Basic Aerodynamics and Flight Mechanics

[pat]

Excellent thread! another book worth reading is "Stick and Rudder" it explains much if this in layman terms.

[n5lp]

Originally Posted by pat

Excellent thread! another book worth reading is "Stick and Rudder" it explains much if this in layman terms.

Interesting that a book published in 1944 is still getting recommendations and I agree that it is a real good book.

I find his advice to pull the nose up and slow down if you are too high on approach works particularly well in RVs.

[David-aviator]

Originally Posted by rfinch

Great! An excellent review of basic aero....now I have someone I can explore a few nooks and crannies with :-)

I've become a bit interested in flutter with respect to the RVs. I talked to a presenter yesterday at the Golden West Flyin, Martin Hollman, owner of Aircraft Designs, Inc. Talking to him after his presentation, he said that the RVs have never had a proper flutter analysis done. To his knowledge Van did pay for an analysis, but it was a cheapo method known for inaccuracy.

Obviously in practice this is not a great problem because RVs do not have a reputation for coming to pieces in flight. Still, I would like to see a trustworthy analysis done of the several RV models using an accepted, modern numerical code. I suppose the time-consuming part would be inputting the structural elements. Perhaps the RV crowd could help with the grunt work?

Of all the gurus in this business, Mr. Hollman is not my favorite.

I remember him being somewhat of a pain in the side years ago when guys were building and flying the canard airplanes created by Burt Rutan. He had criticism of that effort. Now he has taken on another successful effort, how come? Seems like writing books on aviation design is a good idea but questioning the integrity of the most successful home built design ever won't make me go out and buy one of his books. I much prefer "Stick and Rudder" from 1944.

As you say, "....this is not a great problem because RVs do not have a reputation for coming to pieces in flight" which I presume means it is not a problem. So why go to all the trouble and expense of crunching so many numbers to come up with a conclusion it is not a problem?

[David-aviator]

I've read all your stuff here, Steve, and it is good technical information most of which I understand and agree with.

It is curious though in that with all the training I've been subjected to since 1959, there never was discussion or procedure about turning back to the runway with an engine failure. Maybe there's a reason it was not on the agenda - an operational reason.

When we took off in basic training we were cleared very close behind the guy in front of us, like when he broke ground it was go. You could see 2 or 3 or 4 airplanes ahead in the pattern as you rolled day and night. Wouldn't it have been interesting if one of them turned around and came back head on about the time you were ready to break ground?

Same can be said about a departure from a busy place like OSH during the convention. A turn around at that place at that time could be a disaster.

Technically the procedure is doable in any airplane if the pilot knows how it will perform. But I am really hung up on the variables of the maneuver. I'd hate to get half way through the turn seeing nothing but dirt to maintain flight and also seeing the airplane is not going to make it to the runway. Seems like going straight ahead would present more options of where the flight will end and for that reason will stick with that plan.

[rfinch]

Originally Posted by David-aviator

As you say, "....this is not a great problem because RVs do not have a reputation for coming to pieces in flight" which I presume means it is not a problem. So why go to all the trouble and expense of crunching so many numbers to come up with a conclusion it is not a problem?

Fair question.

I'm building a -9A which recommends a max engine power of 160 hp....because of potential flutter problems at high cruise speed. That article got me interested in this flutter business. Not that I'm interested in putting a larger engine in mine, but limits always interest me. Why is the limit at that point? How good was the analysis?

Second, some builders are departing from the kit and plans. This is experimental, after all. In particular, for the -9, it's not uncommon to find builders extending or adding fuel tanks in the wings. Most consider the additional static load, but none consider changes to flutter.

Third, I'd like more practical information about control surface flutter and paint. I have a vague notion, from reading about control surface balancing of certificated aircraft after repainting, that this is important. Yet I can't recall any concern about (re-)balancing control surfaces for the RVs. What should we be looking for? How close are we to flutter? Will the flutter come on gradually, with warning, or suddenly to destruction?

Finally, numerical models are my career and always of interest to me. Very powerful codes are available these days for common PCs which can do quite sophisticated calculations in many fields; collecting data and inputting it is more often the bottleneck.

[jrs14855]

I have been waiting for years for someone to explain why: the Pitts S1S from at or near Vne will fly the length of a 300" runway in a perfect 90 degree bank, and then climb 500 feet, still in a 90 degree bank. It will also make a very rapid 90 degree turn on rudder only with the wings perfectly level. The wings level turn bleeds off speed MUCH more rapidly than the "knife edge". Knife edge is a zero G maneuver. I suspect if the G meter were rotated 90 degrees, it would read 1 G in level knife edge flight. I suspect that the Pitts is using every vertical or near vertical surface for lift in knife edge. Fuselage, wing struts, landing gear and wheel pants. The spring gear Pitts does not knife edge nearly as well as the standard V gear.

[jrs14855]

Leighton Collins started the magazine Air Facts around 1939. Wolfgang Langwiesche wrote for Air Facts, Stick and Rudder was allegedly inspired by the acceptance of Wolfgang's articles in Air Facts. Collins and Langwiesche talked about wind gradient, aileron spins and many similar issues for years before others caught up. Ask any group of pilots today "who was Wolfgang Langwiesche" and you will get a lot of blank looks.

[scsmith]

Hi,
about the comments on knife-edge flight, I believe the poster is correct, the airplane must be getting 1'g worth of lift from all the side area on the airplane. The induced drag to do this is tremendous, because the effective wingspan is so short, but with enough hp, it works.

I also wanted to note that I think "stick and rudder" is indeed a great book. I provided a recommendation for an aerodynamics text, but totally true that you can learn a lot about flight mechanics by reading 'stick and rudder' and it language that will make sense.

Steve

RV-8QB FWF

[scsmith]

Originally Posted by Bill Wightman

Steve, Thanks for taking the time to post all of this. Being a practicing engineer myself ( and ex GD flight test), I wanted to throw in an observation:

The low aspect ratio, rectangular planform of the RV wing does help in terms of torsional rigidity, and therefore would help control structural flutter (not necessarily control surface flutter though) But it does not help in terms of aerodynamic efficiency.

I might point out that although the RV's do spend most of their time in high speed cruise, the rectangle planform still extracts a price in terms of design efficiency.

A tapered planform could produce the same low speed lift with less area, given other factors being equal. That area reduction helps to reduce both parasite and induced drag.

Not to mention a tapered planform reduces roll damping, which leads to smaller ailerons which leads to larger flaps, which further reduces stall speed and/or wing area.

Allowing for a redesign of the flaps, a tapered planform can out perform the RV wing on both ends of the speed spectrum with much less area. Add a newer airfoil, and the results get pretty impressive...

Hi,

All your ideas on improved wing performance are right on. Remember Van's basic premise of all-around sport-flying performance and a safe-handling airplane. So we can't disagree with his decision to use a rectangular wing with lots of area. Also, in the induced drag, rectangular wings actually are not as bad as linear theory would indicate, because of some interesting second-order effects of the large tip side edge at angle of attack (read my thesis!)

However, you are right that if you want to move the design toward more efficient long-range cruise, perhaps at the expense of some 'sportyness', a tapered wing with a bit more span, a fair bit less area, maybe a mild laminar flow airfoil, slotted flaps like the Mooney and RV-10, would be really nice.

But like I said, no fair peaking in my garage!
25 ft span, 98 sq ft, taper ratio =0.6 NLF-0114 section. In principle your comment about improving at both ends of the envelop are right. In design practice, I'm struggling to match stall speed with the reduced area, even with the slotted flap. Partly its because the NLF section doesn't have the CL-max of the 5-digit section. I'll come out of the closet more once my RV-8 is flying with the stock wings first.http://www.vansairforce.com/communit...ons/icon12.gif

Steve.
RV-8QB FWF

[Stephen Lindberg]

Great, now I can't concentrate on fixing the dishwasher, thinking about the speed and handling improvements to be had with a "modern" airfoil, tapered planform, and slotted flaps. More power to you for advancing the state of RVdom. Have you started cutting metal yet? No? Drawn plans, then? I am sure many of us have an intense curiosity about such a project, I know I do.

As a new RV4 owner I have noticed a few things. First, it has very reassuring slow flight characteristics with good aileron control through the stall, with adequate (but not great) aerodynamic stall warning, and a slow stall speed due to adequate wing area. That means I would likely survive an off airport landing in the event of an engine failure. I would not want to own a homebuilt that didn't have these attributes. I know a Glassair III builder who sold it because he felt it lacked this very thing. So question one is: can you keep the stall speed low using slotted flaps on a small area wing? Without excessive weight? I didn't mention complexity or expense because we homebuilders can absorb that gladly.

Another thing about the four I have noticed is that the generous wing area makes for a rough ride at high speed in turbulent air, enough so that I frequently have to slow down. Saves gas, though, so maybe it is all good. I am willing to suffer this loss of speed for the low end handling. I think Van made this choice deliberately. For those who want to go faster on a smaller wing, we eagerly await the results of your project.

Care to comment on the Rocket EVO wing? If I can read between the lines on what I have read about the EVO, it may be a long run for a short slide.

Won't the tail have to be sized upward for the tapered wing? At least on the four the tail size seems to be adequate but not large enough for standard category handling characteristics. In my limited experience, that is the thing that sets the RV4 apart from production airplanes, and the RV4 flying characteristic that an inexperienced pilot will have the most to learn about.

It seems to me that the key to flying fast on the RV4 wing is to keep the induced drag low by keeping it light, and thereby flying at the least angle of attack. I think Van has said as much. Many times.

Sorry if this is a thread hijack. Good luck on your project and please keep all of us informed. Now, back under the sink...