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

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

Originally Posted by BBerson
Is that 10.6 at best L/D speed or cruise speed?
I would think the prop is designed for cruise speed.
yes. at speed for max L/D
Yes. or max speed.

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

Originally Posted by Jan Carlsson
yes. at speed for max L/D
Yes. or max speed.
So, to design a prop for cruise speed, do you input the airplane cruise L/D?

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

Originally Posted by BBerson
So, to design a prop for cruise speed, do you input the airplane cruise L/D?
No, I just use max speed or if knowing cruise speed can calculate max speed, the Best L/D is calculated, and should stay the same in both inputs, just as when calculate for a different alt. L/D is just a control Nr.

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

Toa all,
Well I got an 'e' Scale of from 0.7 to 0.9 from Harloff ( I think) for LSA where as the Scale from Sadraey was for GA Light aircraft which was from 0.0.65 to 0.7. Working through the 'e' Factor using (Sadraey and GAAD, which were both the same for 'Straight Wings') Formula 1.78(1-0.68AR^0.68)-0.64 I get an 'e' of 0.92.

This wasn't intuitive for me, I found it confusing, as when the AR goes down the 'e' goes up and as the RV 3 AR is 4.4, this results in a high 'e'. Give the CDo guide for LSA is between 0.02 and 0.03 I got results of CDo of 0.03 = L/D of 10.3; CDo of 0.25=11.29 and a CDo of 0.02 = L/D of 12.6. The average was 11.4 which was my first WAG - but nothing beats the Designers information for a start point. In this case the HP is higher than in the original RV documentation of max 160 hp- the engine people are looking at 185hp for this test phase, which the RV3 is the mule.

So getting the right sized prop is a good place to start and has been my contribution (to fellow HBA members) using Jan Carlsson's Prop program - I like the comments Jan. Although it's been a fair bit of work I did a number of Spread-sheets which range from Seal level performance to 15,000 ft. to get the 'Big Picture' on expected performance using different L/D ratios. The Prop Program can do this but collating the information from different screens is a manual effort - but well worth the exercise in IMHO.
I hope that information on the calculations help someone.
George

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

Jan,
Just so I'm not missing anything what is meant by your reference of 'Nr" i.e. what is Nr?
George

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

Originally Posted by Lendo
Jan,
Just so I'm not missing anything what is meant by your reference of 'Nr" i.e. what is Nr?
George
It is short for number. it was to long to type.

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

Fly2kads, What's your method of calculating L/D ratio from best Glide Speed.
TIA. George ( down under)

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

Originally Posted by Lendo
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
Sorry, I missed this question for a while. I don't have a good estimate for the interaction between the propwash and the airframe, but if we neglect that, it ought to be possible to come up with something. BTW, that must be a really good prop.

For the first method, it means we're using more thrust horsepower than we thought. .85/.7 = 1.21 x as much drag. That gives an L/D of 10.4, assuming that 30hp at 90mph is right.

We can probably do better if the main gear legs can be reached with a hacksaw in flight. ;-)

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

Originally Posted by BJC
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
Was wondering how you arrived at that number? Is this something you computed yourself, or arrived at from flight testing? I was reading "Engine Out Survival Tactics". The author quoted a glide ratio for the RV-8 of 1.2 nautical miles per 1,000'. If I'm computing this correctly thats about a 7.2:1 ratio.

No disrespect intended. I really want to know.

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

Originally Posted by N804RV
Was wondering how you arrived at that number? Is this something you computed yourself, or arrived at from flight testing? I was reading "Engine Out Survival Tactics". The author quoted a glide ratio for the RV-8 of 1.2 nautical miles per 1,000'. If I'm computing this correctly thats about a 7.2:1 ratio.

No disrespect intended. I really want to know.
Comparisons of estimates are meaningless until you define the gliding state as prop windmilling or prop stopped.

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

Originally Posted by bmcj
Comparisons of estimates are meaningless until you define the gliding state as prop windmilling or prop stopped.
That's why I asked the question. Under what conditions have you observed a glide ratio of better than 9:1? The quote I'm referring to is about engine failure. And, the author doesn't say whether the propellor is stopped or windmilling, or if its a fixed pitch or constant speed propeller in this RV-8. So, I was hoping maybe BCMJ tell me what conditions that estimate is based on.

Seriously, I mean no disrespect. I was just wondering for my own education.

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

Originally Posted by N804RV
Was wondering how you arrived at that number? Is this something you computed yourself, or arrived at from flight testing? I was reading "Engine Out Survival Tactics". The author quoted a glide ratio for the RV-8 of 1.2 nautical miles per 1,000'. If I'm computing this correctly thats about a 7.2:1 ratio.

No disrespect intended. I really want to know.
No disrespect taken; it is good to validate performance claims.

The data is from CAFE. I have more confidence in their HBA data than any other source. See http://cafe.foundation/v2/research_aprs.php then select the RV-8A report.

Keep in mind that there are several glide ratios that could be of interest. One is the ratio for the airframe without a propeller, which should be the same as with a propeller at zero thrust (see the CAFE report on that method) which is a measure of the airframe aerodynamics. Others that may be of interest may be the failed engine ratios, which vary significantly with the type of propeller, i.e., fixed or variable pitch, the propeller pitch, and whether or not the propeller is windmilling.

What conditions were cited for a 7.2 : 1 ratio?

BJC

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

lr27,
That prop efficiency of 85% is the estimate given by the JC Propeller program, not mine.
I was under the impression that 90% was very good (Laminar Airfoils). Jan's props are not Laminar their Clark Y/ RAF 6. combinations
George

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

Originally Posted by BJC
...What conditions were cited for a 7.2 : 1 ratio?

BJC
The author didn't say. It was used in a rough comparison of different aircraft in an engine failure scenario.

My guess is probably a windmilling fixed pitch prop. There was an A36 in the comparison as well. It did 1.7 NM per 1,000 (which is quite a bit better). So, perhaps the fixed pitch RV-8 compared to an A36 with a constant speed prop at the coarsest setting would account for some of that difference.

Thanks for your answer. I'll have to check out those CAFE numbers!

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

Originally Posted by Lendo
lr27,
That prop efficiency of 85% is the estimate given by the JC Propeller program, not mine.
I was under the impression that 90% was very good (Laminar Airfoils). Jan's props are not Laminar their Clark Y/ RAF 6. combinations
George
In my propeller soft there is not used any laminar airfoils because the props must be able to make by hand, axe, draw knife, angle grinder, sandpaper. or CNC and sandpaper.

the L/D of the airfoil itself average on the propeller is something like 50:1 That is 2% loss, if having an very slippery laminar foil, we might think we can get 100:1 or 1% loss, so we can save 1% efficiency, if we can make the 2-6" airfoil perfect, and duck for all bugs and gravel.

Propellers tend to be 75% - 90+% efficient, mostly depending on forward speed, so we have 10-25% losses, mostly induced and rotational losses, Induced loss/drag come from lift, or thrust in this case. we can reduce the induced drag by reducing lift per blade length. until it flutter.

At tip we will have a prop chord of one inch or half inch 5-10% thick looks good if we zoom in the airfoil in a cad program, it will be 0,025 - 0,05 inch thick, at the thickest point.

So there is a reason for everything

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