VansAirForceForums  
Home > VansAirForceForums

-POSTING RULES
-Advertise in here!
- Today's Posts | Insert Pics

Keep VAF Going
Donate methods

Point your
camera app here
to donate fast.


Go Back   VAF Forums > Main > RV General Discussion/News
Register FAQ Members List Calendar Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 09-18-2017, 06:38 PM
Hartstoc's Avatar
Hartstoc Hartstoc is offline
 
Join Date: Aug 2017
Location: Sebastopol,CA
Posts: 368
Default The W&B thing(part 2): IS RV-Fun inversely proportional to RV-Weight?

A recent thread I posted on the importance of accurate W&B docs generated a lively discussion and lots of valuable shared experience from veteran RV builders and pilots, so I thought it might be worthwhile to expand the discussion in a few other directions. If this one gets a similar response, the next will be about the temptation to nudge the GW limit upward a bit during the registration process. Stay tuned.

This thread makes reference to the "selfie-quote" In my VAF signature, which seems to have ruffled a few feathers already. Is "RV-Fun is inversely proportional to RV-Weight" a valid statement? Should it be of concern to all RV builders? It may be a bit cryptic, but I think it IS valid for any given RV configuration of engine, prop, and instrumentation.

I'm not an aerodynamicist, and my hard knowledge here is dwarfed by my ignorance, but I do have a limited qualification to bring up the subject based on my experiences as rookie CAFE Foundation test-pilot/author or flight-engineer/volunteer for most of the CAFE APR series published in Sport Aviation during the 1990's, which included four RV models. During these APR's, we measured stick force gradients, explored stall recovery behavior, and flew a structured handling qualities protocol in multiple loading configurations for each aircraft tested, and the overall experience was a real eye opener.

Actually, I'm betting that you RV-vets will have more interesting things to say than me here, so I'll make just TWO points and cut this loose-

1-Absolute Weight- The rewards for lower empty weight are improved performance at any given load and a higher useful load, both more fun in my book. Fortunately, one strength of all Van's aircraft is that they are intrinsically light in weight if built according to the plans, but the designs are also quite sensitive to excess weight for reasons too complex(and too over my head)to fully address here. Ending up with a finished aircraft that is within Van's guidelines requires incredible discipline every step of the way. Weight goes on a little at a time, and the temptation to add more features and equipment because each will "only add a few ounces" is a slippery slope. Those of you just starting to build are in a position to adopt a very strict attitude about weight and maintain it through the whole process. Is it really worthwhile to prime all those inner surfaces on an airplane that will be babied in a hangar? The tired old saw has some truth: "If you are thinking about adding something to your airplane, toss it in the air. If it does NOT fall to the ground, then it is OK to install it." Every pound saved adds 8-15 miles to your range, depending on the model.

2- Polar Mass- Of course, everything you put on that airplane is subject to gravity, but there can be very compelling reasons to do so. Not everything has the same impact, though. Comfy seats are SO nice to have, and actually not a bad place to splurge a little because they are practically inside the CG range. A CS prop will be a must-have item on the RV-7A I am looking to buy, and the impressive speed range of most RV's simply cries out for CS, but there is a real price beyond dollars to be paid. Unfortunately, the gold-standard Hartzell is heavy and as far forward of CG as you can get. Flying with a forward CG requires extra down-force from the tail, which requires additional up force from the wing, etc.,etc., a drag-inducing positive feedback loop that, carried to the extreme, can compromise control authority in flair, reduce cruise performance efficiency, and make the aircraft sluggish and unpleasant to fly. The lightest weight solution to these problems is a bit of lead as far back in the tail as you can get. Now you have a better balanced airplane for control authority and efficiency, but one that handles a little bit less like a mid-engine sports car and a little bit more like a dumbbell. The prop and ballast have slightly increased resistance to control in the pitch and yaw axis. A heavy paint job would do the same, but with a price in category one too. A very strong argument can be made for saving even 8-10 pounds at the nose with a composite prop if, like me, you must have CS.

Or maybe you are ready to give up the CS advantages for a nice, light FP prop and maybe even a lighter engine. Great, you have just solved the polar mass problem and will have an incredibly nimble airplane that is a delight to fly. Now you must REALLY pay attention to the CG, limit luggage, hang everything you can toward the front, and really fuss over making the paint job light, or even go without, or you may find yourself needing to add one of those 20 pound crush plates behind that lightweight prop. (Or hey- maybe I SHOULD consider a whirlwind? Sheesh!) Every choice has a consequence.

Mind you, I'm not here to be critical of anyone's particular choices, many think added luxury is worth any price. I've just seen disappointment on the faces of too many builders learning the true weight of their aircraft for the first time. The important thing is to make these choices consciously and avoid regret. Not putting something on is a lot easier than taking it off later.

Link to my previous thread mentioned above:
http://www.vansairforce.com/communit...52#post1204252
__________________
Otis Holt-
RV-7A (bought)
Built Monnett Moni
Frmr Test Pilot/Author CAFE APR's:
RV-8A, S-7C, Europa, Glastar.
-2021 VAF donation!!-
"RV-Fun is inversely proportional to RV-Weight!"

Last edited by Hartstoc : 09-20-2017 at 08:46 AM. Reason: Emphasis and clarity
Reply With Quote
  #2  
Old 09-19-2017, 09:11 AM
WA85's Avatar
WA85 WA85 is offline
 
Join Date: Apr 2010
Location: Huntsville, AL
Posts: 167
Default

Great Post - A few additional points

1. There is always a trade off, more weight needs more power or wing area to achieve the same performance...which often leads to more weight. In RVs, its tough to add wing area, so you have to add more power, which leads to...

2. Years ago a WWII pilot told me to learn to fly the wing, not the engine. Possibly too many folks fly the engine.

3. It was be a fun science project to find the optimum RV performance, the CAFE project did some excellent work in several areas. It would be interesting see what combination of engine, prop, avionics, paint, configuration, would deliver the optimum performance closest to Vans original design specifications. In many ways we are already seeing this in the RV-12 and RV-14 kits.

4. It would be interesting to compare wing loading, handling qualities, for a variety of RV configurations, big engine CS prop heavy acft / small engine FP prop light acft, differing GC envelopes. The red bull air races sometimes show the different turn radius and loop diameters of different acft, it would be interesting to see the differences of various RV configurations doing specified maneuvers.
Reply With Quote
  #3  
Old 09-19-2017, 09:33 AM
Hartstoc's Avatar
Hartstoc Hartstoc is offline
 
Join Date: Aug 2017
Location: Sebastopol,CA
Posts: 368
Default

Quote:
Originally Posted by WA85 View Post
Great Post - A few additional points

1. There is always a trade off, more weight needs more power or wing area to achieve the same performance...which often leads to more weight. In RVs, its tough to add wing area, so you have to add more power, which leads to...

2. Years ago a WWII pilot told me to learn to fly the wing, not the engine. Possibly too many folks fly the engine.

3. It was be a fun science project to find the optimum RV performance, the CAFE project did some excellent work in several areas. It would be interesting see what combination of engine, prop, avionics, paint, configuration, would deliver the optimum performance closest to Vans original design specifications. In many ways we are already seeing this in the RV-12 and RV-14 kits.

4. It would be interesting to compare wing loading, handling qualities, for a variety of RV configurations, big engine CS prop heavy acft / small engine FP prop light acft, differing GC envelopes. The red bull air races sometimes show the different turn radius and loop diameters of different acft, it would be interesting to see the differences of various RV configurations doing specified maneuvers.
Excellent points, especially the one about flying the wing! We are taught to be ready with power during practice stall recovery, but I've always favored instead being sensitive to the wing, "feeling" the flow re-attach and lift return(or more importantly, learn to feel the loss of lift and incipient flow detachment). After all, if you got into that too-slow near the ground pickle because the rubber band broke, you are not going to have the power option.
__________________
Otis Holt-
RV-7A (bought)
Built Monnett Moni
Frmr Test Pilot/Author CAFE APR's:
RV-8A, S-7C, Europa, Glastar.
-2021 VAF donation!!-
"RV-Fun is inversely proportional to RV-Weight!"

Last edited by Hartstoc : 09-19-2017 at 10:21 AM.
Reply With Quote
  #4  
Old 09-19-2017, 09:39 AM
rvbuilder2002's Avatar
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,660
Default

Quote:
Originally Posted by Hartstoc View Post
Is "RV-Fun is inversely proportional to RV-Weight" a valid statement?
I would say it depends on what a persons personal definition of fun is while flying.
For many, the fun of their RV is using it to get somewhere fun. In that context, it usually also means being able to carry a lot of stuff with them that they need for that on location fun.
My wife and I are just now beginning to explore that use of our RV-6A via airplane camping (Just got back from 4 days of airplane camping in the San Juan islands, WA). The payload of an RV-6A makes that a challenge (we are slowly accumulating light weight backpacking gear even though we don't do any back packing).

If we change the phrase to "Is RV performance inversely proportional to weight?" The answer is absolutely.

For the record, I agree with your premise though...
When it comes to the pure joy for the pilot while flying an RV, lighter is always better.
__________________
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.

Scott McDaniels
Hubbard, Oregon
Van's Aircraft Engineering Prototype Shop Manager
FAA/DAR
RV-6A (aka "Junkyard Special ")
Reply With Quote
  #5  
Old 09-19-2017, 09:56 AM
Ed_Wischmeyer's Avatar
Ed_Wischmeyer Ed_Wischmeyer is offline
 
Join Date: May 2005
Location: Savannah, GA
Posts: 1,574
Default

The late Alan Tolle built six, I think, RVs, half of them -3s. He said that a lightweight 3 was the most fun.

Then again, when he wanted to go somewhere, he'd fire up the 240 kt Questair Venture. Comparatively speaking, those fly like trucks, which is what you want for an IFR X-C bird.

Ed
__________________
RV-9A at KSAV (Savannah, GA; dual G3X Touch with autopilot, GTN650, GTX330ES, GDL52 ADSB-In)
Previously RV-4, RV-8, RV-8A, AirCam, Cessna 175
ATP CFII PhD, so I have no excuses when I screw up
Vaccines kept me out of the hospital but COVID still cost me a month of living, all told...
Reply With Quote
  #6  
Old 09-19-2017, 11:29 AM
terrye terrye is offline
 
Join Date: Feb 2010
Location: Vancouver, BC, Canada
Posts: 962
Default The W&B thing(part 2): IS RV-Fun inversely proportional to RV-Weight?

Interesting post and I've had this on my mind since I started building in 2004 (yes it's a slow build). These are all compromise choices. I decided to prime my airplane (first real decision in 2004) due to living on the west coast near the ocean. I decided to use the biggest engine allowed in my RV-9A (160 hp) due to living near the mountains and a CS prop to maximize the climb rate as well as lower engine rpm in cruise and a small but actual reduction in fuel consumption. I decided to install nav lights because I already have a night rating and I decided I wanted an IFR panel because I want to pursue an IFR rating in this airplane. Several recent posts about this summers' smoke reinforced this decision. And I'm going to paint the exterior to protect it and not get sun glare off the wings when flying or melt the canopy when parked. And I'm going to install relatively heavy leather seats with seat heaters but decided against the full side panel interior. Just seats, upholstered armrests and front and baggage compartment carpets

I've justified two of these choices (160 hp and CS prop) by realizing Vans demo RV-9A was so equipped. I've justified the IFR panel because I want to grow as a pilot and have the additional utility in the airplane. I've justified the seats because I want to be comfortable (my C150 seats are not particularly comfortable).

But, I also have the advantage of being about 50 lbs lighter than the FAA "standard" pilot weight, so I'm willing to put that extra 50 lbs into the airframe with no guilt. I think an RV-3 with a 300 lb pilot might not be fun and I think an RV-9A that was 50 lbs overweight with a light pilot would still be fun.
__________________
Terry Edwards
RV-9A (Fuselage)
2021/2022 VAF Contribution Sent
Reply With Quote
  #7  
Old 09-19-2017, 11:50 AM
Hartstoc's Avatar
Hartstoc Hartstoc is offline
 
Join Date: Aug 2017
Location: Sebastopol,CA
Posts: 368
Default

Quote:
Originally Posted by terrye View Post
Interesting post and I've had this on my mind since I started building in 2004 (yes it's a slow build). These are all compromise choices. I decided to prime my airplane (first real decision in 2004) due to living on the west coast near the ocean. I decided to use the biggest engine allowed in my RV-9A (160 hp) due to living near the mountains and a CS prop to maximize the climb rate as well as lower engine rpm in cruise and a small but actual reduction in fuel consumption. I decided to install nav lights because I already have a night rating and I decided I wanted an IFR panel because I want to pursue an IFR rating in this airplane. Several recent posts about this summers' smoke reinforced this decision. And I'm going to paint the exterior to protect it and not get sun glare off the wings when flying or melt the canopy when parked. And I'm going to install relatively heavy leather seats with seat heaters but decided against the full side panel interior. Just seats, upholstered armrests and front and baggage compartment carpets

I've justified two of these choices (160 hp and CS prop) by realizing Vans demo RV-9A was so equipped. I've justified the IFR panel because I want to grow as a pilot and have the additional utility in the airplane. I've justified the seats because I want to be comfortable (my C150 seats are not particularly comfortable).

But, I also have the advantage of being about 50 lbs lighter than the FAA "standard" pilot weight, so I'm willing to put that extra 50 lbs into the airframe with no guilt. I think an RV-3 with a 300 lb pilot might not be fun and I think an RV-9A that was 50 lbs overweight with a light pilot would still be fun.
Terry- Congratulations for being so open-eyed about all of your very well though-out choices right from the get-go. This is how great airplanes are built. I've actually been away from aviation for a few years, but back now and looking for a 7A as well thought out as your airplane. My search has revealed what seems like a too-casual attitude about weight among some RV builders these days, and this is what triggered posting these threads. My respect for Dick VanGrunsven as a designer could not be deeper, and discipline about weight is an essential part of his philosophy.
__________________
Otis Holt-
RV-7A (bought)
Built Monnett Moni
Frmr Test Pilot/Author CAFE APR's:
RV-8A, S-7C, Europa, Glastar.
-2021 VAF donation!!-
"RV-Fun is inversely proportional to RV-Weight!"
Reply With Quote
  #8  
Old 09-19-2017, 12:09 PM
BMC_Dave BMC_Dave is offline
 
Join Date: Mar 2017
Location: Seattle, WA
Posts: 288
Default

I mean, your assertion does seem a little arrogant. People build planes for their particular mission. If that includes a leather interior and full AC then who are you to say they're not having as much "fun" as they could be. You know what they say about opinions.

Don't be so quick to judge others for their build choices...
Reply With Quote
  #9  
Old 09-19-2017, 01:01 PM
grubbat's Avatar
grubbat grubbat is offline
 
Join Date: May 2012
Location: Ga
Posts: 718
Default FUN

Fun in my -9 is fantastic acceleration and climb at any altitude with that heavy constant speed prop up front. Also, the additional stability with that prop is nice. As a side note, the efficient wing is good on my pocketbook as well at my age.

That being said, in my old -3, fun was the amazing bird-like feeling which I assume was the direct result of having a very light structure.

Fun in the -6a was the amazing carrier landings that I could drop into for landings with that amazing wing, plus the upside down stuff and the top speed stuff.

I have no feedback on the fun factor in a -10 ......... yet.
__________________
Craig

RV-3 Sold
RV-4 Sold
1968 Twin Comanche - Sold
RV-6a Sold
1965 Comanche 260 - Sold
RV-9 Built and Flying
600A Aerostar - Sold and regretted
2000 Maule MXT-7 Project
1965 Maule M4 Bushwacker projectů till RV-15 shows up
Reply With Quote
  #10  
Old 09-19-2017, 01:19 PM
terrye terrye is offline
 
Join Date: Feb 2010
Location: Vancouver, BC, Canada
Posts: 962
Default The W&B thing(part 2): IS RV-Fun inversely proportional to RV-Weight?

Otis, thanks for the comments. The primer was the only decision made at the start, all the others took quite a bit of thinking/evaluating over several years. Maybe this is one advantage of a "slow build". Having a dual interest in sports cars and airplanes and being an engineer, two of my heros are Dick Van Grunsven and Colin Chapman (I own a 1966 Lotus Elan). Both have lightweight as a central philosophy of design and every time I add weight as I mentioned in my RV or in my Elan (CV joint halfshafts are 8 lbs heavier than the rotoflex/hollow tube original shafts) I get this feeling that the designer is looking over my shoulder.

I think the central point of starting this discussion is to focus builder's minds on the compromises they are making every time they add weight (it's only a few pounds ). Not too long ago Van chastised an RV-10 builder who had added so much stuff that he really didn't have any useful load left, and he was right to do so, and right to chastise the AirVenture judges for giving him a trophy. In my case, I've balanced (pun intended) the capability vs performance vs max gross weight (not an ounce over the designer's recommendation) issues to the best of my ability.

As an aside, I enjoyed all of the CAFE aircraft reports and wish CAFE/EAA were still doing them (some kind of disagreement or fallout prevents this?). They were the only reports I've ever seen that measured airplane performance properly and were central in my decision to build an RV-9A
__________________
Terry Edwards
RV-9A (Fuselage)
2021/2022 VAF Contribution Sent
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 02:44 AM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.