# Issues and Solutions for kit- and plans-based sailplanes and motorgliders

Discussion in 'Soaring' started by TerryM76, Sep 3, 2019.

### Help Support HomeBuiltAirplanes Forum by donating:

1. Sep 10, 2019

### TFF

#### Well-Known Member

Joined:
Apr 28, 2010
Messages:
11,625
3,264
Location:
Memphis, TN
The National Aeroclub of the US is not a “strong” organization. I believe it only exists for FAI licenses because the have to be processed somewhere. It is an interface. The US organizations are AOPA, EAA, SSA and some smaller ones. Private organizations. They may be international now, but intention was really US based clubs and lobbyists. It’s not the same financial struggle to fly a Pawnee here as Europe. Buying cheaper than a EU ultralight. Just keep the tow pilot from shocking the engine when returning. Yea yea fuel cost, but the fuel cost is not European fuel cost. The license cost estimate from zero from my local soaring club is $3800. Probably around$5000 for a PPL.

In the US let’s say you did create interest and thirty potential members showed up for an introductory ride. They expect to fly today. No way they would ever come back without their ride. No way you are making thirty tows not including others in normal operation. I have seen 200 kids flown for first ride in a day with six airplanes. A glider with more performance is not this market.

The market is solo 30 minute flight operation and round trip of 30 min. That is the time allotted to sneak away from the wife on a Saturday morning. You’re expected home after. That is pretty much everyone who owns a plane in the US. Without that ability, in the US, you are out of any sailplane operation option for the average flyer. My wife does not care if I was part of a sailplane club. She would also not put up with me being out there every weekend for eight hours. One hour, sure. Even two. She is not interested in hanging out at a field. If you are eat up with high performance sailplanes, this is not your airplane . Think of it as an alternative to other hobby planes, Pitts or aCub or Long EZ. An option for an interesting plane to own. Not looking for competitive advantage, at least at the base.

gtae07 likes this.
2. Sep 10, 2019

### Victor Bravo

#### Well-Known Member

Joined:
Jul 30, 2014
Messages:
6,071
4,848
Location:
KWHP, Los Angeles CA, USA
Even relatively high performance sailplanes can have "pogo stick" outriggers that are at half-span, and allow the glider to more or less taxi around if powered. I'm not talking about doing power donuts with the tail up like the Super Cub cowboys can do, I mean acceptably manage to get from the ramp to the runway and take off without a wing runner.

The pogo sticks could be removable when the motorglider is operated from "open" airports (ones without taxiways and ground control and such). The pogos could even retract rearward and significantly reduce drag, at the cost of some complexity and weight.

One of the more potentially viable configurations IMHO is still what Fauvel came up with 60-70 years ago, because it can have small wheels under the bottom of the vertical fins underneath the wing. This is, again, "do-able"" but not foolproof. If you turn too tightly on the ground yes it could tip forward and outboard. With a flying wing like a Fauvel, you would definitely want a "nosewheel" instead of "tailwheel configuration anyway... the main gear behind the CG and a steerable nosewheel, plus the little wheels or casters under the fins.

So ifyou used the overall configuration of the higher performance, longer span motorglider Fauvel (AV-43, AV-46????), and used a more modern 2-stroke IC or electric powerplant, you could likely achieve the following:

1) Able to taxi between the runway to the ramp without dragging a wingtip.
2) Reasonably good soaring performance (30-1 or better).
3) Reasonably good takeoff and climb (due to good wing aspect ratio and span loading)
4) No wing runner or towplane to cause problems for the tower
5) 5-10% Reduction in airframe weight (being a nurflugel) can cancel out part or all of the weight of the powerplant (in terms of empty weight).
6) Rear pusher engine "buried" in wing root and rear of "pod" would reasonably minimize engine drag without having to resort to retractable engine, pylon, etc.
7) Engine/motor/battery placement can be arranged so that main wing spar can stay straight while keeping CG where it needs to be.
8) Rearward folding propeller could further reduce drag of warranted, folded blades would not be out in "free stream" air to cause high drag.

3. Sep 10, 2019

### John.Roo

#### Well-Known Member

Joined:
Oct 8, 2013
Messages:
221
157
Location:
You are right (and as I wrote) we have a different situation in EU.
AVGAS costs over 2 EUR / lit. (not gallon), maintennance of GA tow planes is really expensive, only advantage is "relativelly" lower price in compare with new UL/LSA airplane. "Relativelly", because is difficult to compare prices of new airplane and airplane from 1970-1980

Another problem is that we cannot make any modification on certified gliders or motorgliders (adding self launch systems etc.). This is possible only in close cooperation with producer. Self launch gliders (even used) are expensive but there is already second hand market with affordable nice old gliders like ASW15, Cirrus etc. Prices 10-15T EUR are affordable even for young pilots and of course they look for cheap and available way how to get into the air So for us is aerotowing with UL/LSA airplanes definitelly interesting solution.

Best regards!
Martin

4. Sep 10, 2019

### TFF

#### Well-Known Member

Joined:
Apr 28, 2010
Messages:
11,625
3,264
Location:
Memphis, TN
Certified can only be modified by a modification that has been certified in the US. Not as stringent as airplane certification but it still has to pass certain criteria. That is why homebuilts are so big, almost total freedom of modifications. Really the only oversight is why put money that ends up being wasted junk? Economic reason to keep straight. Mostly works, Rotpar. The US just does not want to tie your unknown modified tow plane to something else of unknown qualities. Ok to take yourself down, but not ok to take someone with you.

200,000 private airplanes and 160,000 private pilots in the US; additional 100,000 commercial pilots probably own a good chunk of the private airplanes. The numbers are why LSA does not work well in the US maybe one percent is LSA. https://www.faa.gov/data_research/a...statistics/media/2018-civil-airmen-stats.xlsx If you look at the numbers glider pilots all licenses is two to one over LSA. A surprise. All you hear about is LSA. Getting a chunk of different more common but glider like planes for everyone else is the only way to make a dent in The US. Your competition is the other 200000 airplanes not the 1000 gliders.

5. Sep 10, 2019

### Hot Wings

#### Well-Known MemberHBA Supporter

Joined:
Nov 14, 2009
Messages:
6,352
2,282
Location:
Rocky Mountains
The Fauvel version of your pogo stick. Main point of contact almost in line with the CG.

6. Sep 10, 2019

### PiperCruisin

#### Well-Known Member

Joined:
Jan 18, 2017
Messages:
118
39
Location:
Idaho
Fournier RF-8 config is interesting.

Topaz likes this.
7. Sep 10, 2019

### BBerson

#### Well-Known MemberHBA Supporter

Joined:
Dec 16, 2007
Messages:
12,007
2,332
Location:
Port Townsend WA
I am planning to try that Fauvel undercarriage. (Post 185)
With the pogo or hoop at the 7 foot center section ends and nose wheel/skid. Might attach the pogo to a reinforced flap if I install flaps (pogo retracts with the flap). Going with a pusher prop even though I don't like pusher props. Mostly so I can use a traditional glider cockpit/skid.

8. Sep 10, 2019

### Jay Kempf

Joined:
Apr 13, 2009
Messages:
3,662
929
Location:
Warren, VT USA
The Rf-8 is in the right direction. Instead of just conventional gear retracted into the aft fuselage they went for bicycle plus outriggers, and still the tractor engine installation is probably the conservative way to go but kills it as a glider plus it doesn't get in a 40' hangar. The Grob is probably the best overall optimization of this class and gets exactly the compromised performance at 500fpm, 25-30:1 and can taxi around like an airplane. But still over 40'.

One solution I keep going to is to put two small motors on pylons on the wings or in nacelles pusher with folding props. But that enters into twin operation but keeps the fuselage conventional. Haven't looked into this configuration as a two seater but it makes more sense as a two seater. It's either that or figure out how to package a tractor engine on the nose for soaring somehow?

9. Sep 10, 2019

### BBerson

#### Well-Known MemberHBA Supporter

Joined:
Dec 16, 2007
Messages:
12,007
2,332
Location:
Port Townsend WA
There was guy that retracted his entire engine and prop on the nose with clamshell doors. Looked hideous.
He had a short steel prop for cutting the grass.

10. Sep 10, 2019

### Jay Kempf

Joined:
Apr 13, 2009
Messages:
3,662
929
Location:
Warren, VT USA
Crazy trying to retract a whole motor and prop. I see a lot of that in the flying car world right now.

When I first started to figure out a tandem pusher that could be a clean glider the hard part was to get the fuselage to flow to an aft point (spinner) without screwing up the fuselage forward flow through the wing root and out the back of the fillet without creating tornadoes in the wing roots. That lead to one of the breakthroughs which is to have the entire prop above the wing and well behind but still under a T type tail. Once that breakthrough comes into focus the rest is just fitting the loft over the pink squishy things and getting the CG right. The other thing is you don't want a step ladder to get in and out. Those things drive the design. The rest is pretty conventional.

11. Sep 10, 2019

### BBerson

#### Well-Known MemberHBA Supporter

Joined:
Dec 16, 2007
Messages:
12,007
2,332
Location:
Port Townsend WA
I think if the lower boom keel is only 9" wide it won't have much interference with the low wing.

12. Sep 10, 2019

### Jay Kempf

Joined:
Apr 13, 2009
Messages:
3,662
929
Location:
Warren, VT USA
The one I have pretty well along is a twin boom in T tail config. Gear retracts into the booms.

What I was describing is how to not have the wing boundary layer getting shed into the prop. The one thing you can compromise in a design like this is the low speed powered config. That can have drag problems if you are going to put them somewhere. Landing trim is flapped, gear out, lots of stuff turbulating. As long as it cleans up in climb and cruise trim then it all works (and that assumes you are looking for a higher speed cruise and you need the prop efficiency). But if it doesn't clean up at thermalling speed it also doesn't work. The back of the fuselage is getting a tiny bit of turbulent boundary layer from the root of the with a thermalling speed but there isn't much area back there and it is all turbulent at that point and contracting to the spinner and folding prop. That actually works well if you keep the booms and tail out of the wash which is sorted out. Boom wing intersection flow is very critical as well. Very tricky but easier to get right then fuselage low wing flow.

Using a single boom under the prop works but the tail is in a mess of turbulence. This is true of all T tail sailplanes as well. Junkers did a study of T tailed modern successful sailplanes and almost all had wing root separation or at least boundary layer messing up flow around the bottom of the Vtail. They spent a tone of CFD and flight testing to sort that out to buy back a significant amount of drag. Devil is in the details.

Motorgliders are one of the most interesting engineering compromises that I have run into in aviation.

Topaz likes this.
13. Sep 10, 2019

### BJC

#### Well-Known Member

Joined:
Oct 7, 2013
Messages:
9,512
6,289
Location:
97FL, Florida, USA
A nice change would be for the FAA to define a motorglider in terms of performance rather than span loading.

BJC

14. Sep 10, 2019

### Topaz

#### Super ModeratorStaff Member

Joined:
Jul 30, 2005
Messages:
13,704
5,386
Location:
Orange County, California
Mine will be like this, except the little wheel will be in the back, where it belongs. I just prefer it that way, and it's what I'm used to from flying other gliders.

15. Sep 10, 2019

### proppastie

#### Well-Known Member

Joined:
Feb 19, 2012
Messages:
3,779
1,094
Location:
NJ
Probably less drag

16. Sep 10, 2019

### Topaz

#### Super ModeratorStaff Member

Joined:
Jul 30, 2005
Messages:
13,704
5,386
Location:
Orange County, California
That's for sure! Two optimized "design points." It's a challenge.

17. Sep 10, 2019

### Hot Wings

#### Well-Known MemberHBA Supporter

Joined:
Nov 14, 2009
Messages:
6,352
2,282
Location:
Rocky Mountains
And lets make it even more fun and chop off the tail.

18. Sep 10, 2019

### Topaz

#### Super ModeratorStaff Member

Joined:
Jul 30, 2005
Messages:
13,704
5,386
Location:
Orange County, California
Whaaa??? I'm stupid, not crazy!!!

rdj and gtae07 like this.
19. Sep 10, 2019

### Jay Kempf

Joined:
Apr 13, 2009
Messages:
3,662
929
Location:
Warren, VT USA
Doesn't just stop at aero design points but that adds complexity to the front end of the optimization because when you hit a desired design modification down stream you have to always check back on two cases and make sure it still all works. All the other things come into focus as you climb down into the details and everything is compromised between weight, endurance, cruise speed, flutter, complexity, ergo, CG, and you just have to keep churning back to the beginning and all the way back to the end.

I am not sure how much glider performance you can cram into an otherwise useful airplane. But I think it is a lot. My guess is that you could climb up towards 50:1 if you just got everything right. But it is no easy task. 50:1 and a cruise speed like an older Bonanza or Mooney would be crazy. My numbers show that you could get there just barely with 15 meters with active gust damping. 17 meters is more comfortable because it keeps the aspect ratio up at the required wing area. More active damping. The design for drag reduction in soaring mode would have to be beyond where anyone has gotten yet but it is all possible.

How you package that motor is key. No way to get there with it on the nose that I can come up with. Stemme is the best folding prop and engine package at severe loss of propulsion efficiency and complexity meaning marginal climb (until they put in a turbo). And the wing it HUGE and the cost and overall complexity is tough and that narrow gear. Compromise, compromise, compromise...

Topaz likes this.
20. Sep 10, 2019