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RV-14 Best Glide Speed

JTJones

Well Known Member
I’m trying to fine tune my data on best glide speed in an RV-14. At 95 knots IAS or higher I get some engine backfire which is disconcerting. Engine is an IO390X with a constant speed prop. Anyone have this problem and what have you determined to be best glide speed?
 
When I took transition training, the factory -14A had a 75kt best glide speed; and it also popped (afterfire) when the throttle was pulled to idle on downwind. I was told the popping was normal for some engine configurations.

_Alex
 
The way Mike Seager explained it to me is that the 95KT best glide is longest distance traveled, whereas 75KT is longest duration of flight (time). In my phase one I found both to be pretty true. Couldn't tell much difference between 90-95-100Kts for distance.

I never had the popping issue on power reduction.....
 
The way Mike Seager explained it to me is that the 95KT best glide is longest distance traveled, whereas 75KT is longest duration of flight (time). In my phase one I found both to be pretty true. Couldn't tell much difference between 90-95-100Kts for distance.

I never had the popping issue on power reduction.....

I did a lot of testing and came to the same results as described here 75kts as min sink and 95kts as best glide. Plus or minus 5kts made little difference. Just remember these numbers are for gross weight. I determined the AOA at gross weight best glide. That allows a constant AOA independent of weight.

As for backfire, I would check my idle mixture it is unrelated to glide
 
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I’m trying to fine tune my data on best glide speed in an RV-14. At 95 knots IAS or higher I get some engine backfire which is disconcerting. Engine is an IO390X with a constant speed prop. Anyone have this problem and what have you determined to be best glide speed?

For those trying to ‘fine tune’ their numbers, remember that best glide speed varies with weight. And, in the real world, with wind.
 
Popping as described is pretty common. Start pulling back the throttle earlier and more gradually and I bet the problem goes away. I have yet to hear a good explanation for what causes it, but I noticed the same phenomenon recently with someone starting downhill on a motorbike. Anyone care to explain?

Erich
 
I’m trying to fine tune my data on best glide speed in an RV-14. At 95 knots IAS or higher I get some engine backfire which is disconcerting. Engine is an IO390X with a constant speed prop. Anyone have this problem and what have you determined to be best glide speed?

Assuming you are at idle. Afterfires (unlikely they were back fires) that occur on throttle reduction to idle are often caused by either leaks in the exhaust tube connection at the cylinder or too lean of an idle mixture.

Larry
 
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Thanks for the responses

I am well aware glide performance varies with wind and weight. Thanks for the “afterfire” term.
 
I have a few hours in the factory 14A and also had the afterfires. Mike said completely normal as I recall and the factory plane (with the EXP engine) had a different exhaust setup than standard (ends forward of the tunnel). Wondering if the location of the exhaust exits may be the reason it is more pronounced too as they are pretty much right under your feet.
 
I noticed the same phenomenon recently with someone starting downhill on a motorbike. Anyone care to explain?

As for backfire, I would check my idle mixture it is unrelated to glide

In my experience tuning carbureted motorcycles, ATVs, etc, McGraw is correct.
Popping "afterfires" are usually associated with a lean idle mixture. If the idle mix is too lean, the plugs won't fire the cylinder. Thus the fuel/air charge from the cylinder goes into the exhaust un-burned. After a few misfires, the concentration of fuel/air in the exhaust reaches optimal conditions and ignites causing the pop.
 
Drifting my own thread…

Thanks to everyone for their responses. Now to drift my own thread. So it seems best glide speed is in the neighborhood of 95kias . What glide ratio does that speed equate to in the -14?
 
Hi Jeff, these are the notes I took during phase 1. Looks like about 12/1 glide ratio is a good ballpark figure anywhere between 80 and 95 KIAS. If I'm near gross weight I'll fly closer to 95 KIAS, but no slower than 80 KIAS. Also, I think the glide ratios would be a little lower down in thicker air.

Time/distance/vertical speed to drift from 9,500' to 7,500' (distance is calculated based on TAS). I did the test at fine pitch as well, and at higher airspeeds (90-95 kts) the glide was almost cut in half.

9.5 to 7.5 at 75 KIAS (85 KTAS), coarse pitch = 2:26 = 822 FPM = 3.45 NM = 10.5/1 GR
9.5 to 7.5 at 80 KIAS (91 KTAS), coarse pitch = 2:43 = 736 FPM = 4.12 NM = 12.5/1 GR
9.5 to 7.5 at 85 KIAS (96 KTAS), coarse pitch = 2:29 = 805 FPM = 3.97 NM = 12/1 GR
9.5 to 7.5 at 90 KIAS (101 KTAS), coarse pitch = 2:21 = 851 FPM = 3.95 NM = 12/1 GR
9.5 to 7.5 at 95 KIAS (106 KTAS), coarse pitch = 2:13 = 901 FPM = 3.92 NM = 11.9/1 GR
 
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