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Mar 08, 2014, 08:49 AM
Proud member of LISF and ESL
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Discussion

What is the effect of folded props on glide?


I have seen debates on this topic. Does an electric sailplane suffer significant glide performance degradation from the folded props? I don't think I have seen any detailed studies, but it comes up from time to time.

Well, a full scale sailplane with a FES, front end sustainer, looks a lot like our e-sailplanes. Recent tests actually documented the difference.

You can read about the FES here:
http://www.front-electric-sustainer.com/news.php

You can read the report here:
http://www.front-electric-sustainer....A%20FES_en.pdf
A mix of stills and video of the testing
http://www.front-electric-sustainer.com/video.php


3. Summary and acknowledgements
In four flights on two days the effect of the two propeller blades of Front Electric Sustainer (FES) on the flight performance Lak‐17a FES S5‐3117 was investigated for two flap settings in different speed ranges. The effect of the ventilation state was investigated for one flap position.

The results show a very small increase in aerodynamic drag due to the propeller blades in an expectable order.


If you would like to see a full scale sailplane with a motor in the nose, you can see video here:
http://www.front-electric-sustainer.com/video.php
Last edited by aeajr; Mar 08, 2014 at 08:57 AM.
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Mar 08, 2014, 09:24 AM
Proud member of LISF and ESL
Thread OP
Flight report of a FES equipped sailplane flown in competition
http://soaringcafe.com/2014/02/flyin...ent-2-electro/
Last edited by aeajr; Mar 08, 2014 at 09:43 AM.
Mar 08, 2014, 01:23 PM
Balsa addiction since age 3
ScottSails's Avatar
Thank you for sharing - that was a great read!
Scott
Mar 08, 2014, 08:50 PM
Registered User
R.M. Gellart's Avatar
One word, rubberbands! A rubberband doubled that is run across the blades through the spinner, and the blades I would guess cause some drag, but that rubberband keeps them as tight as they can stay against the fuse. Simple and everyone has props so it is kind of mute, unless you do not use rubberbands and the blades are hanging out.

Marc
Mar 08, 2014, 10:20 PM
Registered User
MikeC's Avatar
I think a 3 or 4 meter model would have at least twice the drag penalty (percentage wise) as a LAK FES.
Mar 09, 2014, 01:13 AM
Proud member of LISF and ESL
Thread OP
Quote:
Originally Posted by MikeC
I think a 3 or 4 meter model would have at least twice the drag penalty (percentage wise) as a LAK FES.
I would suspect you are right, but is twice a very small number still small enough to be a minor issue?
Mar 09, 2014, 10:45 AM
Registered User
Hawksnest's Avatar
Holy Chit, I guess I will design a prop that ejects from the plane at 200m. One use only. LOL
Mar 09, 2014, 10:53 AM
Proud member of LISF and ESL
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Quote:
Originally Posted by Hawksnest
Holy Chit, I guess I will design a prop that ejects from the plane at 200m. One use only. LOL
I have seen that done with a Radian, but not intentionally. Not a pretty sight.
Mar 09, 2014, 11:04 AM
AMA 7224
Leadchucker's Avatar
Quote:
Originally Posted by R.M. Gellart
One word, rubberbands! A rubberband doubled that is run across the blades through the spinner, and the blades I would guess cause some drag, but that rubberband keeps them as tight as they can stay against the fuse. Simple and everyone has props so it is kind of mute, unless you do not use rubberbands and the blades are hanging out.

Marc
Agreed. I use black medium girl's hair rubber bands, they seem to last and are just the perfect pressure if you get the right size. You can get a bag of 5 million for a dollar on fleabay if you don't have any girls about to borrow them from. See loop in attached, please excuse the ignorant drawing.
Latest blog entry: Maxford 27% Gee Bee Model Y ARF
Mar 09, 2014, 11:22 AM
solastagia
kcaldwel's Avatar
The data for the Lak-17 in the paper you posted shows about 2 full points off the best L/D (about 45 to 43, about 5%). That seems reasonably significant to me. That is likely almost entirely due to the flow over the fuselage being tripped to turbulent very early at the nose by the prop blades and spinner gap.

The prop on the full size glider is considerably smaller in proportion to the glider than on most e-gliders. This may cause a larger performance difference on model e-gliders.

The lower Re of the model will make a difference in the performance hit amount as well, but I'm not sure in which direction.

Kevin
Mar 09, 2014, 02:20 PM
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tkallev's Avatar
Quote:
Originally Posted by Hawksnest
Holy Chit, I guess I will design a prop that ejects from the plane at 200m. One use only. LOL
The guys who don't torque down their collets already do that
Mar 09, 2014, 02:57 PM
launch low, fly high
Quote:
Originally Posted by kcaldwel
The data for the Lak-17 in the paper you posted shows about 2 full points off the best L/D (about 45 to 43, about 5%). That seems reasonably significant to me. That is likely almost entirely due to the flow over the fuselage being tripped to turbulent very early at the nose by the prop blades and spinner gap.

The prop on the full size glider is considerably smaller in proportion to the glider than on most e-gliders. This may cause a larger performance difference on model e-gliders.

The lower Re of the model will make a difference in the performance hit amount as well, but I'm not sure in which direction.

Kevin
The performance penalty from the folded prop will reduce in magnitude due to the lower Re of models.

Some of the full scale penalty is due to the man carrying requirement. That large fuselage has a lot of wetted area, and the increase drag due to turbulent flow is magnified as compared to the small amount of surface area on an RC sailplane.

On the other side of the equation is that the model prop is likely a bit bigger, and the hub integration isn't as clean.
Mar 09, 2014, 08:40 PM
Registered User
MikeC's Avatar
Quote:
Originally Posted by aeajr
I would suspect you are right, but is twice a very small number still small enough to be a minor issue?
Minor but noticeable. I remember Michael Selig claiming that a RC pilot can notice as little as a 3%-4% difference in drag.

A friend, with a professional background in flight test evaluation, did intensive tests with two identical hand launch gliders using different flaperon drives and found significant differences (15 second flight times) first from bottom drive, then top drive and finally RDS drive.

I would much rather take a small performance hit then shag highstarts and winch lines.
Mar 09, 2014, 10:00 PM
solastagia
kcaldwel's Avatar
I have no desire to return to winch lines or high starts either, but given the magnitude of the performance loss it may be worth looking at ways to reduce it.

The standard spinners available do not follow the fuselage lines very well. A spinner that was fair to the fuselage lines would help reduce the drag increase. The Vladimir spinners look better than most others I've seen.

http://www.hyperflight.co.uk/product...for-5mm-shafts

The offset yoke spinner designs reduce the frontal area of the blades, and perhaps the area of the blades exposed to the airflow.

There should be a small drag advantage to using smaller propellers. That might impact the climb rate, and also the rpm the propeller has to spin at. Most folding props are rpm limited.

I'm not sure there is much else to be done, unless someone comes up with a design for models like the Stemme S10.

Kevin
Mar 10, 2014, 11:28 AM
Chuck 'Em and Chase 'Em
Fly2High's Avatar
Quote:
Originally Posted by aeajr
Flight report of a FES equipped sailplane flown in competition
http://soaringcafe.com/2014/02/flyin...ent-2-electro/
Ed,

That is an amazing article. You have got to send it out to the guys in the club,. To be honest, if I skipped only a couple sentences here or there, I would swear this was a model article and not a full size one!!

I just love the fact that 'taping' was commented on. Brings us feeling closer to the real thing when we assemble our models.

I am only half way through and I just had to reply that I am really enjoying it.



Thanks Ed

Sincerely,
Frank


P.S. So when are we going to have a scale ALES/ FES contest!!!

So would the name of that contest be a SALFES - Scale Altitude Limited Front Electric Sustainer contest
Last edited by Fly2High; Mar 10, 2014 at 12:11 PM.


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