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

[triumphantduke]

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!

[Sir Newton]

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.

[jaredyates]

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/operating-flight-strength-v-g--v-n-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.

[Nev]

This is a good explanation Jared. So am I understanding correctly that at Va the aircraft will stall at the design load factor ?

This all came about through a discussion on Maximum Rate Turns, and I got to thinking I wasn't happy to demonstrate one from normal cruise speed entry (say 115 KIAS), to then hold it on the buffet in the turn. I think I'll just limit the entry speed for that purpose.

[Steve W]

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.

[Pbruce]

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.

[svyolo]

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.

[Gerhard Rieger]

hi, I found this video explaining how to determine Va very helpful

https://youtube.com/watch?v=ldfbqO4wHyQ&feature=share

[Battson]

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.

[Nev]

Got thinking about this, and the fact that we don't have one published. Va will presumably vary slightly depending on which load factor we use and the applicable stall speed.

Something like this :

2700 lbs 3.8g Vs=48 KTAS Va=94 KIAS
2500 lbs 4.5g Vs=43 KTAS Va=91 KIAS
2300 lbs 5.0g Vs=38 KTAS Va=85 KIAS

Are the units (IAS) correct ? Is the maths correct ?

If so, would it be sensible to use a speed of 90 KIAS as both Va and Rough Air speed ? Or a higher rough air speed ?

nborer

[Nev]

That makes sense Richard. Thanks.

[Battson]

The 2500 lbs is determined by the springs in the gear. Above that weight, they would bottom out with a heavy landing and could cause damage. The structure is good to 2700 lbs.

I think it's the IAS for Va, I can't recall the reason why. Generally V speeds relate to indicated speeds I think??

[Nev]

I was wondering if the calculation is done in TAS then converted back to IAS for our individual aircraft ?

[Mark Goldberg]

Bob designed the 4 place with 2500 lb gross weight. However he has said 2700 lb would be OK for take off using a smooth runway. It is also important to respect the gear leg spread he calls for - especially at gross weight. 72" spread tire center to tire center at "normal" flying weights. Not to exceed 74" at full gross. Mark

[nborer]

Late on this; I didn't see Nev's tag (sorry). For certified airplanes, V_A will be calculated in a V-n diagram in equivalent airspeed (EAS); pretty much it's the same as CAS at these low speeds (well within 1 knot). When it's translated to the AFM or placarded in the cockpit, the "published" V_A should also be given in IAS. It's defined as where the airplane will reach "stall" [***] when it reaches the design load factor, really V_S * sqrt(n_limit) as discussed above. In this case, n_limit is the design limit load factor (limit meaning prior to the application of the structural factor of safety; generally 1.5 for strength and 1.25 for yield - but that is for structural design, not for flight envelope calculations). For most normal category airplanes, n_limit is defined as a 3.8g load at max gross weight; utility will be 4.4g. (The utility category was eliminated in the 2017 rewrite of Part 23 by the FAA and EASA, but I included it for legacy.)

As svyolo points out; a structural designer will consider actual loads, not not load factors (g) for the aerodynamic surfaces. Your airplane may stall at a lower speed at a higher load factor, but the load on the wing won't be higher - the wing can only generate so much lift, regardless of how much the airplane weighs. I always found it curious that AFMs list the maneuver speed limitation as a decreasing number with lower gross weight, since the V-n diagram V_A used for structural certification is at at load (not load factor) defined at maximum gross weight. The only thing I can think of is that the non-aerodynamic loads (engine, crew, passengers, cargo, etc.) are also certified at the same load factor, so even if the airplane is lighter, at higher than the limit load factor, those local loads will be higher, which will cause higher local structural stresses - your wing bolts may not be holding higher loads at lighter weights and higher load factors (since these will be dictated by the maximum lift capability of the wing, which is [mostly] based on the critical angle of attack), but your engine mount bolts will need to take those higher loads and load factors. I'm not a structures guru, so this is speculation, not fact.

***I put asterisks after the term "stall" since the term "stall speed" is associated with V_S, which is sometimes loosely equated with "the speed at which your airplane will stall." For certified airplanes, the two concepts are nearly orthogonal. At max gross weight, most forward CG, and 1 knot/sec deceleration in level flight, V_S will be the speed that the airplane either "experiences an uncontrolled downward pitching motion," "the control reaches the stop," or your stick pusher actuates. Note that none of these requires that the wing actually experiences an aerodynamic stall. The speed that you experience when a "stall" occurs may look very different than V_S. But, V_S is an important speed to know to set other V-speeds, such as V_A (as shown in this thread), V_ref (reference approach speed, typically 1.3*V_S_0), and many others (e.g, minimum climb speed is typically 1.2*V_S_1, rotation speed is at least 1.1*V_S_1, etc.). But again, this is all for certified airplanes. These are good concepts to know for experimental aviation, but don't hold the same weight.