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  #1  
Old 09-25-2022, 10:47 PM
skelrad skelrad is offline
 
Join Date: Mar 2005
Location: Redmond, WA
Posts: 356
Default Explain Gross Weight for Engine Choice

What drives a gross weight changing with an engine size? I guess I expected the recommended gross weight to be the same for an airframe regardless of the engine in it, but the 9A has a different gross weight recommendation depending on the engine.
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RV-9A: Fuselage in progress
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  #2  
Old 09-25-2022, 11:02 PM
RV8JD RV8JD is offline
 
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Posts: 631
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The engine horsepower would make a difference in the airplane's performance depending on maximum gross weight. That's maybe why Van's has the lower gross weight limits with smaller engines, in order to maintain their performance goals for the airplane, e.g., takeoff distance, rate of climb, etc.

However, the Van's RV-9/-9A Weight and Balance Data Quick Reference documents only show the Recommended Gross Weight of 1750 lbs (and the Maximum Utility Category Weight of 1600 pounds), with no mention of any horsepower-dependent recommended gross weight limits.

RV-9:
https://www.vansaircraft.com/wp-cont...9/01/RV9wb.pdf
RV-9A:
https://www.vansaircraft.com/wp-cont.../01/RV9Awb.pdf
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Last edited by RV8JD : 09-25-2022 at 11:35 PM.
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  #3  
Old 09-26-2022, 08:00 AM
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airguy airguy is offline
 
Join Date: Oct 2006
Location: Garden City, Tx
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Default

My DAR granted me 100 pounds over based on my IO-360 and constant speed prop. He said anything more than that, and he wanted to see some engineering and performance test data, but he would grant 100 pounds for the engine/prop combo.

Of course, when I'm loaded close to gross, I'm carrying that excess weight as fuel in the wings very near the CG line too.
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  #4  
Old 09-26-2022, 10:00 AM
rocketman1988 rocketman1988 is offline
 
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Location: Sunman, IN
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Default Interesting

Interesting because as the manufacturer of the aircraft, you can assign any weight you want as long as it is tested during phase 1. Wasn't there a guy that put a 3600 lb gross on an rv-10? I had not heard of a DAR that requires engineering analysis for modifications...until now. The other question is if provided with said analysis, would the DAR have the credentials to verify the analysis?
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  #5  
Old 09-26-2022, 10:18 AM
skelrad skelrad is offline
 
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Location: Redmond, WA
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Huh. I hadn't given it a ton of thought. I just assumed the gross weight was what the airframe could physically "handle," as opposed to also making considerations for performance.
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  #6  
Old 09-26-2022, 10:27 AM
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Ironflight Ironflight is offline
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Quote:
Originally Posted by skelrad View Post
Huh. I hadn't given it a ton of thought. I just assumed the gross weight was what the airframe could physically "handle," as opposed to also making considerations for performance.
You’re not alone! Many pilots are told that gross weight is based solely on what the structure can handle, but in fact, many aircraft over history have their gross weights based on minimum climb performance.

Here in the RV world, with power-to-weight ratios that would be considered ludicrously generous in the Cessna or Piper world, it can be hard to see that - but check out the limiting performance on a C-150 on a hot day in Denver and you’ll see that they can barely climb.

So yeah….give thanks for the overpowered Rv’s, and have compassion for the folks that have to worry very day about density altitude…..
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  #7  
Old 09-26-2022, 11:06 AM
PilotjohnS PilotjohnS is offline
 
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Location: Southwest, USA
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Default Please educate me

Please educate me.
If i want to conduct phase one on my plane at a higher gross weight than what Vans recommends, what are the rules?

If I test it at a higher gross weight, can I now use that number for my gross weight after phase 1?

If analysis is needed, what analysis do i need to show?

With the inherent variability of experimental airplanes, how does one set the gross weight for a entirely scratch built airplane? Does one have to show the detailed mechanical analysis to the DAR for a one -off design to get an airworthiness certificate? Or is the flight testing sufficient?

Thanks in advance,
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WARNING! Information presented in this post is my opinion. All users of info have sole responsibility for determining accuracy or suitability for their use.

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98% done, 2% left to go
To Go: wing mounting, engine baffles, wing tips, move to airport
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  #8  
Old 09-26-2022, 11:27 AM
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Ironflight Ironflight is offline
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Location: Dayton, NV
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Quote:
Originally Posted by PilotjohnS View Post
Please educate me.
If i want to conduct phase one on my plane at a higher gross weight than what Vans recommends, what are the rules?

If I test it at a higher gross weight, can I now use that number for my gross weight after phase 1?

If analysis is needed, what analysis do i need to show?

With the inherent variability of experimental airplanes, how does one set the gross weight for a entirely scratch built airplane? Does one have to show the detailed mechanical analysis to the DAR for a one -off design to get an airworthiness certificate? Or is the flight testing sufficient?

Thanks in advance,
The answer is that it depends on your DAR. DAR’s can refuse to issue you a certificate if they feel that something is unsafe. So it really depends on how your DAR feels about what you’re doing. I had one DAR who felt that since I was an aeronautical engineer, he would trust my judgement (it wasn’t a gross weight thing). I had another that wanted proof from the aircraft designer that the slight increase in gross weight was acceptable for “normal category” operations. Both were reasonable requests, both reasonable responses. If someone asked a DAR to do something ridiculously out of spec with no data to back it up, I would expect them to reject the request.

The important thing is to feel your DAR out - and give him specific requests and data - long before it comes to inspection day, so you can talk about it. This will also determine if you want to proceed with that DAR or not.

The important thing is to recognize that while we’re in an Experimental category, so technically should be able to do whatever we want, we also shouldn’t be doing anything “stupid”. As long as everyone works towards “reasonable”, we should’nt adversely affect the category’s accident record…..
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Editor at Large - KITPLANES Magazine
RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
A&P, EAA Tech Counselor/Flight Advisor
Dayton Valley Airpark (A34)
http://Ironflight.com
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  #9  
Old 09-27-2022, 10:04 AM
PilotjohnS PilotjohnS is offline
 
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Default Thanks

Paul,
thanks for your insight.
I wish there was a more concrete process for establishing a higher gross weight; this loosey-goosey stuff and shopping for a DAR bothers me.
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WARNING! Information presented in this post is my opinion. All users of info have sole responsibility for determining accuracy or suitability for their use.

Dues paid 2022, worth every penny

RV9A- Status:
98% done, 2% left to go
To Go: wing mounting, engine baffles, wing tips, move to airport
www.pilotjohnsrv9.blogspot.com
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  #10  
Old 09-27-2022, 11:47 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Quote:
Originally Posted by PilotjohnS View Post
Paul,
thanks for your insight.
I wish there was a more concrete process for establishing a higher gross weight; this loosey-goosey stuff and shopping for a DAR bothers me.
There is no loosey goosey. To my knowledge, the only one who has done true engineering analysis and destructive testing is Vans and they are crystal clear on what the limit is with no exceptions. Many arm chair Engineers have said "oh that's conservative, so no harm increasing it some." With this kind of engineering, it's all about safety margins between rated and actual structural limits and these margins have to account for all sorts of unplanned things that were not in the structural load engineering. Eat into those margins and fewer of those things can be in place and still avoid structural failure. That said, these things don't happen often and statistics reflect that.

As a builder, you can set what you want, depending upon inspector criteria, but to do so without understanding the consequences puts you and your family at some level of risk. Problem is no one other than Van's can really can quantify that risk and am not even sure they can quantify it, as I believe they are using generally accepted aviation standards for the margins. Though it is possible that they extended those margins further to account for builder flaws that can be expected in EAB that are not generally seen in certified. This is one of my guesses as to why the 6 has a lower GW rating than the 7 (home built spar vs factory built spar), but that is pure speculation and have nothing to base that on.

Larry
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Last edited by lr172 : 09-27-2022 at 12:02 PM.
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