))). At these extreme conditions, aeroelast ic effects and boundary layer separation are pronounced in these flexible f liers operating near the stall in the cold thin air of higher altitude such as the Perlan Project. We have all seen the wing flutter test video of gl iders encountering wing flutter, aileron reversing due to wing twist, etc. The effects of these problems are exacerbated at altitude and high TAS. T he q is low and the TAS high so some believe the density is irrelevant. It is not. Temperature is also a problem, especially with the modulus of ela sticity of carbon fiber. These types of aircraft are very light, but very flexible fliers and have to be thoroughly tested and instrumented due to th e problems associated with aeroelastic and structural effects (as well as a ileron deflection induced by separated flow and the affects of the low OAT) . Or they have to be built heavy but rigid (i.e. the Europa). Aircraft de signed for the long haul of normal operations tend to be quite stiff to avo id these problems and have long operational lives. Finally on aileron flutter. From Dommash - Airplane Aerodynamics: =93Precautions must be observed to prevent destructive flutter or undamped short-period oscillations of control surfaces. These precautions are relat ed to mass balance about the hinge axis, static contour and changes of cont our under air loads, and tightness of tab linkages.=94 =93Mass balance is balance the tab about the hinge axis. There is no rotat ive tendency to induce flutter from mass-inertia effects if the control is balance precisely on the hinge line.=94 See the Europa build manual. Contour Effects: =93All control surfaces should be flat sided or concave. =94 Convex surfaces (flexible fabric that balloons under pressure ((GB rac ers of 1930s)) become unstable and begin an undamped oscillation. See buil d manual on contour. Note: =93Spanwise torsional stiffness in a control surface may also cause flutter.=94 Long or solid hinges and stiff construction are important in c ontrol design. The Europa ailerons have sufficient stiffness and are reaso nably hinged to a rigid wing structure not prone in its flight envelope to aeroelastic effects. So contour your ailerons to the foam shape. Balance them. Keep the hinges and linkages tight and maintained, pad the aileron quick disconnects with no slop, and flutter will not be a problem at any operational limit of the Europa, to include mountain flying up to 25,000 feet for sure. Don=92t fly over 25,000 unpressurized for any extended period, as it is a serious phys iological limit to most people even on oxygen without pressure breathing. Fly your IAS within limits of the flight manual/POH and enjoy yourself. If you want to design drones or manned gliders operating at 100K, that is a different type airplane flying in a different world. To date, only highly funded experimental aircraft fly near that regime. Best Regards, Bud Yerly Sent from Mail for Window s 10 From: Pete Sent: Tuesday, January 9, 2018 12:38 PM Subject: Re: Europa-List: Are Vne and Va IAS or TAS? Not to be argumentative, but as i understand it, Bud's explanation ignores the issue reported/described in the other articles, namely that at altitude there is less flutter damping for the same reasons Bud explains wrt the st ructural limits. So for flutter specifically it is TAS that is the determi ning limit, and not IAS. Taken to the extreme, the U2 sometimes operates n ear "coffin corner" where flutter speed is very near stall speed (IAS is in dicating near stall, but they are so high and the air so thin that they are near the risk of flutter). It is also why the http://www.perlanproject.or g/ glider is heavily i nstrumented to detect the onset of flutter as they go higher and higher in their record attempts. That said, it is comforting to know that Ivan has tested Vd with stick exci tation to 8000 feet, so i would assume that this could be used as the derat ing baseline when going to higher TAS's. Cheers, Pete A239 On Jan 9, 2018, at 7:08 AM, William Daniell > wrote: Bud thanks, sets my mind at rest. William Daniell LONGPORT +57 310 295 0744 On Sun, Jan 7, 2018 at 11:09 PM, Bud Yerly > wrote: Will, I'm still on the road but here goes. Short answer: Q is force, whereas True is just speed. Long Answer: Think of it this way. The mono cruises at 175 TAS at 18,000 with a 914. T he indicated is only 125 ish. You cannot pull enough G to achieve 6 Gs bef ore you stall or have a vertical gust break the plane. Is the mono above VN E, at 175 TAS, NO. Heck you are not at Vno or about 131 KIAS which is your green arc for gust factor(turbulent speed). That gust factor is what you need to be aware of in the turbulence of the m ountains. "Aircraft Performance" by Domash explains it. So does the FAA. For finding high speed affects, the USAF F104 VN diagram is on Wikipedia and shows how Mach affects figure in for high TAS. TAS is important in turn rate, radius, navigation, and determining your Mac h and Q velocity. But it is your Q (dynamic pressure), aka IAS, that affect s, flutter, structural deformation, and your stall and not to exceed speed s. This means what you read on your airspeed indicator is what you need to know for the plane. TAS and Ground Speed affect your pilotage which is a different topic. Again, in mountain flying, you need to know your turn diameter when valley flying, high altitude patterns (wider pattern necessary), lead turns to a r adials etc. (especially In high speed aircraft) and in light aircraft in ve ry high elevations. Engine performance vs airspeed bleed off becomes a fac tor as well. Regards, Bud Yerly Custom Flight Creations From: William Daniell Sent: Friday, January 5, 4:07 PM Subject: RE: Europa-List: Are Vne and Va IAS or TAS? Bud Does this apply even at the upper altitude range ....say 13k or 15k? Will On Jan 4, 2018 22:19, "Bud Yerly" @msn.co m> wrote: Yep Graham, Airplanes only feel air pressure, not the velocity of the molecule alone. Dynamic pressure is =BD Density times Velocity Squared or IAS (actually y ou have calibrated then equivalent) is what the airplane feels. Those RV g uys got all hung up on this and confused everyone. Bottom line, what you read on the airspeed indicator counts. TAS is import ant (actually Mach number) as the skin heats up due to friction which is a different ball of wax. I was always a slow speed aero guy to match my mind .. Regards, Bud Yerly Sent from Mail for Windows 10 From: owner-europa-list-server@matronics.com > on behalf of Pete @zutrasoft.com> Sent: Thursday, January 4, 2018 5:49:37 PM Subject: Re: Europa-List: Are Vne and Va IAS or TAS? Hi Ivan, just to confirm, Vd IAS @8000ft DA? Thanks again! Pete On Jan 4, 2018, at 5:21 PM, @btint ernet.com> @btinternet.com> wrote: All our company aircraft were tested to Vd, 10% over Vne. And not just take n to the speed but then tested [short stick and rudder raps] to see if any flutter mode could be excited at Vd. I have performed these tests at/up to 8000ft . We have never experienced any flutter mode. I also tested the tail plane underbalanced and over balanced with the same results. To my knowled ge we have not had any reported flutter incidence on the entire fleet. As y ou mentioned Pete did exceed Vd on a few occasions. Ivan ________________________________ Message 2 _____________________________________ Time: 03:58:11 PM PST US Subject: Re: Europa-List: Are Vne and Va IAS or TAS? From: Pete Hi Bud, Many thanks for your (as always) comprehensive post. Very much appreciated. Your bolded statement implies that the Europa would be fine to be flown to V NE IAS at 15-18k (altitudes used by some Europa fliers) without risk of flut ter due to the relatively stiff construction (if I have not mis-read). I'm q uite hesitant to ever give that a try, but was searching for the comfortable TAS limits instead... which Ivan has appeared to answer with his proven tes t numbers. Now im wondering if any Europa fliers have become test pilots and flown at V NE IAS higher than Ivan's 8000' during his Vd testing? Just recently, another composite light sport didnt fare as well: https://www.flyingmag.com/technique/proficiency/technicalities-are-you-feeli ng-lucky Cheers and blue skies, Pete :) > On Jan 10, 2018, at 4:15 PM, Bud Yerly wrote: > > p ------------------------------------------------------------------------------------- Other Matronics Email List Services ------------------------------------------------------------------------------------- Post A New Message europa-list@matronics.com UN/SUBSCRIBE http://www.matronics.com/subscription List FAQ http://www.matronics.com/FAQ/Europa-List.htm Web Forum Interface To Lists http://forums.matronics.com Matronics List Wiki http://wiki.matronics.com Full Archive Search Engine http://www.matronics.com/search 7-Day List Browse http://www.matronics.com/browse/europa-list Browse Digests http://www.matronics.com/digest/europa-list Browse Other Lists http://www.matronics.com/browse Live Online Chat! http://www.matronics.com/chat Archive Downloading http://www.matronics.com/archives Photo Share http://www.matronics.com/photoshare Other Email Lists http://www.matronics.com/emaillists Contributions http://www.matronics.com/contribution ------------------------------------------------------------------------------------- These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.