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Max structural cruise and maneuvering speed limitations of the Bearhawk 4/model 5?

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  • Max structural cruise and maneuvering speed limitations of the Bearhawk 4/model 5?

    I’ve tried to search around and am a little surprised not to find mention of what the maneuvering speed is of the Bearhawk 4 place or model 5. I’m assuming I’m missing out on some knowledge. Could someone enlighten me? I’m trying to figure out what would be the maneuvering speed and what would be the max structural cruise speed, in order to think about what how fast you would fly in turbulent air. All I can find is the focus on cruise speed relative to throttle, Vne, and stall speed. Thank you in advance, and I hope I posted this in the appropriate spot!

  • #2
    VNE on the BH5 is listed as 180 mph, All the other speeds will be influenced by the weight of the build & unique to individual planes.

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    • #3
      Bob's Vne number for the original 4-Place is 175 mph, with a 5g limit at 2300 pounds and a 4.5g limit at 2500 pounds, per the first page of the plans. This may be different on the Model B. If it isn't listed on the plans, we might have to ask him.

      I looked back at my flight testing notes to see if I had written down a maneuvering speed, and I didn't. With not doing any aerobatics, I've not come close to full control deflection in any phase other than landing or stall testing, which are both low-speed states. You could write down a single number, but as I understand it, a range would be more realistic, depending on weight and altitude, and I would think CG would make a big difference. But I'm not professionally trained in this area.

      My flight test books are all packed up to move to a new house, but in Vauhan Askue's Flight Testing your Homebuilt he addresses strength testing and envelope expansion as it relates to the v-n diagram. This is one of those parts of his book that is especially applicable to the first flights of a new design, in the realm of helmets and parachutes. He lays out a flight testing plan for slowly expanding into the theoretical envelope of speed vs g, but for a design and application like the Bearhawk, I don't think actual Va limits are one of those things we prove empirically. But I believe a range of maneuvering speeds could be established based on calculations to get you close.

      Here's an article that talks about establishing a v-n diagram:
      http://www.aviationchief.com/operati...-diagrams.html

      Va is the speed at which the stall line intersects the load limit line. If we're only worried about Va and not building the whole diagram, we can apply his formula for increasing stall speed based on load factor, which is VS=VS(1G)√|G| to figure what stall speed would be at Bob's defined 4.5 and 5 g limits. By doing this, instead of drawing the whole stall curve, we're just picking its X value based on our given Y point of interest. For example, here it is at 5 and at 4.5g:

      bearhawkva.jpg
      The chart above basically says, at what theoretical point should the stall speed intersect the load factor limit, based on a particular Vso condition? This chart would apply to any airplane that has a Vso in the column. Keep in mind these numbers don't have any margin, other than Bob's own design margin above 4.5 or 5g, which is his margin and not ours. So if you were wanting to know a maneuvering speed, you could determine your Vso for one condition, apply this formula, subtract whatever margin you're comfortable with, based on whether you have kids or fly with a parachute and helmet, and that would get you close.

      In round numbers, I'd say for our airplane we could call Va 85 knots for 5g or 4.5g and be in the neighborhood. Though I learned through my phase 1 testing that unless it is a pretty aft cg, we'll have flow separation at the elevator hinge line before we reach much more than 2g, and we'll never get close to the load limits. Also, the induced drag would be huge, and I'm not sure how possible it would be to reach anything close to 4g without losing many, perhaps 10s of knots. Maybe a spiral dive, or wide open throttle, or both? These are some of the factors that have made establishing a single Va less of a priority for me.

      I think there is potential for a great Beartracks article on V-n diagrams and how they relate to Bob's designs, if someone wants to write it. If we could get Erbman on board it would be much better than if I tried to do it. He is also qualified and welcome to claim my analysis above to be completely bogus.

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      • #4
        When I was doing my test flights burning off the first 25 hours, the weather was cool and I was seeing speeds in the upper 150’s with the required engine breakin power. Several days I had to return to the airport cause the air was too choppy. Not knowing for sure what a safe Va was, I didn’t feel comfortable.

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        • #5
          I found this on the Professional Pilots Rumour network. Also on a bunch of other independent sources so I believe it to be correct:


          “If we are talking JARs, then all the data is on the JAA site. It is a copy of FARs, so I suppose the same information is on the FAA site.

          JARs give a fairly basic definition of Va, which is a design speed, assessed as basic stall speed times the square root of limiting positive g.

          VB is the design speed for application of gust criteria for certification. The definition is complex, and is in JARs in full. VB is higher than Va. Rough air speed is not a design speed. It is a recommended speed, chosen to give you the best chance in turbulence of not hitting either the low speed or high speed buffet boundaries. Again, JARs explain how it is chosen, but a simple explanation is that it has to be higher than the minimum calculated VB, and can be about half way between Vstall and Vmo/Mmo.

          Although Va, calculated at max. certificated TOW, will not change, published limiting speeds for manoeuvre or for full control deflection will change with aircraft weight, as basic stall speed changes.”


          I’m no expert and I could be way off so don’t quote me, but I think the JAR standards for these speeds originated at the FAA and is identical, at least for the speeds which concern us. Everyone seems to agree that Va is the square root of the load factor times the stall speed (which would change according to weight and CG as well as flap setting). For your Bearhawk, take the calibrated stall speed: say 45 mph times 2.12 (square root of a 4.5G load factor) and you get a maneuvering speed of 95.5 mph calibrated airspeed. That’s pretty low, but it’s a high lift airplane.

          For the rough air penetration speed, it seems like it is specified by the manufacturer. One source I found (but can’t verify) said it must at least 35 knots below Vne. Many others others (verifiable) said that in transport category aircraft, Vb must be established slow enough to withstand a 66fpm gust and preserve a margin above stall speed. How or if that would be of any use to us is beyond my ability to gauge. That particular speed is of great interest to us though, since it’s something we face often (turbulence) and have little control over except to slow down PREEMPTIVELY. I’d love to hear more from those in the know on this group, since it’s latent threat we face on potentially every flight.

          Steve W was spot on in not flying his new 150mph Patrol around doing engine break-in at 75% power on rough days. I’m wild-ass guessing He would have been in the order of 40 mph above a safe rough air penetration speed.

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          • #6
            A big part of this is opinion, and how the designer or manufacturer wants to approach safety/limits. The structure is designed for a certain max load. This load is in force (lbs, kg), not G's. Aluminum used to be 150% safety factor, my knowledge of this being 4 decades old.

            So what is the safe maneuvering speed, which means you stall before you overstress? Depends on your weight. The wing was designed for a certain load. I would take a low weight, Vso, and calculate a maneuvering speed. I would also use that at higher weights, as it is more conservative. At higher weights you will stall at a lot slower speed before over stressing.

            Flying for a living you might fly 500 to a 1000 hours a year. Privately, it is probably 50-200 hours. Being conservative is probably better, as our proficiency is lower. I think Van's uses this approach to Vne and a few other things, and I do not disagree from a safety standpoint, even if it is too conservative theoretically.

            Jared's description is mostly correct. I just dumbed it down to make it easier to understand. Including, for me.

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            • #7
              hi, I found this video explaining how to determine Va very helpful
              https://youtube.com/watch?v=ldfbqO4wHyQ&feature=share

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              • #8
                I have Va to use at two different weights printed on my panel. I am sure I've posted it before. Maybe Va isn't a good search term!

                I feel like I asked Bob and he told me what he would use, but it was 8 years ago so I am not 100% sure.

                From memory, it was 84 KIAS at 1,900 lbs and 96 KIAS at 2,500 lbs. I would have to check to be sure.

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