Vne limit on CH 640

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by geosnooker2000, Jul 4, 2019.

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  1. Jul 4, 2019 #1

    geosnooker2000

    geosnooker2000

    geosnooker2000

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    I emailed the factory to ask "what is the limiting factor on Vne?". I got back a response that was polite enough, but basically said they would prefer not to answer. Then added that basically I would need to exceed the 240 bhp in order to even approach it. But I have some aerodynamic mods in mind, along with a turbo and a variable pitch prop, that might test that theory (stretching the service ceiling up to higher speeds in the flight levels). He attached a spec sheet from their brochure that didn't even have the Vne listed on it. It IS listed on their website as "175mph", but it doesn't say whether that is IAS or TAS. I asked that as well, but got no answer. Basically a "go away boy, ya botha me" (in my best W. C. Fields). Seems a little ham-fisted being listed as mph instead of knots.
    So, would anyone here be familiar enough with the CH640 to care and take an educated guess as to the limiting factor? Flutter maybe? The reason I jump to that conclusion is this video, being that it is basically just a home-built Cherokee 140.
     
  2. Jul 4, 2019 #2

    mcrae0104

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    NASA may struggle with unit conversion from time to time, but you can divide by 1.15, no?
     
  3. Jul 4, 2019 #3

    geosnooker2000

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    Of course. 152kts.
     
  4. Jul 4, 2019 #4

    Marc Zeitlin

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    For an EAB aircraft, does that surprise you? The last thing a seller of a kit or plans wants is for someone to be tickling the dragon. "Here's the limit - deal with it". I might not do exactly the same, but I certainly understand why they do.

    In which case, it's a different plane, and has different limits that you'll need to calculate/test for yourself.

    If flutter limited, for low speed, low altitude aircraft, it's IAS and for high altitude, high speed aircraft, it's Mach #. But that's a simplification - it's actually somewhere between the two, and it's complicated. If it's not flutter limited, then it should just be IAS. But since they won't tell you if it's flutter limited or not, just assume that you need to stay below the service ceiling of the aircraft and below the listed Vne in IAS. Anything else, you're on your own.

    Sure. While many times flutter limits are what sets Vne, many times it's something much more mundane. I don't know the CH640 from a hole in the ground, but I'd guess that they set Vne to 152 KIAS because the maximum speed they could get the thing to in a reasonable full power dive was 167 KIAS at some high(ish) altitude. So, if 167 KIAS is Vd, then Vne is 152 KIAS. Or maybe 167 KIAS is the speed they got to and were scared to go any faster.

    The video you posted was for a twin Comanche, not a 140.

    The Vne of a 140 is 171 mph, so IF the internal structure is similar, that MIGHT be an indication of why the CG640's Vne is close to a Cherokee 140's Vne. Or it might not...

    If the factory isn't going to tell you, neither is anyone else. Aerodynamic and speed mods at your peril.
     
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  5. Jul 4, 2019 #5

    geosnooker2000

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    I realized that was a twin Comanche about the time I clicked on "post reply", but I let it stay because same company, maybe same construction techniques? Probably a poor assumption. Thanks for the input. I still don't see why they wont post whether or not Vne is IAS or TAS. Seems like they would have to.
     
  6. Jul 4, 2019 #6

    wsimpso1

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    Re-read Marc's post...

    They have one airframe. Dive tests take nerve, flutter tests take nerve. They were likely to have shoved the nose way over and let the engine go past redline just to get to 175 mph indicated. Even if you are willing to throw away an airplane, getting out of a broken airframe is not assured. At some point they just said "enough"

    And, given their history ( both in flutter and in people copying them) they have strong reason to keep anyone else from playing near flutter limits on their birds or anything similar or that could be "based" on their birds.

    Billski
     
    Last edited: Jul 4, 2019
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  7. Jul 4, 2019 #7

    BJC

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    Trying to get verified Vne data from most kit suppliers is difficult.

    There is a long thread here about flutter, and whether it is a function of true or indicated airspeed.


    BJC
     
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  8. Jul 4, 2019 #8

    BJC

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    For example, I was very interested in the F1 kit when it first was introduced. I asked them about Vd and Vne, but they avoided a straight forward answer. Then they assured me that the design would not flutter at XXX knots, because the wing was like an RV (that would not flutter at a speed less than XXX) but had the ribs closer together. After a long discussion with them, I concluded that they had no idea of what Vd and Vne are, had not flight tested to Vd, were unwilling to be honest, and that I had no interest in doing business with them.

    The good news is that, AFAIK, no F1 has come apart in flight.


    BJC
     
  9. Jul 4, 2019 #9

    geosnooker2000

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    Ah,ha! Would you please elaborate. I have not heard of them having an "history".
     
  10. Jul 4, 2019 #10

    geosnooker2000

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    Actually there are several. I thought about appending my question to the tail of one of those threads, and then I thought about what one poster said about bring up old threads, and I agree with his point. He said it's not good to do that because the people in that thread that may want to respond or have a stake in it may not be around anymore to defend themselves. But I will say that they were informative, and I read about 4 of them in their entirety.
     
  11. Jul 4, 2019 #11

    BJC

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  12. Jul 4, 2019 #12

    TFF

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    Scott Crossfield was flying that test flight. The guy that flew half of the X-15 flights. When he saw the film he said why did you not not slow me down! Comanches has problems early on as did Bonanzas with flutter. All flying parts can flutter, you just have to make them strong enough to not flutter at speeds you want to fly at.

    As for what your plane can do, if the designer does not know, no one knows. It has not been pushed to know. A lot of airplanes are capable of more and some are on the line. My guess is redline speed with the big engine will be close in level flight, where a smaller engine version would have to dive.

    As for speed in kts or mph, kts is great for navigating as that is what the maps are made in. Mph is great for speed as lay people understand it and the number is bigger so it’s more impressive. Lots of GA planes use MPH because some guy or girl getting out of their car don’t have to convert. Marketing of times gone.
     
  13. Jul 4, 2019 #13

    TFF

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    The Cherokee had pretty much an all star design team. They built a really good plane that pretty much was expanded in form up to a turbine. Lots of variations. Lots of mileage.

    The general homebuilt design of thumb is build a plane that is 2/3s to 3/4 the size of a certified with the same engine. Automatic performance increase. Make sure you put short people in ads so everyone thinks the plane is big. It’s amazing that so many designers are 5-6 and not 6-0. A lot of that is leftover from WW2 with legacy of pilots were preferred to be no taller than 5-9.
     
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  14. Jul 4, 2019 #14

    BoKu

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    Nitpick: Stiff enough.
     
  15. Jul 4, 2019 #15

    TFF

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    If you want a two word explanation.
     
  16. Jul 4, 2019 #16

    flyboy2160

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    If you really want to be fussbudget, professor exam question nit-picky, since you're managing frequencies, you're concerned with specific stiffness - stiffness/mass - since the frequency ~ SQRT (stiffness/mass).

    Making something stiffer by correspondingly increasing its mass doesn't help you manage frequencies.

    And I do mean 'manage.' Sometimes you have to soften something to keep its resonant frequencies away from those of something else.
     
    Last edited: Jul 4, 2019
  17. Jul 4, 2019 #17

    wsimpso1

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    https://www.faa.gov/aircraft/gen_av/light_sport/media/Zodiac.pdf

    They had several customer's aircraft break up in flight. The company insisted that there were build errors, with inadequate control cable tension among others. Bad show, people died, company reputation soiled, structural fix appears to have solved it. If I were them, I would be sensitive indeed.

    Billski
     
  18. Jul 4, 2019 #18

    mcrae0104

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    To be clear, the in-flight breakups were not CH640s, the aircraft the OP is referring to. The CH640 also has a certificated variant, the CH2000 / Alarus. I am not aware of any structural issues with it.
     
  19. Jul 5, 2019 #19

    wsimpso1

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    Just saying that the Zenith folks did have flutter problems with two models of their airplanes, so they might still be very sensitive on the topic...
     
  20. Jul 5, 2019 #20

    pictsidhe

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    Whoever said that "any publicity is good publicity", has not read that report.
     

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