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  #11  
Old 01-17-2012, 11:42 AM
Andy Hill's Avatar
Andy Hill Andy Hill is offline
 
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Hi Ross...

If you read the Builders' Manual carefully, in the Flt Test section, you'll see in the ASI markings table the VNE referred to as TAS i.e. there is no defined IAS limit.

For practical purposes, observe it as IAS at lower levels. Higher up, knock off a suitable amount
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  #12  
Old 01-17-2012, 11:50 AM
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Ironflight Ironflight is offline
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Quote:
Originally Posted by Scorch View Post
Paul, just reading about how you had to throttle back to keep from exceeding the TAS limit on your RV-3 - I'm guessing that you get a TAS display on your G3X?

I'm aware that we have both IAS and TAS limits on our RVs, the IAS being the structural limit and the TAS being the flutter limit. I know the IAS limit on my RV-6 - it's 210 MPH IAS. My question, for you or anyone else who knows: where do we find the TAS (flutter) limit for our RVs? Specifically, for a -6. Is it the same number, 210 MPH, except TAS vice IAS?

I've always observed the IAS limit on my -6, but until your post reminded me, I've been kind of ignorant of its TAS limit. Maybe because I have steam guages with no TAS display...
Kind of like Andy said Scorch - I am sort of extrapolating from what I have learned about the Flutter limit in Van's writings in general. I am basically assuming that the IAS Vne quoted at seal level is the TAS Vne to which the airplane has been tested - that is possibly conservative, but in the absence of data, and with the potential for catastrophic damage when talking about flutter, I'd rather be conservative.

And the EFIS really makes this pretty easy - not only does the G3X show TAS all the time in a little box, you can set the red line on the ASI tape to show where the current (calculated) TAS is relative to IAS. (I know that's a ridiculous sentence....basically, the red line is always at the place you shouldn't exceed RIGHT NOW). It's basically like the barberpole needle on a a turbine airplane's ASI. Time to get Danny to start installing an EFIS Ross!

Paul
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  #13  
Old 01-17-2012, 12:07 PM
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mburch mburch is offline
 
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Quote:
Originally Posted by Ironflight View Post
And the EFIS really makes this pretty easy - not only does the G3X show TAS all the time in a little box, you can set the red line on the ASI tape to show where the current (calculated) TAS is relative to IAS. (I know that's a ridiculous sentence....basically, the red line is always at the place you shouldn't exceed RIGHT NOW). It's basically like the barberpole needle on a a turbine airplane's ASI.
It is indeed a little tricky to describe! But there are a couple of pretty good pictures and descriptions of how this works, on page 63 of the G3X Pilot's Guide and under section 7.3.3 in the G3X Install Manual (pages 7-7 and 7-8).

mcb
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  #14  
Old 01-17-2012, 12:24 PM
Scorch Scorch is offline
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Well, now I AM confused!

So what you guys are saying is, the 210 MPH "redline" for my RV-6 is, and has always been, a TAS restriction, not an IAS restriction, even though it is physically marked on the Airspeed Indicator, which is basically an IAS guage.

So how many of us stone-age steam guage drivers have been blissfully driving around thinking that as long as we kept the needle below the red line we were OK?

And, just to further muddy the waters: Andy, since Vans publishes only a TAS restriction, and not an IAS restriction, does this mean that in our RVs flutter is always more limiting than structural?
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  #15  
Old 01-17-2012, 01:09 PM
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Ironflight Ironflight is offline
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Quote:
Originally Posted by Scorch View Post
So how many of us stone-age steam guage drivers have been blissfully driving around thinking that as long as we kept the needle below the red line we were OK?
Lots.....

But seriously, this has been discussed quite a few times here, and Van has written about it numerous times in the old RVator. The problem, of course, is that you as a pilot have no obligation whatsoever to read any of that stuff, so how would you know?

I think part of this stems back to the fact that we are pretty much all putting on more horsepower than Van originally used in his designs, so we routinely operate closer to the upper end than was originally thought about. With more margin to the limit, it was not a big deal. Now we're all speed demons, and have to be careful - especially in descents from on high...

Paul
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  #16  
Old 01-17-2012, 10:07 PM
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DakotaHawk DakotaHawk is offline
 
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Quote:
Originally Posted by Ironflight View Post
the red line is always at the place you shouldn't exceed RIGHT NOW
Oooohhhh! I like that! Wish my Dynon could do that too!
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  #17  
Old 01-20-2012, 06:35 AM
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Louise Hose Louise Hose is offline
 
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Default The preliminary performance data

Quote:
Originally Posted by Ironflight View Post
Louise has been busy with cruise performance testing, and is reducing the data when she has the chance. I haven’t tried a flat-out max speed run at full throttle on the deck, but based on what I have seen, I expect it to be about 183 knots – redline.
Okay, here are the performance data based on one run at each combination of power setting and elevation . The scientist in me doesn't like to "publish" such preliminary data but Paul thought some of you might be interested in what we are finding. After a little experimenting with the Garmin G3X lean function, we decided to just go with the Lycoming graph for "peak" at a given elevation. So, I leveled out at a fixed elevation, running "rich", and settled at 2400 rpm. I then pulled the mixture back to 75% power (or whatever was the highest available power setting), adjusted the trim to stablize, then put the autopilot in control. Next, the mixture pulled back to the fuel flow recommended for peak by Lycoming for the IO-320: 75%- 8.7gph; 70% - 8.1; 65% - 7.6; 60% - 7.0; 55% - 6.4; 50% - 5.9. Once the fuel flow stabilized, I recorded the TAS (and IAS). (Other data, like OATs, were recorded but haven't yet been factored in.) I then repeated the procedure at each power setting. Here's what the TAS data (in knots and feet of elevation) look like:



Please remember that I'm a neophyte test pilot and be kind in your comments.

Quote:
Originally Posted by Ironflight View Post
I haven’t tried a flat-out max speed run at full throttle on the deck, but based on what I have seen, I expect it to be about 183 knots – redline.
Paul might be able to shed more light on this topic now. Paul?
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Last edited by Louise Hose : 01-20-2012 at 07:12 AM.
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  #18  
Old 01-20-2012, 07:06 AM
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Louise,

You may not be a test pilot but the scientist in you is showing through very well. There's nothing like data to sort out facts from fiction, opinions, and misrepresentations (intentional and unintentional).

While I understand these data are preliminary, the consistency of the slope and the trend toward higher speed with altitude suggest the numbers probably are not far off. I'm, surprised more people don't do this exercise and post the results. This is exactly what I'm looking forward to doing with my RV-9A with the IO-340.

And if I did the calculations right, at 14,000 feet and 50% power, your getting a shade better than 31 mpg. That's really impressive considering your going 163 knots TAS.
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  #19  
Old 01-20-2012, 07:59 AM
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Ironflight Ironflight is offline
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Quote:
Originally Posted by Louise Hose View Post
Paul might be able to shed more light on this topic now. Paul?
Yes...ahem....I did a 1,000' speed run yesterday becasue the ceilings kept me from doing anything up high. Throttle, Prop and Mixture full forward, level at 1,000' msl (near STP) gave me 191 KTAS. We have established that the pitot/static error is +/- 2 knots, so I'd put the same error band on that number.

That's 7 knots over airframe redline BTW - for those thinking of an O-360 on one of these, I'd suggest a tail re-design (for balanced surfaces...).

Paul
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  #20  
Old 01-20-2012, 11:26 AM
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Default For Steam Gauge Drivers...

I wonder if you could have a redline marked on the "true" scale of a true airspeed indicator?
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