# 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.

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