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madriver42(at)gmail.com
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 Posted: Sun May 08, 2011 11:30 am Post subject: 701 Buffet at slow speeds |
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OK Listers,
I have about twelve hours on my 701 - Rotax 912 ULS with Warp Drive three blade (68 inches) set at 13.5 degrees. So far it's the most fun I've had with my clothes on - at least since flying T-38s in the mid sixties...sigh!!
So what's bothering me??
In climb at very low speeds such as 40 - 60 I have a very stable platform with no buffet or oil canning - however when I pull the power back to a lower RPM (2000 - 4000) IN THE PATTERN such as turning base at 65 and slowing to fifty or so on final - and below - I begin to pick up a "rumbling buffet" that seems to increase as the plane slows. I think the key here is POWER OFF OR LOW POWER SETTINGS - The buffet can be felt as a light buffet in the stick and just sounds like a whole lot of oil canning.
I have also slowed the plane at altitude in level flight and get the same results - buffeting and oil canning sounds.
I'm perty much convinced that it is caused by a separation of the air flow over the cockpit and the front part of the top of the fuselage.
When I built the plane I put false ribs in the wings and also added diagonal "L" braces between all fuselage stations in the rear - top, bottom and sides. This seemed to take a lot of the "flex" out of the skin. I also added sound dampener material - stuff used on HVAC ducts - to the back of the baggage compartment wall and under the baggage compartment floor. This also seemed to deaden the panels.
Been noodling with strengthening the top and sides of the fuselage from just behind the top window back to the rear of the baggage compartment - in each bay - with two diagonal "L's". Additionally, putting a single diagonal "L" on the side walls behind the door and then in the next bay in the baggage compartment. Also would add the sound insulating material to the inside top and side. I added the material in the cowl boot area and behind the fire wall and like the looks and it seems to quiet things down.
So what's the problem??? As usual I am trying not to reinvent the wheel and asking what the rest of you 701 - 750'ers have done to control the buffet / clatter.
Contact me off net if you like or leave your response for all to see on the site.
Still grinnin
Phil
Buhl, ID
710, Rotax 912 ULS
[quote][b]
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cscsail(at)gmavt.net
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| Posted: Sun May 08, 2011 12:35 pm Post subject: 701 Buffet at slow speeds |
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Phil,
Mine does exactly the same thing ---no false ribs or x braces.
Gordon
[quote] ---
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n752ms(at)softcom.net
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| Posted: Sun May 08, 2011 1:01 pm Post subject: 701 Buffet at slow speeds |
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Philip
I have no extra braces on my fuselage. The only time I have oil canning is when the ball is off center, at any speed.
I have done my best to get this thing to stall but all it will do is mush in at about 900fpm. Quiet all the time, unless the ball is off.
I put L braces on the back of the baggage compartment, it would oil can on rough fields when landing.
Instead of adding all that weight you might just get a better head set.
701/912ULS 150hrs
Sent from Mark's iPhone
On May 7, 2011, at 7:56 PM, Philip Smith <madriver42(at)gmail.com (madriver42(at)gmail.com)> wrote:
[quote]OK Listers,
I have about twelve hours on my 701 - Rotax 912 ULS with Warp Drive three blade (68 inches) set at 13.5 degrees. So far it's the most fun I've had with my clothes on - at least since flying T-38s in the mid sixties...sigh!!
So what's bothering me??
In climb at very low speeds such as 40 - 60 I have a very stable platform with no buffet or oil canning - however when I pull the power back to a lower RPM (2000 - 4000) IN THE PATTERN such as turning base at 65 and slowing to fifty or so on final - and below - I begin to pick up a "rumbling buffet" that seems to increase as the plane slows. I think the key here is POWER OFF OR LOW POWER SETTINGS - The buffet can be felt as a light buffet in the stick and just sounds like a whole lot of oil canning.
I have also slowed the plane at altitude in level flight and get the same results - buffeting and oil canning sounds.
I'm perty much convinced that it is caused by a separation of the air flow over the cockpit and the front part of the top of the fuselage.
When I built the plane I put false ribs in the wings and also added diagonal "L" braces between all fuselage stations in the rear - top, bottom and sides. This seemed to take a lot of the "flex" out of the skin. I also added sound dampener material - stuff used on HVAC ducts - to the back of the baggage compartment wall and under the baggage compartment floor. This also seemed to deaden the panels.
Been noodling with strengthening the top and sides of the fuselage from just behind the top window back to the rear of the baggage compartment - in each bay - with two diagonal "L's". Additionally, putting a single diagonal "L" on the side walls behind the door and then in the next bay in the baggage compartment. Also would add the sound insulating material to the inside top and side. I added the material in the cowl boot area and behind the fire wall and like the looks and it seems to quiet things down.
So what's the problem??? As usual I am trying not to reinvent the wheel and asking what the rest of you 701 - 750'ers have done to control the buffet / clatter.
Contact me off net if you like or leave your response for all to see on the site.
Still grinnin
Phil
Buhl, ID
710, Rotax 912 ULS
[b]
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ronlee
Joined: 25 Dec 2006
Posts: 141
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| Posted: Sun May 08, 2011 6:47 pm Post subject: Re: 701 Buffet at slow speeds |
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My 701 does the same thing. What makes it do that I have no idea, but I do make use it to control my speed on landings. It's like a built in stall indicator except no conventional stall occurs, just the big controllable mush.
Do I wish the wing profile continued over the cabin, you bet. I truly believe it would be a better plane if it did, as I suspect the buffeting MAY start there. For one thing I could peer into the wing at a sight glass and know exactly how much gas I have. I could run a vent line between the gas tanks to equalize the pressure of the tanks so they would feed evenly, Cessna does it that way. Does the stall (buffeting) start over the cabin long before the rest of the wing, I have no doubt.
All in all I love to fly the 701 very much but believe there is room for improvements in several areas. To be fair you could say that about any airplane though.
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John Bolding
Joined: 23 May 2006
Posts: 281
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| Posted: Mon May 09, 2011 4:19 am Post subject: 701 Buffet at slow speeds |
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Jump over to the "other" 701 yahoo group, LOTS of testing of exactly what
you're wishing for has already gone on.
LO&SLO
John
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bobkat
Joined: 07 Sep 2008
Posts: 143
Location: Bismarck, ND
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| Posted: Mon May 09, 2011 4:43 am Post subject: 701 Buffet at slow speeds |
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Mine does the same thing and I put X braces on all the olarger unsupported
panels. I don't know if it helped or not, but once you get used to it and
realize that something isn't coming apart, you sort of tune it out. I
calculated what my X braces weighed once, and it wasn't all that much, only
a few pounds, though every little bit helps.
Update on my plane - finished painting it and put the wings back on
yesterday. For the time being I'm leaving off the slats and putting on
JG's VGs! Maybe fly it a bit with no slats or VG's and measure all the V
speeds as best I can, and report later on any differences. It looks a bit
strange right now with the slat hangers on the leading edges but no slats.
--
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dons701
Joined: 22 May 2009
Posts: 80
Location: Hershey, PA
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| Posted: Mon May 09, 2011 12:36 pm Post subject: Re: 701 Buffet at slow speeds |
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Ditto for my plane oil canning, except when at half flap or better it is quiet as the airflow must be "pulled" down from the top along the fuselage sides. I am sorting out ideas on how to create my own clear wing surface over the cabin which sounds like the solution to some of the canning issues as well as giving more headroom. There is more to read over at the Yahoo site, just search for beaney mod.. Don B...
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Zenith 701 #76120
Jabiru 2200A #2456 95 hours
Sensenich Wood 64-PJ-36 Prop |
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stolpilot.wilde(at)gmail.
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| Posted: Mon May 09, 2011 5:09 pm Post subject: 701 Buffet at slow speeds |
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Phil: I have almost 200 hours on my 701 and find the plane a little noisy but not too bad. I agree with an earlier poster, use a noise canceling head set and keep the weight off the airplane.
Dan Wilde
N948DW
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