# Thread: L/D Ratio of an RV 3 ?

1. ## L/D Ratio of an RV 3 ?

Does anyone have an RV 3 or know of the Airplanes L/D Ratio?
TIA
George

2. ## Re: L/D Ratio of an RV 3 ?

Since L/D is related to aspect ratio .... and RV3 has an aspect ratio of 4 to 1 .... My first guess is an L/D of 4 to 1.

3. ## Re: L/D Ratio of an RV 3 ?

For reference, the RV-8A has a glide ratio of 9.5 to 1. I would expect the -3 to be fairly close to that.

BJC

4. ## Re: L/D Ratio of an RV 3 ?

There was an L/D graph of an RV3 in one of the Contact books. I'll see if I can dig that up this weekend.

5. ## Re: L/D Ratio of an RV 3 ?

You should be able to back-calculate the L/D, within about 10% or so, from data in the pilot's handbook. I suspect that it's optimum L/D is about 10 or so; this is fairly typical for general aviation aircraft.

6. ## Re: L/D Ratio of an RV 3 ?

I found the chart but it was not L/D, was required hp vs. airspeed. 90 mph required about 30hp.

7. ## Re: L/D Ratio of an RV 3 ?

To all,
I purchased Jan Carlssons's Prop Program and to accurately predict performance the L/D ratio should be known, which acts as a parameter (Limit) to speed calculations (and therefore Prop Diameter/ Pitch). His program does in fact calculate the L/D but if one is trying to push the speed up, it lifts the L/D Ratio to unacceptable limits. As I'm doing a prop Estimate for someone else on HBA, I don't have a clue what the L/D is, so I estimated 11.4 given some other guide I found on the Internet of similar type aircraft. Then I thought I was too conservative and went to 12.7 because of the high HP available and streamlined Cowl. The Original RV 3 had a hp rating of up to 160 hp but this estimate is a little beyond that again - so I figure 12.7 might not be an overestimate in this case. Still if I had a baseline to work with it would be better than nothing.
TIA
George

8. ## Re: L/D Ratio of an RV 3 ?

I would be surprised if the L/D of the RV-3 exceeds 10.

BJC

9. ## Re: L/D Ratio of an RV 3 ?

Originally Posted by Riggerrob
Since L/D is related to aspect ratio .... and RV3 has an aspect ratio of 4 to 1 .... My first guess is an L/D of 4 to 1.
The L/D ratio is related to the wing aspect ratio, but not a direct function of the wing aspect ratio. NACA/NASA published a long list of L/D and A/R for different planes. Gudmundsson have this list in his book, together with some own calculations. I do not have the book in front of me, bur L/D close to 10 have been achieved on airplanes with A/R around five if I remember correct.

A L/D of 8 to 10 on a RV-3 sounds possible, probably with a vide spread from aircraft to aircraft depending on details like wheel fairings, engine baffling, sealing of cockpit/fuselage and other.

10. ## Re: L/D Ratio of an RV 3 ?

Could be measured in flight. Take the sink rate and distance and measure the L/D ratio.

11. ## Re: L/D Ratio of an RV 3 ?

This points to one of the reasons why designing propellers can be very tricky. The underlying methods Jan is using can yield very precise results, but at the same time, they are subject to substantial uncertainties; the drag of the specimen aircraft in question, and the actual power output of that individual powerplant installation chief among them. This is a garbage-in, garbage-out scenario if there ever was one. The more accurate your input, the better your results.

I have seen others on this forum describe propeller design as a black art. I disagree. The physics are well established and the math has been defined. The problem, as Lendo is discovering, is obtaining accurate input data. The "black art" is just lessening the uncertainty and arriving at reasonable estimates. From there, the design process is subject to the same types of tradeoffs as any other aspect of aircraft design.

In this case, if you know the best glide speed for the RV-3, you can determine the L/D ratio from there.

12. ## Re: L/D Ratio of an RV 3 ?

Cafe Foundation has evaluated at least 3 customer built RV's and the factory RV-9. Could probably get I good idea of the RV-3's glide performance there.

13. ## Re: L/D Ratio of an RV 3 ?

If you know what the prop efficiency they assumed is, then that horsepower figure, if accurate, should give you a good idea of L/D. For instance, I'd say, based on nothing but laziness and a wild guess, that the gross weight is 1,000 lbs. Oops, I just checked. It's pretty heavy for a single seat airplane. Real gross is 1100. If it takes 30 hp at 90 mph, with a prop efficiency of 70 percent, that's 11,550 ft-lbs/second at about 132 ft/sec. I.e. about 88 lbs thrust. That's almost 12.6:1. Seems a bit high. If the horsepower was AFTER accounting for prop efficiency, that would be more like 8.8. Throw in something for the drag of a stopped prop, or, worse, a windmilling one.. In any case, this might not be the best L/D speed in the first place. Best L/D should be somewhat above minimum sink. The latter should be somewhere close to speed for fastest climb, maybe a bit less because the best climb figure has to account for what the prop does. Another factor that might change things a little bit is that the propwash means faster air speeds over part of the fuselage.

BTW, using the specs from the Van's Aircraft web site, the aspect ratio is 4.4. (19'11" and 90 square feet)

Another way to go at it, and I suspect this will be considerably fuzzier:
Assume equivalent flat plate area for the fuselage is 1.5 ft^2. (Another wild guess) Dynamic pressure at 90 mph is 20.7 lbs/ft^2. So that's 31 lbs drag . We'll look at the wing as a separate entity, hoping that the interference drag is similar to the drag savings by having part of the wing buried. Totally arbitrary and due to laziness on my part. The wing loading is 1100/90 = 12.2 lbs/ft^2. 12.2/20.7 = .58 for the Cl. I'm assuming that span efficiency is 0.85. cdi=.58^2/(pi*4.4*.85) or 0.0286 .0286 * 20.7 *90 is 53.3 lbs drag, which is higher than I expected. If we assume the wing Cd is .012 (another wild guess), then .012*20.7*90 is 22.4 lbs drag. So that makes a total of 107 lbs drag, or L/D = 10.3 So at least we're in the same ballpark. And maybe the higher reported figure means the RV-3 is somewhat cleaner than most. If I cheated on any of the guesses, I don't remember doing so.

If someone else can prove there are holes in the above reasoning, I'd appreciate it if you did.

14. ## Re: L/D Ratio of an RV 3 ?

Ir27 -You right 12.6 or 12.7 was my second guess but 10.3 seems a little low, it's fairly clean with a Powersport Cowl, so maybe my first WAG of 11.4 is more like it.
Your right we have 90 sq.f.t wing area, 1,100 lbs., T/O weight, and 185 hp, the prop efficiency is about 85% - how dos that affect your maths.
George

15. ## Re: L/D Ratio of an RV 3 ?

Originally Posted by BJC
I would be surprised if the L/D of the RV-3 exceeds 10.

BJC
Me too.

Pretty easy to calculate with CD0 Oswald and AR.

Likely in the 8-9 range without prop and considerably below 8 with a stopped prop.

Good planes have AR>>30

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